Abstract The examination of fossil assemblages of Coleoptera (beetles) in North America is a relatively new but rapidly growing research area. This chapter is an attempt to use suites of fossil Coleoptera to establish the nature and timing of different paleoenvironments at the margins of the Laurentide Ice Sheet as it advanced and retreated from the Great Lakes region. Some of the work is synthesized from previous publications, but some important data are from unpublished sites. Before the various sites illustrated in Figure 1 are discussed, certain assumptions must be restated that are implicit in the study of Quaternary insect assemblages. The first of these is that beetle species can be identified by characters that are not always used by neotaxonomists. The latter frequently describe species by using portions of the exoskeleton that are destroyed or scattered following the death of the organism. In many cases a specific determination cannot be made with the available fossil components, but even narrowing the identification to a species group often is sufficient to provide valuable ecological data. The second assumption is that species have not evolved during the Pleistocene. Early in the century a general belief prevailed that intense speciation occurred during periods of glaciation. This was one of the greatest barriers to the development of paleoentomology, and it was not until the work of Henriksen (1933), Lindroth (1948), and Coope (1959, 1968; Coope and others, 1961) that this belief was shown to be fallacious. Beetles recovered from Quaternary deposits can be matched precisely with modern species, although they may occur currently thousands of kilometers away from the fossil locality. Examples of major late Pleistocene geographic shifts are demonstrated in papers by Coope (1973) and Hammond and others (1979). Some general statements can be added to the two basic tenets stated above. Paleoenvironments or paleoclimates are not established by use of individual species, but rather of species assemblages. Inherent is the assumption that we know the full modern distribution of a species (or at least the range limits), and that adequate ecological data are available. We assume that the ecology of the fossil species is the same as that of the modem species, and that little or no physiological change has occurred during this period. Publications commenting on some of the points mentioned above include two excellent summaries by Coope (1970, 1979). North American reviews are provided by Ashworth (1979), Matthews (1977, 1980), Morgan and Morgan (1980a, 1980b), and Morgan and others (1983a).

ABSTRACT Landscape features that formed when buried ice melted and overlying sediment collapsed are abundant and widespread in the part of Wisconsin’s Northern Highland region glaciated by the Wisconsin Valley Lobe and the western part of the Langlade Lobe. Stagnation and burial of ice of the Wisconsin Valley Lobe are documented by broad tracts of hummocky moraine topography that record the position of the maximum extent of the lobe, and by extensive pitted and collapsed heads-of-outwash and outwash plains deposited during recession. Recession of the Wisconsin Valley Lobe was characterized by episodes of stagnation interspersed with episodes of readvance, documented by small west-east–trending heads-of-outwash. Advances of the western margin of the Langlade Lobe deposited large northwest-southeast–trending heads-of-outwash characterized by extensive areas of pitted and collapsed outwash plains with obscure but recognizable ice-contact faces. Following recession of the Wisconsin Valley and Langlade Lobes, the Ontonagon Lobe advanced out of the Superior Basin and over sediment containing abundant buried ice. Permafrost and debris cover combined to delay the melting of buried ice and the formation of the postglacial landscape. Regional correlation of ice-margin positions, combined with geomorphic and stratigraphic relationships, indicates that ice buried in north-central Wisconsin persisted in some places for up to 5000 yr or more following the recession of active ice.

During the last part of the Wisconsin Glaciation, central Wisconsin was occupied by a proglacial lake called Lake Wisconsin. Lake Wisconsin formed when the Green Bay Lobe of the Laurentide Ice Sheet reached the Baraboo Hills in south-central Wisconsin, damming the lowland to the north, which is now occupied by the south-flowing Wiscon-sin River. At its greatest extent the lake was 115 km long, and its greatest depth was 50 m. Lake Wisconsin occupied a number of separate but interconnected topographic basins, including a main basin and seven smaller basins to the south of the main basin. During most of its history, Lake Wisconsin drained to the northwest through one of several outlets, all of which emptied into the Black River, a tributary of the Mississippi River. Sometime before the maximum expansion of the Green Bay Lobe during the Wisconsin Glaciation, Lake Wisconsin emptied southward through the Devils Lake outlet to the Wisconsin River, a tributary of the Mississippi River. At the end of Lake Wisconsin’s existence, it drained to the southeast out the Alloa outlet, to the Wisconsin River. The southern basins drained, at various times, through various combinations of a dozen separate outlets. Wind, slope wash, soil creep, cryoturbation, and solifluction have destroyed most of the beach ridges and terraces of Lake Wisconsin. Based on elevations of the few remaining beaches plus the elevations of the lake outlets, ice-rafted erratics, offshore deposits, the break in slope at the west edge of the outwash plain on the east side of the main basin, and shore-ice collapse trenches, we have been able to reconstruct three shorelines; from oldest to youngest, these are: the Wyeville shoreline, which has been tilted S45°W at 0.1 m/km; the Johnstown shoreline, tilted S60°W at 0.6 m/km; and the Elderon shoreline, tilted S55°W at 0.3 m/km. They intersect each other along a northwest-southeast line through the northwestern outlet. The shorelines are tilted because the Earth’s crust had been depressed under the weight of the glacier but later rebounded with the removal of the weight. The Wyeville shoreline probably formed about 19,000 years ago when Lake Wis-consin first came into existence as the glacier expanded into central Wisconsin during the last part of the Wisconsin Glaciation. The Johnstown shoreline formed about 15,000 years ago when the glacier was at its maximum extent and the Earth’s crust was at its maximum depression. The Elderon shoreline formed about 14,000 years ago, just before the lake finally drained, after the crust had begun to rebound as the glacier wasted back near the end of the Wisconsin Glaciation. Tens of meters of offshore sand was deposited in the deep basins of glacial Lake Wisconsin. Much of this sand is included in the Big Flats Formation. The sand was deposited in part by density currents derived from glacial meltwater rivers. In most areas, one or more beds of offshore silt occur within the sand sequence; the uppermost one (the New Rome Member) was probably deposited somewhat before the Johnstown Phase of glaciation. The final drainage of Lake Wisconsin was probably catastrophic. As the Green Bay Lobe wasted from the east end of the Baraboo Hills, the narrow ice dam was breached by water from the lake, causing the water level in the Lewiston basin to drop tens of meters, probably within a few days. This caused water from the main basin and other southern basins to flood into the Lewiston basin through a breach in the Johnstown moraine; this flood at least partly cut the sandstone gorges at the Wisconsin Dells.X

The Midcontinent Rift System is a major tectonic feature that formed about 1.1 to 1.2 b.y. ago in Precambrian crust that had a long and complex history but that was a rigid crustal block for several hundred million years prior to rifting. The rift formed at a high angle to the generally east-northeast-trending structural grain of the older rocks and crossed the boundary between two very different geologic provinces. The northern part of the rift, including all the rift in the immediate vicinity of Lake Superior, formed on rocks of the Superior Province or where Superior Province rocks are basement for Proterozoic supracrustal rocks. Available geochronologic data suggest that these rocks are not older than 3.1 b.y. and that they have remained a rigid crustal block since 2.6 b.y. ago. To the south, the rift developed in rocks that may be as old as 3.8 b.y. and that have since undergone repeated orogenesis at 2.6, 1.85, and 1.76 b.y. ago. Intrusion of the 1,5-b.y.-old, non-orogenic Wolf River Batholith in central Wisconsin marks the last major event prior to Keweenawan rifting 1.1 b.y. ago. The boundary between the Superior Province and rocks to the south seems to have had a pronounced effect on the nature of the Midcontinent Rift System. Where the rift occurs in rocks of the Superior Province, it is about 150 km wide and is complex. Where it is in rocks of the more complex terrane to the south, it is much narrower (about 90 km wide) and has a simpler shape. The width of the rift changes abruptly at the contact of the two terranes. Although orientation of the rift does not seem to be strongly influenced by the older northeast-trending fabric that it crosscuts, the orientation and location of the rift seem to have been controlled by pre-Keweenawan faults and fractures and a major intrusive body in central Wisconsin. Lineament analysis of gravity and magnetic data and Landsat imagery, as well as orientation of major dike swarms and mapped faults show that individual rift segments are either parallel to or colinear with these features. A major elliptical gravity low in central Wisconsin suggests the presence of a large body of subjacent granite. The arcuate shape of the rift encircles this gravity low, suggesting that the inferred granite served as a tectonic guide during rift formation. Some important constraints can be placed on the width and direction of rifting, even though a unique solution for the width of the rift is not available from gravity and magnetic data. Geometric limitations suggest that the rift formed by roughly north-south separation parallel to a major preexisting tectonic zone, herein named the Trans-Superior Tectonic Zone, which bisects the arc formed by the two arms of the rift. Correlation of pre-Keweenawan features across the rift suggests that maximum separation was about 50 km in a north-south direction. But the presence of compressional features along the rift, such as the Keweenawan and Isle Royale faults, makes a unique solution impossible.

ABSTRACT The Pleistocene Okanogan lobe of Cordilleran ice in north-central Washington State dammed Columbia River to pond glacial Lake Columbia and divert the river south across one or another low spot along a 230-km-long drainage divide. When enormous Missoula floods from the east briefly engulfed the lake, water poured across a few such divide saddles. The grandest such spillway into the Channeled Scabland became upper Grand Coulee. By cutting headward to Columbia valley, upper Grand Coulee’s flood cataract opened a valve that then kept glacial Lake Columbia low and limited later floods into nearby Moses Coulee. Indeed few of the scores of last-glacial Missoula floods managed to reach it. Headward cutting of an inferred smaller cataract (Foster Coulee) had earlier lowered glacial Lake Columbia’s outlet. Such Scabland piracies explain a variety of field evidence assembled here: apparently successive outlets of glacial Lake Columbia, and certain megaflood features downcurrent to Wenatchee and Quincy basin. Ice-rafted erratics and the Pangborn bar of foreset gravel near Wenatchee record late Wisconsin flood(s) down Columbia valley as deep as 320 m. Fancher bar, 45 m higher than Pangborn bar, also has tall foreset beds—but its gravel is partly rotted and capped by thick calcrete, thus pre-Wisconsin age, perhaps greatly so. In western Quincy basin foreset beds of basaltic gravel dip east from Columbia valley into the basin—gravel also partly rotted and capped by thick calcrete, also pre-Wisconsin. Yet evidence of late Wisconsin eastward flow to Quincy basin is sparse. This sequence suggests that upper Grand Coulee had largely opened before down-Columbia megaflood(s) early in late Wisconsin time. A drift-obscured area of the Waterville Plateau near Badger Wells is the inconspicuous divide saddle between Columbia tributary Foster Creek drainage and Moses Coulee drainage. Before flood cataracts had opened upper Grand Coulee or Foster Coulee, and while Okanogan ice blocked the Columbia but not Foster Creek, glacial Lake Columbia (diverted Columbia River) drained over this saddle at about 654 m and down Moses Coulee. When glacial Lake Columbia stood at this high level so far west, Missoula floods swelling the lake could easily and deeply flood Moses Coulee. Once eastern Foster Coulee cataract had been cut through, and especially once upper Grand Coulee’s great cataract receded to Columbia valley, glacial Lake Columbia stood lower, and Moses Coulee became harder to flood. During the late Wisconsin (marine isotope stage [MIS] 2), only when Okanogan-lobe ice blocked the Columbia near Brewster to form a high lake could Missoula floodwater from glacial Lake Missoula rise enough to overflow into Moses Coulee—and then only in a few very largest Missoula floods. Moses Coulee’s main excavation must lie with pre-Wisconsin outburst floods (MIS 6 or much earlier)—before upper Grand Coulee’s cataract had receded to Columbia valley.

The Superior Lobe of the Wisconsin glaciation was initially localized by the deep lowlands of the Lake Superior Basin, cut in relatively nonresistant late Precambrian red sandstone. It advanced southwest out of this lowland and crossed a low divide leading to the Minneapolis Lowland, which is underlain by Precambrian and Cambrian sandstones. The conspicuous drumlins of central Minnesota, and the rugged St. Croix Moraine that borders the drumlins on the west and loops across the Minneapolis Lowland, delimit the major stillstand of the Superior Lobe. Discovery of red drift with diagnostic rock types from the Lake Superior Basin (agate, amygdaloidal basalt, red and purple felsite, red sandstone) in southwestern Minnesota indicates that the Superior Lobe once extended farther southwest down the Minneapolis Lowland to the Minnesota River valley and beyond during a pre-Wisconsin or early Wisconsin glaciation. As the Superior Lobe wasted from the St. Croix Moraine, a series of sharp subparallel tunnel valleys were cut into the drumlin plain and even into the underlying bedrock by subglacial streams driven to high velocity by the hydrostatic pressure resulting from the load of many hundreds of meters of active ice. Subsequent thinning and stagnation of the Superior Lobe opened the tunnel valleys to atmospheric pressure and converted the subglacial streams from major erosional streams to small depositional streams, which formed discontinuous eskers along many of the tunnel valleys. After distant retreat the Superior Lobe readvanced three times out of its basin, twice after proglacial lakes had produced a supply of red clay to be overridden. These latter two readvances may represent surges of the ice lobe resulting from the buildup of basal meltwater behind the frozen toe of the ice lobe. The Des Moines Lobe, originating in the Red River Valley of Manitoba and western Minnesota, moved southeastward down the Minnesota River valley and thence northeastward up the Minneapolis Lowland, overriding a segment of the St. Croix Moraine and extending across the state to Wisconsin in the form of the Grantsburg Sublobe. Its drift is characteristically gray to yellowish-brown and highly calcareous. The ice incorporated masses of Superior Lobe drift as it overrode the St. Croix Moraine, stringing them out to produce a complex of foliated red and gray drift. The Grantsburg Sublobe blocked the Mississippi River and other drainage from central Minnesota to form glacial Lake Grantsburg about 16,000 yrs ago. By that time the Superior Lobe had withdrawn completely from central Minnesota, for it supplied meltwater (and red clay) only on the east, down the St. Croix River and its upper tributaries. Meanwhile, the main Des Moines Lobe, which thus far supplied ice only to the Grantsburg Sublobe, thickened sufficiently to spill southward out of the Minnesota Valley across a low divide into Iowa. This produced the lobe that reached Des Moines 14,000 yrs ago. Beheading of the Grantsburg Sublobe in this manner caused the stagnation of the latter, which then wasted to form the Anoka Sandplain in its stead. The St. Louis Sublobe protruded from the Des Moines Lobe in northwestern Minnesota at a later date (about 12,000 yrs ago). Its meltwater flowed down the St. Louis River toward Lake Superior, but it was diverted southward into the St. Croix drainage by the still-existing Superior Lobe. Final wastage of the entire Des Moines Lobe produced glacial Lake Agassiz in northwestern Minnesota and adjacent North Dakota and Manitoba.