Lee Clayton

A GLACIAL PROCESS-FORM MODEL

The glacial geomorphology of the interior of North America can be interpreted using a glacial process-form model that requires a quarrying zone near the glacial margin (the glacier is frozen to its bed in most places, excess pore-water pressure is dissipated by marginal seepage, and the glacial flow is upward) and an abrasion zone behind the margin (the bed is thawed and the glacier is sliding, excess pore-water pressure is not dissipated, and the glacial flow is horizontal or downward). Most erosion takes place in the quarrying zone, producing well-homogenized englacial sediment, most of which becomes superglacial mudflows as the glacier melts.

Evidence against pervasively deformed bed material beneath rapidly moving lobes of the southern Laurentide Ice Sheet

Abstract During the Wisconsin Glaciation, the ice lobes of the southern part of the Laurentide Ice Sheet moved rapidly as the result of elevated subglacial water pressure. The rapid movement was probably not accompanied by wide-spread pervasive deformation of unfrozen material under the ice. The till stratigraphy of much of this area is well known, and it is clear that the stratigraphic sequence is generally intact. Therefore, such deformation, if it occurred, would have been restricted to the till sheet that was currently being deposited. However, this seems unlikely as well, because the till of the region commonly contains lenses and clasts of unlithified bedded sand that should have been destroyed by pervasive shearing. If unfrozen deformed-till layers were widely present, they must have been confined to the thin upper parts of till sheets.

Late Wisconsin landform distribution and glacier-bed conditions in Wisconsin

Abstract The late Wisconsin Laurentide Ice Sheet advanced across permafrost and reached its maximum extent in Wisconsin between about 18,000 and 15,000 years ago. Deep permafrost persisted in southern Wisconsin until about 14,000 years ago and in northern Wisconsin until about 13,000 years ago. We suggest that during maximum glaciation a zone about 5 km wide in the south and 20 km wide in the north along the margin of the late Wisconsin glacier was frozen to its bed. Meltwater from farther behind the margin, where the bed was at least locally thawed, cut a series of closely spaced tunnel channels through the frozen-bed zone. These channels most likely formed episodically, and they were the source for much of the meltwater-stream sediment deposited in broad outwash plains beyond the ice margin. Frozen-bed conditions near the margin also likely contributed to increased upward shearing of sediment and the accumulation of thicl supraglacial sediment in northern areas. Up ice from the frozen-bed zone the glacier bed was at least locally thawed in a zone about 75 km wide. Extensive drumlin fields formed in the area of the bed that was thawed. By about 13,000 years ago permafrost melted in northern Wisconsin and thawed-bed conditions probably extended to the ice margin throughout Wisconsin and adjacent areas. After about 13,000 years ago in northern Wisconsin the glacier was sliding on its bed and forming drumlins out to the ice margin, and thick supraglacial sediment no longer accumulated.

Tectonic depressions along the hope fault, a transcurrent fault in North Canterbury, New Zealand

Abstract Tectonic depressions or compressive upbulges have developed at most bends in the Hope Fault, a major Pleistocene transcurrent fault in North Canterbury. The topographic expression of the depressions can be used to give otherwise unobtainable information about fault geometry and movement.

Correlation of late Wisconsin glacial phases in the western Great Lakes area

Late Wisconsin glacial phases of the Superior Lobe in Wisconsin and adjacent Minnesota are tentatively correlated across northern Wisconsin and adjacent Michigan to phases of the Green Bay and Lake Michigan Lobes. Spanning the period from about 26,000 yr B.P. to about 9500 yr B.P., 7 phases of the late Wisconsin Laurentide Ice Sheet are discussed, but only 2 of these are precisely dated. Ice advanced out of the basins of Lakes Superior and Michigan and stabilized during the St. Croix-Hancock phase, the most extensive late Wisconsin phase in most of the area, between about 18,000 and 15,000 yr ago. Following the St. Croix-Hancock phase, the general wasting of the ice was interrupted by stillstands or readvances of the ice margin during the intermediate and Mountain phases. The ice then wasted into the Superior basin before readvancing, over a landscape containing ice masses from earlier phases, during the Winegar-early Athelstane phase. This phase is believed to have occurred just prior to growth of the Two Creeks Forest in eastern Wisconsin. The ice then wasted back and advanced three more times during the Marenisco-late Athelstane, Porcupine (about 11,000 yr B.P.), and Marquette (about 9900 yr B.P.) phases.

The Mosbeck Site: a paleoenvironmental interpretation of the late quaternary history of Lake Agassiz based on fossil insect and mollusk remains

The Mosbeck Site is in the southern part of the Lake Agassiz basin in northwestern Minnesota. The stratigraphic section at the site consists of seven lithologic units, which are from bottom to top (A) unsorted, pebbly, sandy, silty clay, (B) coarse gravel, (C) silty sand, (D) peat, (E) fine sand, (F) interbedded sand and gravel, and (G) unbedded dirty gravel. The lower few centimeters of unit E are unoxidized and contain black spruce and tamarack driftwood, which has been radiocarbon dated at 9940 ± 160 BP (I-3880). Units C-E contain numerous, well-preserved insect and mollusk remains. These fossils have been compared with modern species, and at least 76 insect and 15 mollusk taxa are present. Assuming that their ecological tolerances have changed little in the past 10,000 years, they provide valuable information about the environment of Lake Agassiz. Few of the insects are now found in the region, indicating that the environment has changed. With few exceptions the species present indicate that the climate and vegetation at the time were similar to the present-day climate and vegetation of southeastern Manitoba. The lithology and faunal contents of the sediment are interpreted as follows. Unit A is Late Wisconsinan glacial sediment. Unit B is a lag concentrate formed by wave action during a regressive phase of Lake Agassiz. Unit C is the sediment of a small body of water that formed when the level of Lake Agassiz had dropped below the site. The banks were covered with a spruce forest. Open water gave way to swampy conditions, and unit D was formed. Both units C and D were deposited during the low-water Moorhead Phase of Lake Agassiz. Units E and F are shoreline sediment deposited as the lake level rose, drowning the vegetation. Unit G is modern ditch spoil.

Effects of Ground-Water Seepage on Fluvial Processes

Field observations of a small stream have indicated that seepage of water into or out of a stream may greatly alter stream competence. Theoretically, water seeping through the stream bed exerts a drag on quartz grains that changes stream competence by the factor 1.65/(1.65 − i ) where i is the seepage gradient. Experiments in laboratory flumes, however, indicate that seepage through a stream bed, despite its effect on effective grain density, causes no change in competence in gaining streams nor in losing streams that lack a mud seal. Possibly the expected change in competence is partially counteracted by the accompanying changes in form drag and surface drag on the grains. Flume experiments also show that upward seepage reduces the steepness of bed forms and decreases bed roughness, whereas downward seepage steepens the bed forms and increases bed roughness, but these changes appear to be insufficient to cause any measurable change in the slope of the water surface. Downward seepage in the presence of sufficient suspended sediment, however, can lead to the formation of a mud seal on the surface of a stream bed; this results in a great increase of the effective density of the bed sediment and may result in the total elimination of sediment entrainment from the bed.

Paleolimnology of late quaternary deposits: seibold site, north dakota.

A unique late Quaternary lacustrine deposit has been discovered recently on the Missouri Coteau of North Dakota. A diverse, extremely well-preserved biota of more than 160 species has been recovered primarily from an organic mud deposited about 9500 years before the present. The lacustrine body shallowed gradually as the climate became drier.

Stratigraphy and origin of an area of hummocky glacial topography, northern Wisconsin, U.S.A.

Abstract The Winegar Phase of the Wisconsin Glaciation produced a band of hummocky glacial topography in northern Wisconsin. Some hummocks are composed of flow till and somewhat sorted till-like debris-flow sediment. These hummocks formed when ice melted beneath a cover of debris that flowed to its present position. Other hummocks contain interlayered lodgement and meltout till, flow till, debris-flow and slopewash sediment, lake, and meltwater-stream sediment. These hummocks formed where sediment was stacked near the ice margin. In other hummocks till and supraglacial debris-flow and slopewash sediment deposited during the Winegar Phase form a thin veneer over older meltwater-stream sediment. These hummocks formed when ice buried in pre-Winegar Phase stream sediment melted, and the overlying sediment collapsed.

Bison trails and their geologic significance

Bison trails are conspicuous on aerial photographs of many parts of the mid-continent grasslands. Bison trails have been mistaken for glacial disintegration trenches, joints, and faults. The trails probably had an influence on the present drainage pattern of the Great Plains.