Abstract This one-day field trip highlights research into the late Cenozoic evolution of topography in the Appalachian Mountains through geomorphic observations in the Cullasaja River basin, located in Macon County, North Carolina. Herein we present observations, data, and modeling results that challenge the paradigm of the Appalachians as a dead and slowly decaying orogen. Previous studies on the Cenozoic evolution of topography in the Appalachians are reviewed, showing that the post-orogenic history of eastern North America is best described by spatial and temporal changes in topographic relief, erosion rate, and sediment flux. When these data sets are placed in the context of other geologic and geophysical studies, they indicate that the Appalachians are a dynamic mountain range. We review previous studies in the Cullasaja basin that document and characterize the magnitude of base-level fall, relief production, and ensuing landscape response to such changes. These studies show that topographic relief within the basin was relatively subdued in the Miocene and subsequently has been rejuvenated ~160%, transforming the landscape into the rugged mountains we recognize today. We highlight hillslope and fluvial geomorphic observations that show landscape disequilibrium associated with ongoing adjustment to ~500 m of late Cenozoic base-level fall. Potential mechanisms for topographic rejuvenation of the Appalachians, such as climate change and epeirogenic uplift, are discussed using available field observations.

Abstract This one-day field trip highlights recent research into the late Holocene geomorphic evolution and land use history of Piedmont stream valleys near Raleigh, North Carolina. European settlers began building water-powered milldams in the eastern United States in the 1600s, and dam construction continued until the early twentieth century. At the same time, regional-scale land clearing associated with agriculture and development increased upland erosion rates 50–400 times above long-term geologic rates. Much of the eroded sediment was subsequently aggraded on floodplains and impounded behind milldams. This trapped "legacy" sediment, commonly mistaken for natural floodplain deposition, has gone largely unrecognized until recently. This study focuses upon 1st to 4th order streams in W.B. Umstead State Park that drain into the Neuse River basin. There are seven water-powered milldam locations within the park and adjacent areas. Geomorphic mapping demonstrates that upland soil erosion and valley bottom sediment aggradation was substantial following European-American land acquisition and their conversion of large amounts of forest land for agricultural purposes. We observe three distinct sedimentary units in stream bank exposures that are corroborated by 14 C dating. Pre-European sediments range from ca. 4400–250 yr B.P. and consist of quartz-rich axial stream gravels and off-channel organic rich clays. Two legacy sediment units are differentiable; pre and post-dam, and range in age from ca. 300–100 yr B.P. The pre-dam sediments consist primarily of fluvial sands, and are interpreted as channel aggradation in response to soil erosion from upland land clearing prior to dam construction. Post-dam sediments are distinguished by finer grain size and sedimentology consistent with slackwater deposition, including sandy "event" layers, interpreted to be the result of large floods into the former mill ponds. Stream bank magnetic susceptibility (MS) measurements exhibit large and consistent increases at and above the pre-European-legacy sediment contact, suggesting that MS is a suitable proxy for legacy sediment identification along North Carolina Piedmont streams. Estimates of aggraded legacy sediment from two stream reaches in Umstead State Park indicate that the volume of eroded upland soils is approximately balanced by valley bottom sediment aggradation, and that area-averaged depth of upland soil loss was equivalent to 3–15 cm across this part of the Piedmont. We evaluate the current impact of legacy sediment erosion on stream water quality by capturing the total suspended sediment load (TSS) during discharge events using ISCO samplers at 5 sites on Reedy and Richland Creek. We document a TSS increase as water passes through reaches containing milldam deposits. This suggests that modern stream water impairment in the Piedmont may result where milldams were constructed and legacy sediments impounded. The field trip concludes by examining an active beaver (Castor canadensis) pond–wetland meadow complex above the historic Yates Mill pond. Beavers may prove to be valuable assets in the restoration of Piedmont stream systems still suffering from centuries of poor land and soil management.

Textures and mineral assemblages of metamorphic rocks of the Tobacco Root Mountains are consistent with metamorphism of all rocks during the Big Sky orogeny (1.77 Ga) at relatively high pressure ( P >1.0 GPa) followed by differential reequilibration on a clockwise P-T path at lower pressures (0.6–0.8 GPa). The highest pressures are documented by coarse-grained kyanite and orthopyroxene in aluminous orthoamphibolites, which require P ≥ 1.0 GPa. Other higher-pressure mineral assemblages of note include kyanite + orthoamphibole and kyanite + K-feldspar. Abundant textural evidence for partial melting in pelitic and basaltic rocks includes leucosomes, very large (several cm across) porphyroblasts of garnet, and an absence of primary (foliation-defining) muscovite. Partial to complete overprinting of the coarse-textured, high-pressure assemblages by lower-pressure assemblages and textures occurred across the Tobacco Root Mountains, especially where assisted by deformation and the availability of water. In aluminous rocks, sillimanite bundles typically replace kyanite, and garnet may be rimmed by cordierite + orthopyroxene symplectite or, in quartz-absent rocks, sapphirine + spinel + cordierite symplectite. Orthoamphibolites with partial pseudomorphs of garnet by cordierite are common. Garnet necklaces surround orthopyroxene in orthopyroxene-plagioclase gneisses, whereas orthopyroxene + plagioclase pseudomorphs of garnet occur in nearby hornblende amphibolites. These features appear to require nearly isobaric cooling at pressures near 0.8 GPa, followed by nearly isothermal decompression at temperatures near 700 °C. The resulting P-T path is believed to be the result of tectonic denudation late in the orogenic cycle. Quartz-plagioclase-garnet-hornblende amphibolites occur throughout the Tobacco Root Mountains. Near-rim mineral compositions from these rocks have been used to calculate T s of 650–750 °C at P s of 0.7–0.9 GPa across the terrane. There is no systematic variation in calculated P and T between units nor geographically within units; differences appear to reflect variations in thermometer closure possibly due to the availability of water during cooling. Field relations involving metamorphosed mafic dikes, as well as geochronological data from monazite and zircon, demonstrate that some rocks were first metamorphosed at high temperatures and pressures at 2.45 Ga. However, we have not identified mineral assemblages that can be assigned unequivocally to this earlier event.

Abstract We use Quaternary stratigraphy to reconstruct landscape evolution and measure tectonic deformation of the Olympic Mountains section of the Pacific Northwest Coast Range. An important motivation for understanding orogenesis here, and throughout the Coast Range, is the concern about the relationship of active deformation to seismic hazards associated with the Cascadia subduction zone. There is also much interest in apportioning the nature of the deformation, whether cyclic or permanent, whether it involves mainly shortening parallel or normal to the margin, and how the deformation on the pro- versus retrowedge sides of the orogen compare. Pre-Holocene stratigraphy and structure provide the only records of sufficient duration to separate long-term permanent deformation from earthquake-cycle elastic deformation. For this reason, active-tectonic studies have focused on deformation of Quaternary deposits and land-forms, which are best preserved along the Pacific Coast and offshore on the continental shelf. At least four major glacial advances are recorded in the valley and coastal deposits along the western margin of the Olympic Peninsula. Both numeric and relative dating, including soils of these deposits, establish a stratigraphic anchor that is used to document the relationship between margin parallel and margin normal deformation in the Olympic Mountains, which, on a geologic time scale (>10 3 yr), seems to be the fastest deforming part of the Cascadia forearc high. The glacial stratigraphic framework is extended to fluvial terraces of the Clearwater drainage, which remained unglaciated during the late Pleistocene and Holocene, preserving a record of river incision, with each terrace recording the shape and height of past long profiles. We assess how fluvial terraces are formed in this tectonically active setting and then use features of the terraces to estimate incision rates along the Clearwater long profile. The long fluvial history preserved in the Clearwater ensures that the unsteady deformation associated with the earthquake cycle is averaged out, leaving us with a record of long-term rock uplift as well as horizontal shortening. We show, however, that the earthquake cycle may play an important role in terrace genesis at the millennial time scale.