lecture_16 - LECTURE 16 GLACIAL HISTORY OF THE GREAT LAKES...

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LECTURE 16 - GLACIAL HISTORY OF THE GREAT LAKES BASIN PHYSICAL SETTING The Great Lakes basin includes a lowland of Paleozoic age and an upland of Precambrian age. Lowland characterized by broad, low morainal ridges , low relief , and mantle of glacial sediments in places over 350 m thick. Upland characterized by distinct bedrock-dominated topography formed as a result of bedrock structures and differential erosion. Glacial sediments thin. PREGLACIAL LANDSCAPE Prior to glaciation basin was subjected to long-term subareal erosion and included two major drainage systems , the Laurentian and Teays-Mahomet. Element of the Laurentian system can be traced in the subsurface from the western end of the Ontario basin to the Georgian Bay basin. Drained into e St. Lawrence valley and probably extended into the Lake Michigan and Lake Superior basins. Elements of the Teays-Mahomet system occur in northern Ohio, Indiana and Illinois. This system drained into the Mississippi valley . Some sinkholes , underground streams and caves probably also existed, particularly with respect to the Niagara Dolomite . ORIGIN OF THE LAKE BASINS The Great Lakes basins owe their origin mainly to channeling of ice along major bedrock valley systems that existed prior to glaciation.
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Lakes Michigan, Huron and Erie basins underlain mostly by erodible Devonian shales and limestones . Lake Ontario basin underlain mainly by Ordovician shale.
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