Encyclopedia of the Great Plains

GLACIATION

During the Pleistocene epoch (from two million to 10,000 years ago), continental glaciers invaded the Great Plains only in the northern portions; nevertheless, their effects on the entire region were profound. Glacial ice repeatedly blocked the rivers that drained eastward, forming ice-marginal lakes and diverting the rivers southward. Wind deflated sand and silt from the floodplains of the rivers that carried sediment-laden meltwater from both the enlarged glaciers in the Rocky Mountains and the continental ice sheet, creating fields of dunes and depositing a loess mantle on the uplands. Permafrost (perennially frozen ground) developed around the ice margin where strong winds swept the blanket of snow from the surface. Lowered temperatures along the glacier margin and southward caused major changes in distribution of the biota of the entire region.

The Laurentide ice sheets that developed in eastern and central Canada expanded to reach the Great Plains at least seven times between about 2.2 million years ago and less than 10,000 years ago, when the margin of the last glacier melted from the area. After each period of glaciation, ice disappeared long enough for soil profiles to form on the sediments left behind.

The early glaciations were the most extensive. They have been dated approximately by the relationships of the tills they deposited in reference to four ashfall deposits, three from the Yellowstone Caldera, and one from Bishop, California. One till lies beneath the oldest of these ash lenses, dating back more than 2 million years, and at least four till units lie between ash lenses deposited from 1.27 million to 610,000 years ago. All of these are now referred to as pre-Illinoian tills. The outer margin of one or more of these early continental glaciers extended into northeastern Kansas and eastern Nebraska, then angled northwestward across the Dakotas and northern Montana, where it approached the piedmont glaciers of the Rocky Mountains.

All the major rivers that drain eastward from the Rocky Mountains built alluvial fans where they left the mountain front and deposited gravel and sand along their routes toward the northeast and east. Uplift in the late Cenozoic period (around two million years ago) resulted in downcutting by those rivers, so that terraces, identified by soil profile, preservation, relative height, and presence of ash lenses, exist along the valleys. Stream diversions and piracy have since changed the courses of many streams in the Central and Northern High Plains. The Missouri River, which formerly drained northeastward into the Hudson Bay Lowland, was ponded several times by ice dams and ultimately diverted into its present course. The ancestral Platte River, its upper course rejuvenated by uplift in the Rocky Mountains and its discharge augmented by increased precipitation and, perhaps, meltwater from late Pliocene mountain glaciers, carried a vast amount of coarse gravel eastward across eastern Wyoming and into west-central and northern Nebraska, where it accumulated as the Broadwater and Long Pine Formations. Across the Central Plains, the ancestral Platte shifted its course repeatedly throughout the Pleistocene in response to local uplift (the Chadron Arch), aggradation, and glacial blocking, finally following its present course after having spread sheets of gravel across wide areas of east-central Nebraska. In the Southern Plains, sand and silt, probably blown from the floodplains of the Pecos and Canadian Rivers, accumulated in a sequence that becomes finer northeastward across western Texas and Oklahoma (Blackwater Draw Formation) and reaches as far as south-central Nebraska. Deposition began more than 1.4 million years ago and continued intermittently until less than 118,000 years ago, with paleosols (ancient soils) recording periods of stability.

Glaciations took place between 610,000 years ago and the first of the Illinoian glaciations, 300,000 years ago, but left no recognizable record in the Great Plains. Nor have tills of Illinoian age been identified with certainty in the Great Plains, although the ice undoubtedly reached the region, and tills of that age should exist buried beneath the deposits of the last, or Wisconsinan, glaciation.

The most complete record of the effects of glaciation in the Great Plains, as elsewhere, is that of the late Wisconsinan ice sheet, which reached its maximum extent about 20,000 years ago and covered the region north of a diagonal line from southeastern South Dakota to southern Alberta. Lobes of Wisconsinan ice expanded into lowlands from the James River basin to the upper Milk River basin in Montana. Meltwater flooded down the Missouri River Valley, as well as down rivers that were fed by alpine glaciers in the Rocky Mountains. Strong northwesterly winds deflated sand and silt from these ribbons of outwash to accumulate as dunes along the rivers and loess over much of the upland.

Strong winds associated with both glaciers and dry interglacial conditions generated dunes from alluvial sediments deposited in central Nebraska along an abandoned mid-Pleistocene route of the Platte River. Late Wisconsinan winds built massive dunes from these sand fields, which were reactivated repeatedly during dry phases over the last 10,000 years. The strong northwesterly winds eroded troughs and ridges in the zone adjacent to the ice margin and swept the zone free of snow so that permafrost developed. Lakes formed where drainage was blocked by the ice. The largest of these, Lake Agassiz, covered more than half of Manitoba and extended into Saskatchewan, Ontario, Minnesota, North Dakota, and the northeast corner of South Dakota. This huge lake drained through the Minnesota River Valley until the retreating ice margin had uncovered outlet routes to the east. By about 11,000 years ago, the ice margin lay along the north side of Lake Winnipeg, although the Laurentide ice sheet did not entirely disappear until about 6,000 years ago.

See also WATER: Kettle Lakes; Lake Agassiz.

William J. Wayne University of Nebraska-Lincoln

Morrison, Roger B., ed. Quaternary Nonglacial Geology: Conterminous U.S., The Geology of North America, vol. K-2. Boulder CO: Geological Society of America, 1991.

Sibrava, Vladimir, D. Q. Bowen, and G. M. Richmond, eds. Quaternary Glaciations in the Northern Hemisphere. Oxford: Pergamon Press, 1986.