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Abstract

Distinctive suites of landslides occur in five stratigraphic-structural provinces in the Foothills area of southwestern Alberta. The Porcupine Hills are characterized by slumps and earthflows on slopes steepened by fluvial activity. The ridges of the Rocky Mountain Foothills have a low frequency of landsliding due to slope angles that are generally lower than bedding-plane dip angles. Extensive landsliding around the Mokowan Butte upland is likely due to shearing of bedrock beneath the Lewis thrust and glacial oversteepening. Glaciolacustrine valley fills form the floors of interridge valleys within the Foothills. These glaciolacustrine sediments fail as rotational slumps and flows. Rockslides and rock avalanches cluster along major thrust faults in the eastern Rocky Mountains. Glacial steepening and the exposure of cliff-forming Proterozoic and Paleozoic carbonates and clastics overlying recessive clastics, particularly along thrust faults, are identified as significant destabilizing factors. A mass-wasting feedback loop is suggested, cliff-forming massifs driving failure in underlying recessive rocks, which in turn triggers failures in the massif. Creep is suspected as a factor in footwall slope instability. Landsliding has likely been a prime agent in the retreat of the mountain front for at least the past 2.6 m.y. Recession rates of 0.2 cm/yr can be computed for this period.

Introduction

Surficial and bedrock geology of the Foothills and adjacent parts of the Rocky Mountain Front Ranges to the west and Interior Plains to the east were remapped in southwestern Alberta at 1:50 000 scale between 1993 and 1996 in a part of the Geological Survey of Canada's National Geoscience Mapping Program (NATMAP; Fig. 1) (Holme, 1998; Jackson, 1995, 1998a, 1998b, 1998c, 1998d, 1998e; Jackson and Leboe, 1998; Leboe, 1996, 1998a, 1998b; Little, 1995, 1998a, 1998b, 1998c). These maps supersede reconnaissance-scale maps of the area (1:250 000 to 1:125 000 scale; Stalker, 1957, 1959, 1962; Jackson, 1987a). The remapping of the surficial geology of this region provided the opportunity to systematically map landslides for the first time. Landsliding has been shown to be a common and widespread potential geologic hazard in this region (Jackson, 1995).

Figure 1.

Location map of southeastern Alberta study area. Numbers refer to locations of subsequent figures.

Figure 1.

Location map of southeastern Alberta study area. Numbers refer to locations of subsequent figures.

The purpose of this chapter is twofold: first, to summarize the distribution and styles of landsliding within a large area of the eastern margin of the Canadian Cordillera (∼12 000 km2), and second, to explore the relationships between landslide occurrence and the stratigraphy, structure, surficial geology, and landscape evolution during the late Tertiary and Quaternary Periods.

GEOLOGIC FRAMEWORK

The study area is underlain almost entirely by sedimentary bedrock units ranging in age from Proterozoic to early Tertiary. It is discontinuously capped by late Pleistocene glacial drift (Table 1). Precambrian and Paleozoic sedimentation occurred in passive continental margin basins along what was then the west coast of North America. Mesozoic and early Tertiary sedimentation took place in foreland basins created by continental accretion and orogeny to the west. Tectonic compression from the west eventually deformed the entire sedimentary wedge into a fold and thrust mountain belt during the Late Cretaceous and early Paleogene (Gabrielse et al., 1992; Stott et al., 1992). Orogeny ended in the Paleocene. The area discussed in this chapter is divisible into five stratigraphic-structural provinces (Fig. 2).

Figure 2.

Regional characterization of landslides and associated bedrock and/or structural provinces. Bedrock geology is generalized after Douglas (1950, 1952) and Norris (1955, 1993). Locations: OMR is area of active slumping in glaciolacustrine sediments and Late Cretaceous bedrock along Oldman River; SWC is area of extensive earth flows from glaciolacustrine sediments and argillaceous bedrock units (Fig. 4); LvT is Livingstone thrust; LT is Lewis thrust; TMF is Turtle Mountain fault; OMD is Oldman Dam; OMG is Oldman gorge, created by glacial diversion of Oldman River; CNP is Crowsnest Pass; CNR is Crowsnest River; CM is Chief Mountain; PH is Porcupine Hills; MB is Mokowan Butte; BR is Belly River; FS is Frank slide. A—A′, B—B′, C—C′, D—D′ and E—E′ locate section lines shown in subsequent figures.

Figure 2.

Regional characterization of landslides and associated bedrock and/or structural provinces. Bedrock geology is generalized after Douglas (1950, 1952) and Norris (1955, 1993). Locations: OMR is area of active slumping in glaciolacustrine sediments and Late Cretaceous bedrock along Oldman River; SWC is area of extensive earth flows from glaciolacustrine sediments and argillaceous bedrock units (Fig. 4); LvT is Livingstone thrust; LT is Lewis thrust; TMF is Turtle Mountain fault; OMD is Oldman Dam; OMG is Oldman gorge, created by glacial diversion of Oldman River; CNP is Crowsnest Pass; CNR is Crowsnest River; CM is Chief Mountain; PH is Porcupine Hills; MB is Mokowan Butte; BR is Belly River; FS is Frank slide. A—A′, B—B′, C—C′, D—D′ and E—E′ locate section lines shown in subsequent figures.

Table 1.

Generalized Stratigraphy of the Foothills and Rocky Mountains, Southwestern Alberta

AgeStratigraphic unitLithologyThickness (m)Comments
CenozoicTertiaryPorcupine Hills FormationSandstone and shale1200Sandstone beds form cliffs or steep slopes
Willow Creek FormationSandstone and shale600Recessive, usually involved in landsliding or badland erosion
CretaceousSt. Mary River Formation900As above
Belly River FormationQuartz and chert Sandstone1200Forms small cliffs where competent
Alberta GroupSiltstone, shale, and local resistant sandstone700Recessive, resistant competent sandstones form ridges
PaleozoicPermianRocky Mountain GroupQuartzite and dolomitic sandstone210Recessive to cliff forming
CarboniferousRundle GroupDolomite, limestone, shale, anhydrite250Forms 200 m cliffs
Banff FormationLimestone, shale, siltstone, chert190Recessive, carbonate units form small cliffs
Exshaw FormationSiltstone, limestone20Recessive
Palliser FormationLimestone, dolomite200Cliff forming
DevonianAlexo FormationLimestone, dolomite90Cliff forming
Mt. Hawk FormationArgillaceous limestone, dolomite120Recessive, grades laterally into the biohermal Southesk Formation, which forms cliffs
Fairholm FormationLimestone, dolomite200Recessive to cliff forming
Precambrian (Proterozoic)Purcell Supergroup3400–6000
Roosville FormationSiltstone Quartzite Dolomite50Recessive to cliff forming
Phillips FormationSiltstone Dolomite400–500As above
Gateway FormationAs above1000Recessive
Shepherd Formation300Recessive
Purcell LavaChoritized andesite200–300Recessive
Syeh FormationDolomite800Cliff forming
AgeStratigraphic unitLithologyThickness (m)Comments
CenozoicTertiaryPorcupine Hills FormationSandstone and shale1200Sandstone beds form cliffs or steep slopes
Willow Creek FormationSandstone and shale600Recessive, usually involved in landsliding or badland erosion
CretaceousSt. Mary River Formation900As above
Belly River FormationQuartz and chert Sandstone1200Forms small cliffs where competent
Alberta GroupSiltstone, shale, and local resistant sandstone700Recessive, resistant competent sandstones form ridges
PaleozoicPermianRocky Mountain GroupQuartzite and dolomitic sandstone210Recessive to cliff forming
CarboniferousRundle GroupDolomite, limestone, shale, anhydrite250Forms 200 m cliffs
Banff FormationLimestone, shale, siltstone, chert190Recessive, carbonate units form small cliffs
Exshaw FormationSiltstone, limestone20Recessive
Palliser FormationLimestone, dolomite200Cliff forming
DevonianAlexo FormationLimestone, dolomite90Cliff forming
Mt. Hawk FormationArgillaceous limestone, dolomite120Recessive, grades laterally into the biohermal Southesk Formation, which forms cliffs
Fairholm FormationLimestone, dolomite200Recessive to cliff forming
Precambrian (Proterozoic)Purcell Supergroup3400–6000
Roosville FormationSiltstone Quartzite Dolomite50Recessive to cliff forming
Phillips FormationSiltstone Dolomite400–500As above
Gateway FormationAs above1000Recessive
Shepherd Formation300Recessive
Purcell LavaChoritized andesite200–300Recessive
Syeh FormationDolomite800Cliff forming

Porcupine Hills

Porcupine Hills is a flat-summit upland that spans the eastern margin of the Foothills and the western limit of the Interior Plains. It is underlain by flat-lying to gently dipping strata of latest Cretaceous and Paleocene shale and sandstone with minor coal, conglomerate, and coquinoidal limestone of the Porcupine Hills, Willow Creek, and St. Mary River Formations (Table 1; Douglas, 1950). Shales and some sandstone units within the Willow Creek and St. Mary River Formations are bentonitic and erodible, and readily form badland topography where exposed. Relief ranges to 350 m from crest to eastern and western margins, but local relief is usually <100 m.

Foothills

Mathews' (1986) definition of the Foothills included the entire disturbed belt east of the Lewis and McConnell thrusts (south and north of Crowsnest Pass, respectively). For the purposes of this chapter, the western limit of the Foothills is placed at the eastern limit of exposures of Paleozoic or Proterozoic rocks that are brought to the surface along the Lewis, Livingstone, and Turtle Mountain thrusts. Foothills relief commonly ranges to 600 m. The Foothills, as defined here, are characterized by asymmetric ridges underlain by steeply dipping Jurassic to Late Cretaceous sandstone, shale, and conglomerate. These are variably folded and repeated along westward-dipping thrust faults (Douglas, 1950; McMechan and Thompson, 1992).

Mokowan Butte

The Mokowan Butte upland, and nearby uplands along the front of the Rocky Mountains are a part of the Foothills that was formerly beneath the Lewis thrust before erosion stripped away the overlying thrust sheet (McMechan and Thompson, 1992; Fig. 3). Rocks below major thrusts are usually covered by glacial drift, landslide deposits, or other types of colluvium. Where they are exposed, they are extensively sheared and deformed. Ross (1959) and Ross and Wezak (1959) estimated the thickness of the zone of sheared and broken rock below the Lewis thrust to range from ∼100 to as much as 600 m. Much of Mokowan Butte is suspected as being a part of this sheared zone based upon the extensive landsliding in this area (see following) and scattered exposures of sheared and broken rock (D. Lebel, 1995, personal commun.).

Figure 3.

Geologic cross section (vertical exaggeration ×2) looking north across Lewis and Clark Ranges (generalized from McMechan and Thompson, 1992). T is Tertiary clastics west of Flathead fault, M is folded and thrust-faulted Mesozoic clastics, and P are folded and thrust-faulted carbonates and clastics. PC is thrust-faulted Precambrian (Proterozoic) carbonates, clastics and minor volcanics. Reconstruction of eroded rock above Lewis thrust is based upon complete sections preserved west of Flathead fault. Thickness of rock removed from Foothills is based upon coal maturation studies. X is hypothetical point where ROR (contrasting resistant over recessive lithologies) was first exposed during regional erosion following orogeny. Z is hypothetical point where contemporary ROR configuration became exposed and sapping retreat of mountain front, that continues today, began. B—B′ indicates location to Sofa Mountain-Mokowan Butte profile shown in Figure 10.

Figure 3.

Geologic cross section (vertical exaggeration ×2) looking north across Lewis and Clark Ranges (generalized from McMechan and Thompson, 1992). T is Tertiary clastics west of Flathead fault, M is folded and thrust-faulted Mesozoic clastics, and P are folded and thrust-faulted carbonates and clastics. PC is thrust-faulted Precambrian (Proterozoic) carbonates, clastics and minor volcanics. Reconstruction of eroded rock above Lewis thrust is based upon complete sections preserved west of Flathead fault. Thickness of rock removed from Foothills is based upon coal maturation studies. X is hypothetical point where ROR (contrasting resistant over recessive lithologies) was first exposed during regional erosion following orogeny. Z is hypothetical point where contemporary ROR configuration became exposed and sapping retreat of mountain front, that continues today, began. B—B′ indicates location to Sofa Mountain-Mokowan Butte profile shown in Figure 10.

Drift-filled intra-Foothills valleys

The large valleys and extensive lowland areas within the Foothills, particularly those immediately west of the Porcupine Hills, are underlain by extensive fillings of glaciolacustrine clay and clayey silt (e.g., Fig. 2, OMR, areas designated by L; Fig. 4). These sediments were deposited in lakes formed by the damming of these valleys by glacial ice during the late Pleistocene (Douglas, 1950; Alley and Harris, 1974; Jackson, 1980; Holme et al., 2000). These fills deeply bury bedrock and, with respect to the occurrence of landslides, constitute a distinct terrain.

Figure 4.

Slumps and earthflows in area of Porcupine Hills and extensive failures in glacial lake sediments, upper reaches of South Willow Creek, and its major tributaries. Slumps in bedrock occur in sandstone and mudstone.

Figure 4.

Slumps and earthflows in area of Porcupine Hills and extensive failures in glacial lake sediments, upper reaches of South Willow Creek, and its major tributaries. Slumps in bedrock occur in sandstone and mudstone.

Rocky Mountains

This province is formed of thrust sheets of Proterozoic and Paleozoic carbonate and clastic rocks. This province includes those portions of the Foothills defined by Mathews (1986) as having Paleozoic units thrust over Mesozoic ones. Individual thrust faults such as the Lewis thrust (Fig. 2) can be traced for hundreds of kilometers. Relief from ridge top to mountain valley commonly exceeds 1000 m. Two distinct subprovinces occur within the area studied. South of the Crowsnest Pass, mountain-forming Proterozoic Belt and/or Purcell clastic, carbonate, and minor volcanic rocks are brought to the surface as low-angle thrust sheets (Fig. 3). Remnants of the largely eroded Paleozoic sedimentary cover are locally present. Purcell rocks are thrust over Upper Cretaceous clastic rocks along the Lewis thrust at the mountain front (Douglas, 1952; Price, 1962, 1965). North of the Crowsnest Pass (Figs. 48), mountain-forming Paleozoic carbonates are brought to surface along low to moderately dipping northwest-southeast-imbricated thrust faults (Douglas, 1950; Norris, 1955, 1993). The nature of faulting ranges from single faults beneath imbricated thrust blocks to numerous faults spaced tens of hundreds of meters apart. Rocks of Triassic to Late Cretaceous age form footwalls. Scattered exposures indicate that footwall rocks range from apparently undisturbed to intricately crumpled and broken (Ross, 1959). The nature of deformation in footwall rocks usually cannot be readily determined because they are buried by talus and landslide colluvium.

Figure 5.

Landslides and related features along Livingstone Range in area of Todd Creek. Cross section: P is cliff-forming Mississippian limestone and dolostone of Rundle Group; M is Cretaceous coal-bearing clastics (generalized from Norris, 1955; no vertical exaggeration); LT is Livingstone thrust. Relief from mountain summit to valley bottoms to east is ∼900 m. Air photo: Livingstone thrust (not marked) is at base of cliff-forming limestone and dolostone of Rundle Group. Adjacent forested slopes are underlain by predominantly Cretaceous coal-bearing clastics. BP is bedding plane rockslide in Rundle Group; RA is rock avalanche in Paleozoic bedrock; MS is bedding plain rockslide in Mesozoic clastics; EC is area of closely spaced gulleys cutting headward into Mesozoic clastics and undermining overlying Paleozoic carbonates; EM is landslide complex in Mesozoic clastics undermining overlying Paleozoic carbonates (Canada National Airphoto Library A23234-11. A23234-12). C is the western end of section C–C′.

Figure 5.

Landslides and related features along Livingstone Range in area of Todd Creek. Cross section: P is cliff-forming Mississippian limestone and dolostone of Rundle Group; M is Cretaceous coal-bearing clastics (generalized from Norris, 1955; no vertical exaggeration); LT is Livingstone thrust. Relief from mountain summit to valley bottoms to east is ∼900 m. Air photo: Livingstone thrust (not marked) is at base of cliff-forming limestone and dolostone of Rundle Group. Adjacent forested slopes are underlain by predominantly Cretaceous coal-bearing clastics. BP is bedding plane rockslide in Rundle Group; RA is rock avalanche in Paleozoic bedrock; MS is bedding plain rockslide in Mesozoic clastics; EC is area of closely spaced gulleys cutting headward into Mesozoic clastics and undermining overlying Paleozoic carbonates; EM is landslide complex in Mesozoic clastics undermining overlying Paleozoic carbonates (Canada National Airphoto Library A23234-11. A23234-12). C is the western end of section C–C′.

Figure 6.

Complex landslides and related mass-wasting features and deposits, upper Willow Creek and Livingstone River basins in area of Livingstone thrust fault. Cross section (no vertical exaggeration): thrust faults depicted as individual features are actually complexes of closely spaced faults. P is predominantly Paleozoic carbonates; M is Mesozoic shale, sandstone, and coal; RS is rock avalanche located on air photo. Air photo: RS is rock avalanche in glacially steepened mountain pass; BP is bedding-plane failure in Paleozoic bedrock. Relief of rock avalanche from source area to toe of deposit is ∼270 m. Maximum local relief is ∼730 m (Canada National Airphoto Library A18306172, A18306173).

Figure 6.

Complex landslides and related mass-wasting features and deposits, upper Willow Creek and Livingstone River basins in area of Livingstone thrust fault. Cross section (no vertical exaggeration): thrust faults depicted as individual features are actually complexes of closely spaced faults. P is predominantly Paleozoic carbonates; M is Mesozoic shale, sandstone, and coal; RS is rock avalanche located on air photo. Air photo: RS is rock avalanche in glacially steepened mountain pass; BP is bedding-plane failure in Paleozoic bedrock. Relief of rock avalanche from source area to toe of deposit is ∼270 m. Maximum local relief is ∼730 m (Canada National Airphoto Library A18306172, A18306173).

Figure 7.

Cross section showing the structural and stratigraphic setting of the summit fissure on Mt. Livingstone (generalized from Jackson and Lebel, 1998). Vertical exaggeration ×2.5: M is Mesozoic shale, sandstone, and coal; P is predominantly Paleozoic carbonates. Thrust faults depicted are commonly complexes of closely spaced faults. Mesozoic units are complexly folded. Photos: A is oblique aerial view looking south. B is ground view looking north from “V” at south end of fissure. Total width of cracking is ∼5 m.

Figure 7.

Cross section showing the structural and stratigraphic setting of the summit fissure on Mt. Livingstone (generalized from Jackson and Lebel, 1998). Vertical exaggeration ×2.5: M is Mesozoic shale, sandstone, and coal; P is predominantly Paleozoic carbonates. Thrust faults depicted are commonly complexes of closely spaced faults. Mesozoic units are complexly folded. Photos: A is oblique aerial view looking south. B is ground view looking north from “V” at south end of fissure. Total width of cracking is ∼5 m.

Figure 8.

Reconstruction of the face of Turtle Mountain using profiles 4 and 6 of Daly et al. (1912). No vertical exaggeration. Profiles are ∼250 m apart and trend east-west normal to long axis of Turtle Mountain. Profile 4 is located immediately northwest of the detachment scar of the 1903 Frank Slide (profile 6) and is representative of the prefailure profile. Glacial limit is projected based on adjacent surficial geology mapping (Jackson et al., 1996).

Figure 8.

Reconstruction of the face of Turtle Mountain using profiles 4 and 6 of Daly et al. (1912). No vertical exaggeration. Profiles are ∼250 m apart and trend east-west normal to long axis of Turtle Mountain. Profile 4 is located immediately northwest of the detachment scar of the 1903 Frank Slide (profile 6) and is representative of the prefailure profile. Glacial limit is projected based on adjacent surficial geology mapping (Jackson et al., 1996).

REGIONAL LANDSCAPE EVOLUTION

Slope failure takes place when resisting forces within a slope become less than driving forces. Landscape evolution since the building of the Rocky Mountains generally has acted to reduce resisting forces through weathering and increase driving forces through stream incision and undercutting of slope toes. Denudation has been balanced by regional uplift. If it had not, the carbonate-dominated Rocky Mountains would have been leveled by solution alone in 12–14 m.y. (Ford et al., 1981). The compensatory uplift has apparently been caused by isostatic adjustment in response to regional erosional stripping rather than compressive tectonic forces: the most recent faulting in the region was immediately postorogenic normal faulting along the Flathead fault (Fig. 3) during the Paleogene (Ross, 1959; Price, 1965). Furthermore, the region appears seismically quiescent; no earthquakes above magnitude 3 having been recorded since seismic monitoring of the region was established in 1955 (Sweeny et al., 1992), with the notable exception of a magnitude 4.7 earthquake in the area of Crowsnest Pass in 1984 (Drysdale and Horner, 1984). Coal maturation studies estimate that 2–3 km of rock have been removed by erosion from the Interior Plains east of the study area since the Paleocene (Nurnowski, 1984; Hacquebard, 1987). Stratigraphic studies west of the Flathead fault (Fig. 3) estimate the removal of at least 6 km of Mesozoic and Paleozoic rock since orogeny (Price, 1965; McMechan and Thompson, 1992). However, estimates of the rate of valley deepening can be made for no more than the past several million years. This period is also coincident with the onset of periodic montane glaciation, which has served to steepen valley sides as well as deepen them. Ford et al. (1981), on the basis of the age of dripstone in phreatically formed caverns, estimated a minimum rate of 0.04 m/k.y. for valley deepening in the Crowsnest Pass area of the Rocky Mountains since the last magnetic reversal (ca. 0.78 Ma; Cande and Kent, 1995).

Recent investigations of the late Tertiary to mid-Pleistocene Kennedy Drift (Cioppa et al., 1995; Barendregt et al., 1991) permits an estimate of the maximum rate of valley incision over a period about three times longer. The Kennedy Drift caps Mokowan Butte and other remnants of the late Miocene or Pliocene Flaxville erosional surface east of the Rocky Mountain Front in the area of the International Border (Fig. 10; Alden, 1932; Ross, 1959). If an age of ca. 2.6 Ma is assumed for the oldest drift unit in the Kennedy Drift (Cioppa et al., 1995; Barendregt et al., 1991), and that the Flaxville surface approximates the surface of the Interior Plains at that time, then the ∼430 m of relief between the summit of the Mokowan Butte upland and the bottom of the adjacent Belly River valley was cut at a rate of 0.16 m/k.y. This should be considered a maximum limiting estimate because the valley of the Belly River may have been partly cut at the time of deposition of the basal unit of the Kennedy Drift.

Figure 10.

Topographic profile between Sofa Mountain and Mokowan Butte area (vertical exaggeration ×2.5). Flaxville erosion surface is capped by Kennedy Drift (basal age estimated as 2.6 Ma). Incision of the Flaxville surface partly or entirely postdated the deposition of Kennedy Drift. Profile shows hypothetical mountain front position at time of deposition of Kennedy Drift assuming 0.2 cm/yr recession rate. PC is Proterozoic Purcell Supergroup; M is Mesozoic sandstone and shale.

Figure 10.

Topographic profile between Sofa Mountain and Mokowan Butte area (vertical exaggeration ×2.5). Flaxville erosion surface is capped by Kennedy Drift (basal age estimated as 2.6 Ma). Incision of the Flaxville surface partly or entirely postdated the deposition of Kennedy Drift. Profile shows hypothetical mountain front position at time of deposition of Kennedy Drift assuming 0.2 cm/yr recession rate. PC is Proterozoic Purcell Supergroup; M is Mesozoic sandstone and shale.

The Foothills region was extensively glaciated by montane piedmont glaciers and continental provenance glacial ice during the last (late Wisconsinan) ice age (ca. 20–10 ka; Little, 1995; Jackson et al., 1996; Leboe, 1996). The incursion of continental ice into the region was unique to the late Wisconsinan (Jackson et al., 1997, 1999), but montane glaciers mantled the Rocky Mountains many times earlier during the Quaternary Period (Richmond, 1965). During the last glaciation, river diversion by glaciers caused dramatic local valley cutting and deepening, slope destabilization, and postglacial migration of knickpoints upstream. For example, the 80-m-deep gorge of the Oldman River, now the site of the Oldman River Dam (Fig. 2), was cut through Upper Cretaceous sand-stone and shale in early postglacial time due to the burial of the preexisting Oldman River valley by drift and subsequent displacement of the Oldman River (Leboe, 1996). Upstream migration of the knickpoint created by this diversion has caused tens of meters of incision through unstable glacial lacustrine sediments.

LANDSLIDE MAPPING AND CLASSIFICATION

Landslides were identified and mapped almost entirely through air-photo interpretation using ∼1:50 000 scale air photographs. They were mapped regardless of apparent activity or inactivity. Landslide type was assigned according to the type of past movement and the material (soil or rock) after the system of Varnes (1978). Any reconnaissance classification of landslides is by necessity subjective and many failures are apparently complex landslides that combine several failure types. In these cases, the predominant failure type was chosen to characterize the failure. A brief description of categories recognized is presented in the following. Smaller scale mass-wasting phenomena such as small-scale rockfalls (e.g., Gardner, 1982) or channelized debris flows (Jackson, 1987b) are not addressed in this summary.

Rotational slump

Rotational slumps were distinguished where there was evidence of the slope failure having, at least partly, rotated about an axis so that the upper part of the failure had dropped and the lower part had risen relative to the unfailed slope. Within the study area, the slumping process commonly remolds material forming the toe of the slump to the extent that it may continue to move downslope as an earth flow, as depicted by Selby (1982, his Fig. 6.5).

Earthflow

Earthflows are tongue-shaped or lobate in plan. Their profiles gradually thicken in a downslope direction from a concave source area, commonly a rotational slump failure, to a bulbous terminus (Fig. 4). The lack of brittle cracking of the surface of these failures and the presence of transverse ridges reflects movement through internal ductile deformation as well as by sliding as a mass along a basal failure plane. They commonly occur on slopes of 10° or less and lack evidence of rapid movement. Past or present movement of these failures, where they appear to be active, appears to be by very slow or discontinuous creep.

Rockslide

Rockslides are detached planar masses of bedrock that have slid along a well-defined relatively linear surface in contrast to rotational slumps (Fig. 5). Included in this category is a continuum of failures, from those where the failed mass has largely moved as a unit and is relatively unbroken (rock glide), to those where the failed mass has disintegrated due to differential slip rates within the mass, or progressive but discontinuous propagation of failure upslope. Within the study area, these failures usually appear to be associated with bedding planes or unconformities.

Rock avalanche

These landslides (Fig. 5 and 9) result from cliff collapse in mountainous areas. These are rapid failures: velocities ranging from 16 to 77 m/s have been recorded or reconstructed (Mollard, 1977; Plafker and Erikson, 1978; Jackson and Isobe, 1990). Former cliffs are entirely reduced to bouldery rubble. These failures are notable for their anomalous mobilities and fluid-like flow behaviors, including superelevation around bends in their paths, runup of slopes, and the ability to become an airborne stream when launched over a ramp (Hsu, 1975; Eisbacher, 1979; Mollard, 1977; Jackson and Isobe, 1990). Runout distances and run up heights on opposite valley walls greatly exceed those predicted by directly measured sliding friction values of rock (Hsü, 1975). The cause of the low effective sliding friction angles of rock avalanches has been debated (see Melosh, 1987, for a summary). However, it appears that low effective sliding friction angles become operative when a volumetric threshold in the range of 0.1–1 × 106 is crossed (Eisbacher and Clague, 1984; Melosh, 1987). The Frank slide (Fig. 2, FS) with an estimated volume of 36.5 × 106 m3 is the best example within the study area. It occurred in 1903, killing 70 persons and partly burying the town of Frank. Maximum travel of the debris was 3 km (Cruden, 1976). It climbed a maximum of 140 m vertically up the adjacent mountainside from the base of Turtle Mountain (Daly et al., 1911). Other smaller rock avalanches (105–106 m3 range) are common in the study area and show similar mobility. The northern of the two rock avalanches in Figure 5 displays superelevation on the outside of its curved travel path.

Figure 9.

Rockslides in stratigraphic successions of resistant, ridge forming units overlying recessive ones. Stereo pair: white letters identify rockslides (A—E), black or white letters in italics identify end points of cross sections shown below, and bold black letters denote physiographic and drainage features. Rockslide A: limestones and dolomites of Devonian Fairholme Group (Df) and Middle Cambrian Windsor Mountain and Elko Formations (Ce) (cross section F—F′) are failing on shale of underlying Middle Cambrian Gordon Formation (Ggo). Rockslides B, C, D: complex landslides involving ridge-forming Flathead Formation quartzite (Cft) and recessive underlying shale, dolostone, and sandstone of Proterozoic Roosville (Hro) and Phillips (Hph) Formations, Purcell Supergroup. Flathead Formation overlies Proterozoic succession along angular unconformity. Rockslide E remains to be studied in more detail. Other units shown (formations of Purcell Supergroup): Hga is Gateway Formation (mafic lava, slate, silt-stone, and dolomite), Hsh is Shepard Formation (dolomite). Locations on stereo pair: MC is Mill Creek; EC is East Castle River; WR is Windsor Ridge; WM is Windsor Mountain (Canada National Airphoto Library A23232262, A23232263). Cross sections (no vertical exaggeration) modified slightly form those prepared by D. Lebel, Geological Survey of Canada.

Figure 9.

Rockslides in stratigraphic successions of resistant, ridge forming units overlying recessive ones. Stereo pair: white letters identify rockslides (A—E), black or white letters in italics identify end points of cross sections shown below, and bold black letters denote physiographic and drainage features. Rockslide A: limestones and dolomites of Devonian Fairholme Group (Df) and Middle Cambrian Windsor Mountain and Elko Formations (Ce) (cross section F—F′) are failing on shale of underlying Middle Cambrian Gordon Formation (Ggo). Rockslides B, C, D: complex landslides involving ridge-forming Flathead Formation quartzite (Cft) and recessive underlying shale, dolostone, and sandstone of Proterozoic Roosville (Hro) and Phillips (Hph) Formations, Purcell Supergroup. Flathead Formation overlies Proterozoic succession along angular unconformity. Rockslide E remains to be studied in more detail. Other units shown (formations of Purcell Supergroup): Hga is Gateway Formation (mafic lava, slate, silt-stone, and dolomite), Hsh is Shepard Formation (dolomite). Locations on stereo pair: MC is Mill Creek; EC is East Castle River; WR is Windsor Ridge; WM is Windsor Mountain (Canada National Airphoto Library A23232262, A23232263). Cross sections (no vertical exaggeration) modified slightly form those prepared by D. Lebel, Geological Survey of Canada.

PATTERNS OF LANDSLIDE OCCURRENCE IN THE CONTEXT OF REGIONAL GEOMORPHIC EVOLUTION

Each of the physiographic regions previously described have characteristic suites of slope failures. These are described in the following and their general distribution is presented in Figure 2; particularly notable areas of occurrence are presented in Figures 49. Detailed analysis of the causes of each failure is beyond the scope of this chapter. This discussion identifies factors that have led to landsliding in the context of long- and short-term geomorphic evolution.

Porcupine Hills

Slope failures within the Porcupine Hills are predominantly rotational slumps and earthflows that originate within shale units. Because the slumps and earthflows co-occur and intergrade, they are designated by a single symbol in Figure 2 (diamond). They occur along valley sides within higher relief areas of the Porcupine Hills (Fig. 4) and along streams where they are undercutting valley sides. These failures are ultimately triggered by fluvial incision of this upland. However, glaciofluvial erosion is an indirect contributing factor: major melt-water channels were cut along the east and west margins of the Porcupine Hills during the last glaciation. This has lowered local base level, causing streams with headwaters within the Porcupine Hills to incise and destabilize adjacent slopes.

Foothills (bedrock uplands)

Landslides originating entirely within bedrock are relatively uncommon within the ridges of the Foothills compared to the Porcupine Hills to the east and the Rocky Mountains to the west. The largest landslides in the Foothills are rockslides along dip slopes where slope angles and bedding dip angles are close in value (Fig. 5, MS).

Several factors account for the relatively low density of landsliding in the Foothills.

1. Sandstone and shale units are generally more competent within the Foothills (excluding the Mokowan Butte area) and less expansive than the younger formations in the Porcupine Hills.

2. Relief is generally less than within the Rocky Mountains.

3. Bedding inclinations are predominantly steeper than slope angles. Such underdip slopes have been shown to be stable in the region (Cruden and Hu, 1993).

4. Ridges of Mesozoic clastics are not surmounted by thrust sheets of massive Paleozoic or Proterozoic rocks (as in the Rocky Mountains), which impart significant driving forces into lower slopes in that region (see following).

5. Although the area was inundated by as much as 300 m of ice during the last glaciation, there is little evidence of glacial erosion of ridges. Points where ice flow was confined and erosion and slope steepening was intense as a result were probably rare compared to the Mokowan Butte area and parts of the Rocky Mountains (see following).

Mokowan Butte upland

The Mokowan Butte upland (Fig. 2, MB; Fig. 10) is the most extensive area of contiguous landsliding in the entire region. Landsliding has consumed, or is in the process of consuming, almost all of this upland with the exception of a small area of the summit. No geotechnical investigations have been carried out to obtain detailed information on the mode and depth of failure within the slopes of Mokowan Butte or the degree to which they are currently active. Slope failures in this area are complex landslides and are comparable in style to the active and inactive earthflow complexes described by Swanson and Swanston (1977) in the Oregon Cascades. Failure around summit areas appears to be by rotational slumping. However, these failures grade down slope into ridged and undulatory topography similar to that seen on earthflows. Failure and movement occurs on slopes as low as 3–12°.

The role that bedrock structure (bedding and joint orientation) plays in the great extent of landsliding around this upland is not clear. Structure can only be extrapolated through the thick cover of glacial drift and landslide colluvium. Consequently, Lewis thrust-related shearing and weakening of the sandstone, shale, and coal underlying Mokowan Butte is strongly suspected but not currently demonstrable. However, the recent glacial history of Mokowan Butte is well known (Alden, 1932; Little, 1995), and it may have been a significant factor in destabilizing slopes: the west side of Mokowan Butte (Fig. 10) was scoured by a montane glacier that advanced from south to north down the Belly River valley across 49°N at the climax of the last glaciation. The east side of Mokowan Butte was scoured by the margin of the Laurentide ice sheet, which flowed south across 49°N subparallel to Foothills structure during the same glaciation. Erosion by the passing ice may have oversteepened these slopes, leading to landsliding when the ice left the area.

Foothills (drift-filled interridge valleys)

The glaciolacustrine sediments that make up much of the glacial drift fill in interridge valleys have mean in situ moisture contents above the plastic limit, and values sometimes approach the liquid limit. They creep noticeably in many artificial cuts (Jackson, 1987a, p. 20–26). They have failed massively along deeply incised meltwater channels and along contemporary stream valleys, which have been incised due to glacially induced lowering of the local base level. Landsliding is initiated as rotational slumping and is transformed into earthflows. The most extensive failures have occurred in the upper reaches of South Willow Creek in the Langford Creek map area (Fig. 4). Failure occurs in glaciolacustrine sediments where driving forces in slopes are increased due to glaciofluvial or fluvial erosion of slope toes. Failure is not restricted to glaciolacustrine sediments. Glaciolacustrine sediments and underlying Upper Cretaceous bedrock commonly fail together in large slump complexes along steep-sided stream valleys or meltwater channels that have been incised through glaciolacustrine sediments and into the underlying Upper Cretaceous bedrock. Slump complexes along the Oldman River between its confluence with Callum Creek and Crowsnest River are notable examples of this process (Fig. 2, OMR).

Rocky Mountains

The Rocky Mountains are dominated by rockslides and rock avalanches.

Rockslides.

Dip-slope rockslides are widespread in the Rocky Mountains and occur in environments similar to those in the Foothills; i.e., where slope angles are close to or exceed bedding dip angles (Figs. 5 and 6). However, rockslides and rock avalanches commonly cluster above, below, and across contacts where resistant, usually massive cliff-forming carbonate units or thick, resistant clastic units overlie recessive clastics. These montane-scale resistant over recessive successions are referred to as ROR herein. RORs are the result of major overthrust faulting that has superposed Paleozoic or Proterozoic carbonate units upon recessive clastics (Figs. 2, 3, 410) or conformable or unconformable depositional superposition of ROR units (Cruden, 1976; Stepanek, 1992; Jackson and Lebel, 1998). Particularly notable depositional RORs that have given rise to rockslides occur in Lower Cambrian successions and superposition across the angular unconformity between the Paleozoic and Precambrian Purcell Supergroup rocks (Fig. 9; see following).

Rock avalanches.

The Frank slide (Fig. 2 in FS; Fig. 8) is the largest rock avalanche in the study area, and occurred within a structurally created ROR. It was originally interpreted as a failure across bedding and along joint planes in the Mississippian limestone and dolostone of the Rundle Group above the Turtle Mountain thrust fault (Daly et al., 1912). Subsequent reexaminations have concluded that it was bedding failure along an overturned anticline or a toppling failure of the Mississippian limestone and dolostone of the Rundle Group (Cruden and Krahn, 1973; Jones, 1993). The role of coal mining in triggering the Frank slide has long been controversial (Daly et al.. 1912; Stead and Benko, 1998).

The most recent large failure in the Rocky Mountains in this region occurred in July 1992 several kilometers south of the study area in Montana when part of north face of Chief Mountain (Fig. 2, CM; Fig. 11) collapsed in a large mobile rock avalanche estimated to be in the 10 × 106 m3 range (Schuster et al., 1995). Many smaller scale rock avalanches have occurred along the Livingstone Range, principally from the Rundle Group (Fig. 5, RA). Some of these have occurred during the past several thousand years judging from their unvegetated states. However, other failures date back to late glacial time: an extensive rock avalanche deposit in the 1 × 106 to 1 × 107 m3 volume range fell on to stagnant glacial ice in late glacial time. It has many kettle holes that were created when the underlying glacial ice melted. It is also partly overlain by a moraine from a late glacial cirque advance (Fig. 12).

Figure 11.

North face of 2768-m-high Chief Mountain. Montana, showing 1993 rock-avalanche deposits and failure area within dolomite of Siyeh Formation of Purcell Supergroup. Lewis thrust is located at base of steep face of mountain. Slopes below Lewis thrust are composed of Mesozoic clastic rocks and are failing as landslides.

Figure 11.

North face of 2768-m-high Chief Mountain. Montana, showing 1993 rock-avalanche deposits and failure area within dolomite of Siyeh Formation of Purcell Supergroup. Lewis thrust is located at base of steep face of mountain. Slopes below Lewis thrust are composed of Mesozoic clastic rocks and are failing as landslides.

Figure 12.

Approximate limits of rock-avalanche deposit that fell during waning stages of last ice age. A is source area of rock avalanche; L is limit of rock avalanche deposits; M is moraine built by subsequent advance of niche glacier. Summit of Livingstone Range in area of failure is ∼600 m above toe of rock-avalanche deposit. It is ∼1700 m in length and 600 m in width.

Figure 12.

Approximate limits of rock-avalanche deposit that fell during waning stages of last ice age. A is source area of rock avalanche; L is limit of rock avalanche deposits; M is moraine built by subsequent advance of niche glacier. Summit of Livingstone Range in area of failure is ∼600 m above toe of rock-avalanche deposit. It is ∼1700 m in length and 600 m in width.

ANTECEDANT CONDITIONS FOR ROCK AVALANCHES AND ROCKSLIDES IN THE STUDY AREA

Geomorphic, structural, and stratigraphic antecedent conditions are apparent at failures in the Rocky Mountains. These are described in the following sections.

Glacial erosional steepening and rock avalanches

Glacial steepening of slopes formed in massive, cliff-forming bedrock units such as the Rundle Group can be identified as an antecedent condition at every rock avalanche within the study area. The following examples illustrate this point. A rock avalanche near the northern end of the Livingstone Range (Fig. 5, RA) occurred on the north side of a narrow east-west mountain pass through the Livingstone Range. Failure occurred in a thrust-faulted anticline in Mesozoic clastics (Norris, 1993) exposed on the north side of a narrow east-west gap through the Livingstone Range. The walls of the gap were steepened by an outlet glacier flowing from the Livingstone valley eastward across the Livingstone Range to the Willow Creek basin. Farther south along the Livingstone Range, small recent failures such as those indicated by RA in Figure 5 occur in the steep walls of small cirques in the Livingstone Range.

However, the most notable example is the Crowsnest River valley at Turtle Mountain, site of the Frank slide. The glacier that occupied the Crowsnest River valley at the climax of the last ice age drained thousands of square kilometers of ice caps that covered the Rocky Mountains west and east of the Continental Divide. Much of its flow was through the 1-km-wide gap to the north of Turtle Mountain. Flow rates of the glacier through this bottleneck must have been many times its velocity a few kilometers up the Crowsnest River valley. The relatively rapid flow of ice scoured the northeast face of Turtle Mountain to an elevation of ∼1800 m (∼520 m above the adjacent valley floor; Jackson et al., 1996). This undercut the remaining 330–400 m of Turtle Mountain (Fig. 8), creating a steep rock face with slope angles between 50° and 68° (Daly et al., 1912). As indicated in Figure 8, the 1903 failure involved the upper part of Turtle Mountain, which was undercut by glacial erosion. This relationship of the failed mass to the former glacial limit has not been noted in past analyses of this famous rock avalanche.

Depositional RORs and rockslides

Rockslides cluster along RORs: one of the best examples in the study area is the extensive complex of rockslides occurring in depositionally created RORs in the Windsor Ridge area (Fig. 9, WR). Here, gently dipping massive dolostone and limestone of the Middle Cambrian Elko Formation and Devonian Fairholme Group are slowly sliding over the underlying green shale of the Cambrian Gordon Formation along an apparently linear failure plane (Fig. 9, landslides A and B; Jackson and Lebel, 1998). Sliding has progressed upslope, causing the west side of Windsor Ridge to fail locally as immense rotational blocks that have joined the slowly creeping rockslide. The summit is cut by tension cracks. Nearby and lower in the section, the ridge-forming quartzite of the Cambrian Flathead Formation is being torn apart by failure in gently dipping underlying recessive Proterozoic slate, dolomite, sandstone, and volcanics.

Structural RORs, rockslides and retreat of the Rocky Mountain Front

Structurally formed RORs occur widely along the front of the Rocky Mountains and are loci for landslides. They are most commonly cliff-forming Proterozoic or Paleozoic units thrust faulted upon Cretaceous and Jurassic foreland basin clastic units (Figs. 2, 3, 58). Thick thrust sheets of resistant Mesozoic clastics also form RORs here and elsewhere in the Rocky Mountains (Fig. 6; Stepanek, 1992). The concentration of landsliding in this structural and stratigraphic setting has been recognized on both sides of the International Border (Ross, 1959; Carrara, 1990; Jackson and Lebel, 1998) and in analogous settings in the Alps and other mountainous regions of Europe (Stepanek, 1992; Reitner et al., 1993; Ivanovic, 1998; Lotter et al, 1998). In addition to being innately more erodible than the rocks of the overlying thrust sheet (ROR top), the footwall clastics (ROR base) have been sheared and broken by thrust faulting. The rocks of the ROR top act as a rigid and erosionally resistant cap rock over the weaker ROR base units (Figs. 5 and 13). Failure in the recessive ROR base ultimately undermines the resistant ROR top and causes failure by small-scale rockfall, rock avalanche, toppling, or rockslide processes. This undermining or progressive sapping, was first recognized by Ross (1959, p. 103), who identified it as the process leading to the retreat of the Rocky Mountain Front.

Figure 13.

Diagrammatic representation of sapping process. 1: Late Tertiary uplift and stream incision exposed resistant over recessive (ROR) successions of resistant and massive Precambrian or Paleozoic carbonates (A) juxtaposed over folded and sheared Mesozoic clastics and coal (B) along thrust faults or complexes of closely spaced thrust faults. Differential erosion of recessive units, loading by overlying massive units, stress release, and possibly ductile creep cause landsliding in Mesozoic rocks, which undermine resistant cliff-forming carbonates. 2: Undermining results in rockfall and cliff collapse in overlying resistant unit and retreat of mountain front. Debris may accumulate or be transported downslope by underlying landsliding. 3: Periodic glaciation cleans slopes and steepens rock faces, further accelerating landslide activity in both levels of ROR during interglaciations (1).

Figure 13.

Diagrammatic representation of sapping process. 1: Late Tertiary uplift and stream incision exposed resistant over recessive (ROR) successions of resistant and massive Precambrian or Paleozoic carbonates (A) juxtaposed over folded and sheared Mesozoic clastics and coal (B) along thrust faults or complexes of closely spaced thrust faults. Differential erosion of recessive units, loading by overlying massive units, stress release, and possibly ductile creep cause landsliding in Mesozoic rocks, which undermine resistant cliff-forming carbonates. 2: Undermining results in rockfall and cliff collapse in overlying resistant unit and retreat of mountain front. Debris may accumulate or be transported downslope by underlying landsliding. 3: Periodic glaciation cleans slopes and steepens rock faces, further accelerating landslide activity in both levels of ROR during interglaciations (1).

However, periodic glaciation also has a role in the sapping process. The lack of any landslide deposits predating the last glaciation suggests that landslide deposits that accumulate during Quaternary interglaciations are totally removed during the subsequent glaciations by montane glacial advances. Removal of landslide deposits by periodic glaciation acts to make the sapping process self perpetuating.

Retreat of the Rocky Mountain Front by sapping was initiated when regional erosion first exposed RORs (Figs. 10 and 13). The Livingstone Range in the headwaters of Todd Creek presents one of the best examples of ROR-related recession in progress (Fig. 5). Amphitheater-shaped basins are actively cutting into the Mesozoic clastics that underlie the lower slopes of the Livingstone Range through intense gullying (area west of EC) or by landsliding within the ROR base (Fig. 5, landslide EM). Both processes result in the undermining of the massive Rundle Group, which caps the Livingstone Range.

Load-induced ductile creep

Where RORs are exposed along glacially steepened valleys or the mountain front, the resistant ROR top imparts significant downward- and outward-loading forces on ROR base rocks. This is in addition to the release of a horizontal component of in situ stress from within the ROR due to exposure of formerly buried rocks along a free face. Both are driving forces within the ROR base. Resistance to these driving forces would be particularly low within variably sheared Mesozoic sandstone, shale, and coal of ROR bases below thrust faults. The possibility of ductile creep within the footwall rocks as a long-term destabilizing process is suggested by this imposition of driving forces on low-strength rocks. If ductile creep is active, it would play a role in initiating and maintaining failure in the ROR base and would also cause the cliff-forming upper half of an ROR to spread and founder. The 1992 rock avalanche from Chief Mountain, Montana (Schuster et al., 1995; Fig. 11), is the type of failure expected as the culmination of this process. Tension cracks along summits parallel to the mountain front are indicators of underlying creep. Active tension cracks within the Rundle Group along the summit of Mount Livingstone (Fig. 7; Jackson and Lebel, 1998) are examples of such features. Similar phenomena have been reported farther north in the Rocky Mountains by Stepanek (1992) and Simmons and Cruden (1980). RORs analogous to those in the study area exist along the front of the eastern Alps, where carbonate nappes are thrust over clastic rocks of the Alpine flysch (see Eisbacher and Clague [1984] for a summary of Alpine tectonostratigraphic belts). For example, near Gmuden, Austria, office-building—size pinnacles of limestone progressively detach from the nappe front and have toppled during historic time. Microearthquakes recorded in that area are attributed to creep movement beneath these pinnacles (D. van Husen, 1994, personal commun.).

The hypothesis that load-induced ductile creep is occurring in RORs within the Rocky Mountains will require testing in future studies. The installation of small area seismic networks capable of locating and analyzing microseismic tensional fracture events and systematic mapping of summit tension crack patterns, similar to work carried out in the Alps (e.g., Lotter et al., 1998), will be key components of such studies.

DISCUSSION

The association of characteristic suites of slope failure types with structural and stratigraphic domains leads to some observations about the role of landsliding in the geomorphic evolution of the study region.

Rocky Mountains

This study indicates that westward retreat of the mountain front is underway wherever the montane-scale ROR successions are exposed along slopes within and along the front of the Rocky Mountains. The retreat of the mountain front was hastened once erosion of the Rocky Mountains exposed the Proterozoic-Mesozoic ROR along the Lewis thrust (Ross, 1959). This erosional level is roughly equivalent to the Flaxville-Blackfoot erosion surface, which has a limiting age of ca. 2.6 Ma (Fig. 6). Klippen of Purcell Supergroup rocks are as much as 5.6 km from the mountain front northeast of Mokowan Butte area (Fig. 2; Douglas, 1950). Assuming a minimum age for the Flaxville-Blackfoot surface of ca. 2.6 Ma and that the mountain front was continuous with the easternmost klippen when the Interior Plains was at the level of the Flaxville surface, the subsequent rate of recession of the mountain front has been ∼0.2 cm/yr. If the 3.2–5 km distance from the Chief Mountain klippe (Figs. 2 and 9) to the adjacent mountain front is used, a comparable rate of 0.1–0.2 cm/yr is determined. This suggests an average of ∼10–20 m of recession since the end of the Pleistocene.

No surveys of the thickness of rockfall material below the mountain front exist to verify this retreat rate. Other studies of the retreat rates of mountain sides in glaciated northern climates have arrived at rates an order of magnitude less (e.g., Gray, 1972). Conversely, local retreat rates since deglaciation have greatly exceeded these values where rockfall avalanches have occurred (e.g., Turtle and Chief mountains). These long-term rates of mountain front retreat should be regarded as limiting values in the absence of further investigations.

Mountain-front retreat rates and long-term landslide frequency reflect two long-term sets of destabilizing processes. The first processes include regional (isostatic) uplift and resultant fluvial incision and erosion. These constitute a feedback loop in which erosion is compensated for by uplift, and uplift in turn triggers more stream incision. This process has exposed RORs and initiated cliff retreat by sapping (Fig. 13). The sapping process is innate to ROR architecture of the Rocky Mountains and has paused only when the region was glacier covered during the periodic glaciations of the late Tertiary and Quaternary (Karlstrom, 1987; Cioppa et al., 1995).

The second destabilizing process is periodic glacial erosion. The link between glacial oversteepening of resistant slopes and the occurrence of major rockfall avalanches documented here leaves little doubt that glacial erosion during the last ice age is still a causative factor in the contemporary frequency of slope failure. A survey by Cruden and Hu (1993) suggested that landslide frequency for at least one type of failure (dip slope) has decreased since deglaciation. If this is the case for all landsliding, sapping along RORs determines a steady state or residual landslide frequency innate to the stratigraphic and structural architecture of the Rocky Mountains (Fig. 14). This residual rate would be approached as the effects of glacial erosion diminish with time.

Figure 14.

Hypothetical plot showing changes in landslide frequency per unit area over tens of thousands of years following glaciation in Rocky Mountains. Failure due to glacial steepening of resistant over recessive successions (ROR) decreases with time until basal value is reached due to inherently unstable ROR architecture. This cycle was repeated during each Quaternary interglaciation.

Figure 14.

Hypothetical plot showing changes in landslide frequency per unit area over tens of thousands of years following glaciation in Rocky Mountains. Failure due to glacial steepening of resistant over recessive successions (ROR) decreases with time until basal value is reached due to inherently unstable ROR architecture. This cycle was repeated during each Quaternary interglaciation.

Foothills (bedrock uplands), Porcupine Hills, and Mokowan Butte

Landslide frequency in many of the Foothills uplands and the Porcupine Hills is probably most significantly controlled over the long term by isostatic adjustments in response to erosion, which induces stream incision and resulting slope adjustment. The steepening of slopes by direct glacial erosion in these areas is probably a less important factor in determining landslide frequency than in the Rocky Mountains or the Mokowan Butte upland. Valleys are generally broad between Foothills ridges, so areas of intense glacial flow rates were rare. Furthermore, ice overtopped most ridges, and much of the Porcupine Hills, so that glacial undermining around former nunataks is generally not a destabilizing factor, because few nunataks existed. Glaciation has destabilized slopes indirectly through glacially induced changes to local base level: slope toes have been undermined as a result.

The Mokowan Butte upland differs from the rest of the Foothills and Porcupine Hills in that glacial oversteepening of slopes may have had a significant effect in predisposing it to extensive landsliding.

Foothills (intraridge drift-filled valleys)

The thick fills of silt and clay in many interridge valleys were deposited in glacial lakes. The cutting of glaciofluvial meltwater channels and glacially induced stream incision has created slopes and destabilized them in otherwise flat-lying, thick glaciolacustrine valley fills. The lack of any sediments predating the last glaciation within this domain (Jackson et al., 1996; Leboe, 1996) indicates that these fills do not persist sufficiently long enough for stream incision induced by long-term regional uplift to be a factor in slope instability.

SUMMARY

The Foothills region of southwestern Alberta is naturally divisible into stratigraphic-structural domains where distinctive suites of landslide types predominate. The Porcupine Hills are dominated by rotational slumps and earthflows that originate within shale units along valley sides. Complex landslides, which appear to be rotational slump-earthflow complexes, mantle much of the slopes of the Mokowan Butte upland. The ubiquitous landsliding in this area may have been conditioned by glacial oversteepening by flow of the Laurentide ice sheet on its east and the Belly River valley glacier on its west and to an unknown extent by its structural history.

Landslides originating entirely within bedrock are relatively uncommon within the Foothills belt compared to the Porcupine Hills to the east, and the Rocky Mountains to the west, with the exception of the Mokowan Butte area. The largest landslides in the Foothills are rockslides along dip slopes, where slope angles and bedding dip angles are close in value. However, slope angles are usually less than bedding dip angles within slopes in the Foothills region.

Drift-filled interridge valleys are characterized by rotational slump failures in thick glaciolacustrine silt and clay accumulations along former meltwater channels and contemporary stream valleys that have undergone glacially induced incision. The Rocky Mountains are dominated by rock avalanches and rockslides. Slope failures of both categories cluster above, below, and across montane-scale resistant over recessive (ROR) successions, usually massive cliff-forming carbonate units over recessive clastics. These either result from depositional succession or are created by overthrusting. The resistant upper part of the ROR imparts a load on the lower recessive level of the ROR, creating driving forces. Ductile creep may also occur. Future studies are suggested to investigate the ductile-creep hypothesis. Glacial erosional steepening of slopes formed in massive, cliff-forming bedrock units such as the Rundle Group is identified as an antecedent condition at every rock avalanche within the study area.

Retreat of the Rocky Mountain front is underway through landslide-driven sapping wherever RORs are exposed along steepened slopes. Estimates of the maximum rate of recession for the past several million years range from than 0.1 to 0.2 cm/yr. These rates reflect instability driven by regional uplift and resultant fluvial incision together with glacial erosion. Landslide-driven sapping creates a basal frequency of landslide activity upon which glacially induced landslide frequency is superimposed.

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Acknowledgments

This is dedicated to the memory of Ben Page of Stanford University. He is fondly remembered as a teacher and friend.

I gratefully acknowledge the help and field collaboration of Daniel Lebel. This paper was markedly improved by reviews by Greg Brooks and an anonymous reader and by the advice and criticism of Steve Evans.

Figures & Tables

Figure 1.

Location map of southeastern Alberta study area. Numbers refer to locations of subsequent figures.

Figure 1.

Location map of southeastern Alberta study area. Numbers refer to locations of subsequent figures.

Figure 2.

Regional characterization of landslides and associated bedrock and/or structural provinces. Bedrock geology is generalized after Douglas (1950, 1952) and Norris (1955, 1993). Locations: OMR is area of active slumping in glaciolacustrine sediments and Late Cretaceous bedrock along Oldman River; SWC is area of extensive earth flows from glaciolacustrine sediments and argillaceous bedrock units (Fig. 4); LvT is Livingstone thrust; LT is Lewis thrust; TMF is Turtle Mountain fault; OMD is Oldman Dam; OMG is Oldman gorge, created by glacial diversion of Oldman River; CNP is Crowsnest Pass; CNR is Crowsnest River; CM is Chief Mountain; PH is Porcupine Hills; MB is Mokowan Butte; BR is Belly River; FS is Frank slide. A—A′, B—B′, C—C′, D—D′ and E—E′ locate section lines shown in subsequent figures.

Figure 2.

Regional characterization of landslides and associated bedrock and/or structural provinces. Bedrock geology is generalized after Douglas (1950, 1952) and Norris (1955, 1993). Locations: OMR is area of active slumping in glaciolacustrine sediments and Late Cretaceous bedrock along Oldman River; SWC is area of extensive earth flows from glaciolacustrine sediments and argillaceous bedrock units (Fig. 4); LvT is Livingstone thrust; LT is Lewis thrust; TMF is Turtle Mountain fault; OMD is Oldman Dam; OMG is Oldman gorge, created by glacial diversion of Oldman River; CNP is Crowsnest Pass; CNR is Crowsnest River; CM is Chief Mountain; PH is Porcupine Hills; MB is Mokowan Butte; BR is Belly River; FS is Frank slide. A—A′, B—B′, C—C′, D—D′ and E—E′ locate section lines shown in subsequent figures.

Figure 3.

Geologic cross section (vertical exaggeration ×2) looking north across Lewis and Clark Ranges (generalized from McMechan and Thompson, 1992). T is Tertiary clastics west of Flathead fault, M is folded and thrust-faulted Mesozoic clastics, and P are folded and thrust-faulted carbonates and clastics. PC is thrust-faulted Precambrian (Proterozoic) carbonates, clastics and minor volcanics. Reconstruction of eroded rock above Lewis thrust is based upon complete sections preserved west of Flathead fault. Thickness of rock removed from Foothills is based upon coal maturation studies. X is hypothetical point where ROR (contrasting resistant over recessive lithologies) was first exposed during regional erosion following orogeny. Z is hypothetical point where contemporary ROR configuration became exposed and sapping retreat of mountain front, that continues today, began. B—B′ indicates location to Sofa Mountain-Mokowan Butte profile shown in Figure 10.

Figure 3.

Geologic cross section (vertical exaggeration ×2) looking north across Lewis and Clark Ranges (generalized from McMechan and Thompson, 1992). T is Tertiary clastics west of Flathead fault, M is folded and thrust-faulted Mesozoic clastics, and P are folded and thrust-faulted carbonates and clastics. PC is thrust-faulted Precambrian (Proterozoic) carbonates, clastics and minor volcanics. Reconstruction of eroded rock above Lewis thrust is based upon complete sections preserved west of Flathead fault. Thickness of rock removed from Foothills is based upon coal maturation studies. X is hypothetical point where ROR (contrasting resistant over recessive lithologies) was first exposed during regional erosion following orogeny. Z is hypothetical point where contemporary ROR configuration became exposed and sapping retreat of mountain front, that continues today, began. B—B′ indicates location to Sofa Mountain-Mokowan Butte profile shown in Figure 10.

Figure 4.

Slumps and earthflows in area of Porcupine Hills and extensive failures in glacial lake sediments, upper reaches of South Willow Creek, and its major tributaries. Slumps in bedrock occur in sandstone and mudstone.

Figure 4.

Slumps and earthflows in area of Porcupine Hills and extensive failures in glacial lake sediments, upper reaches of South Willow Creek, and its major tributaries. Slumps in bedrock occur in sandstone and mudstone.

Figure 5.

Landslides and related features along Livingstone Range in area of Todd Creek. Cross section: P is cliff-forming Mississippian limestone and dolostone of Rundle Group; M is Cretaceous coal-bearing clastics (generalized from Norris, 1955; no vertical exaggeration); LT is Livingstone thrust. Relief from mountain summit to valley bottoms to east is ∼900 m. Air photo: Livingstone thrust (not marked) is at base of cliff-forming limestone and dolostone of Rundle Group. Adjacent forested slopes are underlain by predominantly Cretaceous coal-bearing clastics. BP is bedding plane rockslide in Rundle Group; RA is rock avalanche in Paleozoic bedrock; MS is bedding plain rockslide in Mesozoic clastics; EC is area of closely spaced gulleys cutting headward into Mesozoic clastics and undermining overlying Paleozoic carbonates; EM is landslide complex in Mesozoic clastics undermining overlying Paleozoic carbonates (Canada National Airphoto Library A23234-11. A23234-12). C is the western end of section C–C′.

Figure 5.

Landslides and related features along Livingstone Range in area of Todd Creek. Cross section: P is cliff-forming Mississippian limestone and dolostone of Rundle Group; M is Cretaceous coal-bearing clastics (generalized from Norris, 1955; no vertical exaggeration); LT is Livingstone thrust. Relief from mountain summit to valley bottoms to east is ∼900 m. Air photo: Livingstone thrust (not marked) is at base of cliff-forming limestone and dolostone of Rundle Group. Adjacent forested slopes are underlain by predominantly Cretaceous coal-bearing clastics. BP is bedding plane rockslide in Rundle Group; RA is rock avalanche in Paleozoic bedrock; MS is bedding plain rockslide in Mesozoic clastics; EC is area of closely spaced gulleys cutting headward into Mesozoic clastics and undermining overlying Paleozoic carbonates; EM is landslide complex in Mesozoic clastics undermining overlying Paleozoic carbonates (Canada National Airphoto Library A23234-11. A23234-12). C is the western end of section C–C′.

Figure 6.

Complex landslides and related mass-wasting features and deposits, upper Willow Creek and Livingstone River basins in area of Livingstone thrust fault. Cross section (no vertical exaggeration): thrust faults depicted as individual features are actually complexes of closely spaced faults. P is predominantly Paleozoic carbonates; M is Mesozoic shale, sandstone, and coal; RS is rock avalanche located on air photo. Air photo: RS is rock avalanche in glacially steepened mountain pass; BP is bedding-plane failure in Paleozoic bedrock. Relief of rock avalanche from source area to toe of deposit is ∼270 m. Maximum local relief is ∼730 m (Canada National Airphoto Library A18306172, A18306173).

Figure 6.

Complex landslides and related mass-wasting features and deposits, upper Willow Creek and Livingstone River basins in area of Livingstone thrust fault. Cross section (no vertical exaggeration): thrust faults depicted as individual features are actually complexes of closely spaced faults. P is predominantly Paleozoic carbonates; M is Mesozoic shale, sandstone, and coal; RS is rock avalanche located on air photo. Air photo: RS is rock avalanche in glacially steepened mountain pass; BP is bedding-plane failure in Paleozoic bedrock. Relief of rock avalanche from source area to toe of deposit is ∼270 m. Maximum local relief is ∼730 m (Canada National Airphoto Library A18306172, A18306173).

Figure 7.

Cross section showing the structural and stratigraphic setting of the summit fissure on Mt. Livingstone (generalized from Jackson and Lebel, 1998). Vertical exaggeration ×2.5: M is Mesozoic shale, sandstone, and coal; P is predominantly Paleozoic carbonates. Thrust faults depicted are commonly complexes of closely spaced faults. Mesozoic units are complexly folded. Photos: A is oblique aerial view looking south. B is ground view looking north from “V” at south end of fissure. Total width of cracking is ∼5 m.

Figure 7.

Cross section showing the structural and stratigraphic setting of the summit fissure on Mt. Livingstone (generalized from Jackson and Lebel, 1998). Vertical exaggeration ×2.5: M is Mesozoic shale, sandstone, and coal; P is predominantly Paleozoic carbonates. Thrust faults depicted are commonly complexes of closely spaced faults. Mesozoic units are complexly folded. Photos: A is oblique aerial view looking south. B is ground view looking north from “V” at south end of fissure. Total width of cracking is ∼5 m.

Figure 8.

Reconstruction of the face of Turtle Mountain using profiles 4 and 6 of Daly et al. (1912). No vertical exaggeration. Profiles are ∼250 m apart and trend east-west normal to long axis of Turtle Mountain. Profile 4 is located immediately northwest of the detachment scar of the 1903 Frank Slide (profile 6) and is representative of the prefailure profile. Glacial limit is projected based on adjacent surficial geology mapping (Jackson et al., 1996).

Figure 8.

Reconstruction of the face of Turtle Mountain using profiles 4 and 6 of Daly et al. (1912). No vertical exaggeration. Profiles are ∼250 m apart and trend east-west normal to long axis of Turtle Mountain. Profile 4 is located immediately northwest of the detachment scar of the 1903 Frank Slide (profile 6) and is representative of the prefailure profile. Glacial limit is projected based on adjacent surficial geology mapping (Jackson et al., 1996).

Figure 10.

Topographic profile between Sofa Mountain and Mokowan Butte area (vertical exaggeration ×2.5). Flaxville erosion surface is capped by Kennedy Drift (basal age estimated as 2.6 Ma). Incision of the Flaxville surface partly or entirely postdated the deposition of Kennedy Drift. Profile shows hypothetical mountain front position at time of deposition of Kennedy Drift assuming 0.2 cm/yr recession rate. PC is Proterozoic Purcell Supergroup; M is Mesozoic sandstone and shale.

Figure 10.

Topographic profile between Sofa Mountain and Mokowan Butte area (vertical exaggeration ×2.5). Flaxville erosion surface is capped by Kennedy Drift (basal age estimated as 2.6 Ma). Incision of the Flaxville surface partly or entirely postdated the deposition of Kennedy Drift. Profile shows hypothetical mountain front position at time of deposition of Kennedy Drift assuming 0.2 cm/yr recession rate. PC is Proterozoic Purcell Supergroup; M is Mesozoic sandstone and shale.

Figure 9.

Rockslides in stratigraphic successions of resistant, ridge forming units overlying recessive ones. Stereo pair: white letters identify rockslides (A—E), black or white letters in italics identify end points of cross sections shown below, and bold black letters denote physiographic and drainage features. Rockslide A: limestones and dolomites of Devonian Fairholme Group (Df) and Middle Cambrian Windsor Mountain and Elko Formations (Ce) (cross section F—F′) are failing on shale of underlying Middle Cambrian Gordon Formation (Ggo). Rockslides B, C, D: complex landslides involving ridge-forming Flathead Formation quartzite (Cft) and recessive underlying shale, dolostone, and sandstone of Proterozoic Roosville (Hro) and Phillips (Hph) Formations, Purcell Supergroup. Flathead Formation overlies Proterozoic succession along angular unconformity. Rockslide E remains to be studied in more detail. Other units shown (formations of Purcell Supergroup): Hga is Gateway Formation (mafic lava, slate, silt-stone, and dolomite), Hsh is Shepard Formation (dolomite). Locations on stereo pair: MC is Mill Creek; EC is East Castle River; WR is Windsor Ridge; WM is Windsor Mountain (Canada National Airphoto Library A23232262, A23232263). Cross sections (no vertical exaggeration) modified slightly form those prepared by D. Lebel, Geological Survey of Canada.

Figure 9.

Rockslides in stratigraphic successions of resistant, ridge forming units overlying recessive ones. Stereo pair: white letters identify rockslides (A—E), black or white letters in italics identify end points of cross sections shown below, and bold black letters denote physiographic and drainage features. Rockslide A: limestones and dolomites of Devonian Fairholme Group (Df) and Middle Cambrian Windsor Mountain and Elko Formations (Ce) (cross section F—F′) are failing on shale of underlying Middle Cambrian Gordon Formation (Ggo). Rockslides B, C, D: complex landslides involving ridge-forming Flathead Formation quartzite (Cft) and recessive underlying shale, dolostone, and sandstone of Proterozoic Roosville (Hro) and Phillips (Hph) Formations, Purcell Supergroup. Flathead Formation overlies Proterozoic succession along angular unconformity. Rockslide E remains to be studied in more detail. Other units shown (formations of Purcell Supergroup): Hga is Gateway Formation (mafic lava, slate, silt-stone, and dolomite), Hsh is Shepard Formation (dolomite). Locations on stereo pair: MC is Mill Creek; EC is East Castle River; WR is Windsor Ridge; WM is Windsor Mountain (Canada National Airphoto Library A23232262, A23232263). Cross sections (no vertical exaggeration) modified slightly form those prepared by D. Lebel, Geological Survey of Canada.

Figure 11.

North face of 2768-m-high Chief Mountain. Montana, showing 1993 rock-avalanche deposits and failure area within dolomite of Siyeh Formation of Purcell Supergroup. Lewis thrust is located at base of steep face of mountain. Slopes below Lewis thrust are composed of Mesozoic clastic rocks and are failing as landslides.

Figure 11.

North face of 2768-m-high Chief Mountain. Montana, showing 1993 rock-avalanche deposits and failure area within dolomite of Siyeh Formation of Purcell Supergroup. Lewis thrust is located at base of steep face of mountain. Slopes below Lewis thrust are composed of Mesozoic clastic rocks and are failing as landslides.

Figure 12.

Approximate limits of rock-avalanche deposit that fell during waning stages of last ice age. A is source area of rock avalanche; L is limit of rock avalanche deposits; M is moraine built by subsequent advance of niche glacier. Summit of Livingstone Range in area of failure is ∼600 m above toe of rock-avalanche deposit. It is ∼1700 m in length and 600 m in width.

Figure 12.

Approximate limits of rock-avalanche deposit that fell during waning stages of last ice age. A is source area of rock avalanche; L is limit of rock avalanche deposits; M is moraine built by subsequent advance of niche glacier. Summit of Livingstone Range in area of failure is ∼600 m above toe of rock-avalanche deposit. It is ∼1700 m in length and 600 m in width.

Figure 13.

Diagrammatic representation of sapping process. 1: Late Tertiary uplift and stream incision exposed resistant over recessive (ROR) successions of resistant and massive Precambrian or Paleozoic carbonates (A) juxtaposed over folded and sheared Mesozoic clastics and coal (B) along thrust faults or complexes of closely spaced thrust faults. Differential erosion of recessive units, loading by overlying massive units, stress release, and possibly ductile creep cause landsliding in Mesozoic rocks, which undermine resistant cliff-forming carbonates. 2: Undermining results in rockfall and cliff collapse in overlying resistant unit and retreat of mountain front. Debris may accumulate or be transported downslope by underlying landsliding. 3: Periodic glaciation cleans slopes and steepens rock faces, further accelerating landslide activity in both levels of ROR during interglaciations (1).

Figure 13.

Diagrammatic representation of sapping process. 1: Late Tertiary uplift and stream incision exposed resistant over recessive (ROR) successions of resistant and massive Precambrian or Paleozoic carbonates (A) juxtaposed over folded and sheared Mesozoic clastics and coal (B) along thrust faults or complexes of closely spaced thrust faults. Differential erosion of recessive units, loading by overlying massive units, stress release, and possibly ductile creep cause landsliding in Mesozoic rocks, which undermine resistant cliff-forming carbonates. 2: Undermining results in rockfall and cliff collapse in overlying resistant unit and retreat of mountain front. Debris may accumulate or be transported downslope by underlying landsliding. 3: Periodic glaciation cleans slopes and steepens rock faces, further accelerating landslide activity in both levels of ROR during interglaciations (1).

Figure 14.

Hypothetical plot showing changes in landslide frequency per unit area over tens of thousands of years following glaciation in Rocky Mountains. Failure due to glacial steepening of resistant over recessive successions (ROR) decreases with time until basal value is reached due to inherently unstable ROR architecture. This cycle was repeated during each Quaternary interglaciation.

Figure 14.

Hypothetical plot showing changes in landslide frequency per unit area over tens of thousands of years following glaciation in Rocky Mountains. Failure due to glacial steepening of resistant over recessive successions (ROR) decreases with time until basal value is reached due to inherently unstable ROR architecture. This cycle was repeated during each Quaternary interglaciation.

Table 1.

Generalized Stratigraphy of the Foothills and Rocky Mountains, Southwestern Alberta

AgeStratigraphic unitLithologyThickness (m)Comments
CenozoicTertiaryPorcupine Hills FormationSandstone and shale1200Sandstone beds form cliffs or steep slopes
Willow Creek FormationSandstone and shale600Recessive, usually involved in landsliding or badland erosion
CretaceousSt. Mary River Formation900As above
Belly River FormationQuartz and chert Sandstone1200Forms small cliffs where competent
Alberta GroupSiltstone, shale, and local resistant sandstone700Recessive, resistant competent sandstones form ridges
PaleozoicPermianRocky Mountain GroupQuartzite and dolomitic sandstone210Recessive to cliff forming
CarboniferousRundle GroupDolomite, limestone, shale, anhydrite250Forms 200 m cliffs
Banff FormationLimestone, shale, siltstone, chert190Recessive, carbonate units form small cliffs
Exshaw FormationSiltstone, limestone20Recessive
Palliser FormationLimestone, dolomite200Cliff forming
DevonianAlexo FormationLimestone, dolomite90Cliff forming
Mt. Hawk FormationArgillaceous limestone, dolomite120Recessive, grades laterally into the biohermal Southesk Formation, which forms cliffs
Fairholm FormationLimestone, dolomite200Recessive to cliff forming
Precambrian (Proterozoic)Purcell Supergroup3400–6000
Roosville FormationSiltstone Quartzite Dolomite50Recessive to cliff forming
Phillips FormationSiltstone Dolomite400–500As above
Gateway FormationAs above1000Recessive
Shepherd Formation300Recessive
Purcell LavaChoritized andesite200–300Recessive
Syeh FormationDolomite800Cliff forming
AgeStratigraphic unitLithologyThickness (m)Comments
CenozoicTertiaryPorcupine Hills FormationSandstone and shale1200Sandstone beds form cliffs or steep slopes
Willow Creek FormationSandstone and shale600Recessive, usually involved in landsliding or badland erosion
CretaceousSt. Mary River Formation900As above
Belly River FormationQuartz and chert Sandstone1200Forms small cliffs where competent
Alberta GroupSiltstone, shale, and local resistant sandstone700Recessive, resistant competent sandstones form ridges
PaleozoicPermianRocky Mountain GroupQuartzite and dolomitic sandstone210Recessive to cliff forming
CarboniferousRundle GroupDolomite, limestone, shale, anhydrite250Forms 200 m cliffs
Banff FormationLimestone, shale, siltstone, chert190Recessive, carbonate units form small cliffs
Exshaw FormationSiltstone, limestone20Recessive
Palliser FormationLimestone, dolomite200Cliff forming
DevonianAlexo FormationLimestone, dolomite90Cliff forming
Mt. Hawk FormationArgillaceous limestone, dolomite120Recessive, grades laterally into the biohermal Southesk Formation, which forms cliffs
Fairholm FormationLimestone, dolomite200Recessive to cliff forming
Precambrian (Proterozoic)Purcell Supergroup3400–6000
Roosville FormationSiltstone Quartzite Dolomite50Recessive to cliff forming
Phillips FormationSiltstone Dolomite400–500As above
Gateway FormationAs above1000Recessive
Shepherd Formation300Recessive
Purcell LavaChoritized andesite200–300Recessive
Syeh FormationDolomite800Cliff forming

Contents

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