User:UBeR/Glacial history of North America

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Four major periods of the current ice age within the Pleistocene epoch in the United States are recorded by broad sheets of till and complex moraines, separated by ancient soils and layers of wind-blown silt. Striations, drumlins, eskers, and other glacial features show that almost all of Canada, the mountain areas of Alaska, and the eastern and central United States, down to the Missouri and Ohio Rivers, were covered with ice. There were three main zones of accumulation, the largest of which was centered over Hudson Bay. Ice advanced radially from there, northward to the Arctic islands and southward into the Great Lakes area. A smaller center was located in the Labrador Peninsula. Ice spread south from this center into what are now the New England states. In the Canadian Rockies, to the west, valley glaciers coalesced into ice caps. These grew into a single ice sheet, which then moved westward into the Pacific shores and eastward down the Rocky Mountain foothills, until it merged with the large sheet from Hudson Bay.

Throughout much of central Canada, the glaciers eroded from 15 to 25 meters (49 to 82 ft) of regolith and solid bedrock. This material was transported to the glacial margins and accumulated as ground moraine, end moraines, and outwash in a broad belt from Ohio and Montana. In places, the glacial debris is more than 300 m (984 ft) thick, but the average thickness is about 15 m (49 ft). Meltwater carried sediment down the Mississippi River, and much of the fine-grained sediment was transported and redeposited by wind.

Modification of drainage systems
Before glaciation, the landscape of North America was eroded mainly by running water. Well-integrated drainage systems collected runoff and transported it to the ocean. Much of North America was drained by rivers flowing northeastward into Canada because the features of the regional slope throughout the north-central part of the continent was to the northeast. The preglacial drainage patterns are not known in detail. Before glaciation, the major tributaries of the upper Missouri and Ohio rivers were part of a northeastward-flowing drainage system. This system also included the major rivers draining the Canadian Rockies, such as the Saskatchewan, Athabasca, Peace, and Liard rivers. It emptied into the Arctic Ocean, probably through Lancaster Sound and Baffin Bay, and an eastern drainage was out of the Saint Lawrence River.

As the glaciers spread over the northern part of the continent, they effectively buried the trunk streams of the major drainage systems, damming up the northward-flowing tributaries along the ice front. This damming created a series of lakes along the glacial margins. As the lakes overflowed, the water drained along the ice front and established the present courses of the Missouri and Ohio rivers. A similar situation created Lake Athabasca, Great Slave Lake, and Great Bear Lake, and their drainage though the Mackenzie River. This process established the present drainage pattern over much of North America.

There is extensive and convincing evidence of these changes in South Dakota. There, The Missouri River flows in a deep, trench-like valley, roughly parallel to the regional contours. All important tributaries enter from the west. East of the Missouri River, preglacial valleys are now filled with glacial debris, marking remnants of preglacial drainage. The pattern of preglacial drainage is also supported by discoveries of huge, thick, deltaic deposits in the mouth of Lancaster Sound and in Baffin Bay. These deposits are difficult or impossible to explain as results of the present drainage pattern because no major drainage system currently empties into those areas.

Beyond the margins of the ice, the hydrology of many streams and rivers was profoundly affected, either by increased flow from meltwater or by the greater precipitation associated with the glacial epoch. With the appearance of the modern Missouri and Ohio rivers, water that formerly emptied into the Arctic and Atlantic oceans was diverted to the Gulf of Mexico through the Mississippi River. Other streams became overloaded and their valleys partly filled with sediment. Still others became more effective agents of downcutting, as a result of glacial sediment, and their valleys deepened. Although the history of each river is complex, the general effect of the glaciation on the rivers was to produce thick alluvial fill in their valleys; the fill is now being eroded to form stream terraces.

Lakes
The Pleistocene glaciation created more lakes than all other geologic processes combined. The reason is because a continental glacier completely disrupts the preglacial drainage system. The surface over which the glacier moved was scoured and eroded by the ice, leaving myriad closed, undrained depressions in the bedrock. These depressions filled with water and became lakes.

Farther south, in the north-central United States, lakes formed in a different manner. There, the surface covered by glacial deposits of ground moraine and end moraines. Throughout Michigan, Wisconsin, and Minnesota, these deposits formed closed depressions that soon filled with water to form tens of thousands of lakes. Many of these lakes still exist. Others have been drained or filled with sediment, leaving a record of their formed existence in peat bogs, lake silts, and abandoned shorelines.

Very large lakes were created along the glacial margins. The ice was about 3,000 m (9,8043 ft) thick near the centers of maximum accumulation, but it tapered toward the glacier margins. Crustal subsidence was greatest beneath the thickest accumulation of ice. As the ice melted, rebound of the crust lagged behind, producing a regional slope toward the ice. This sloped formed basins that have lasted for thousands of years. These basins became lakes or were invaded by the ocean. The Great Lakes were formed primarily in this way.