Publisher's Synopsis
Excerpt from Lake Agassiz: A Chapter in Glacial Geology
The superficial material which'overlies the bed-rock with 111 this northern glaciated area has everywhere beeh ploughed up and worked over by the slowly moving ice-sheet, and at its disappearance was left in a deposit of, clay, sand, gravel and boulders, mixed in one confused mass, which 18 called till. Except in the valleys, as 'of the Mississippi at Minneapolis, where streams have assorted the'se materials and spread them in layers, the till forms the surface of nearly all of Minne sota, its thickness being from 50 to 250 feet. It is the Stony and gravelly clay, in which cellars and wells are dug; and it forms a sheet of such great extent and. Thickness that about half of the counties of western Minnesota contain no exposures of the underlying older rocks.
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