NE-SAW-JE-WON 



debris on a city dump. Obviously, this material is a collection 

 of all sorts of unrelated odds and ends of rock material from 

 the varied surfaces over which the ice had passed. The melt 

 water sorted out some of the debris and carried it away from 

 the ice front, building up deposits called outwash plains or 

 aprons, the coarser gravel being left near the moraines and 

 the finer gravel, sand and silt being carried farther away. 

 The moraines may be likened to entrenchments thrown up 

 for protection by the retreating ice in the warfare between 

 heat and cold. They mark the lines of halt and reinforcement, 

 followed by giving-way and further retreat, as the warming 

 climate slowly forced the glaciers from the continent. A rapid 

 retreat, when the forces of heat were great and when melting 

 was more powerful than the forward push, caused the rock 

 debris to be deposited in gently undulating plains of com- 

 mingled sand, clay and stones called till. These deposits, known 

 as ground moraine or till plains, are similar on a large scale to 

 the dusty, clayey deposit left on the surface after a persistent 

 winter snowbank has melted. Formation of the till plain was 

 stopped at each halt, as the ice built another backstep moraine. 

 Long roughly parallel lines of morainal hills, stretching from 

 Pennsylvania to Illinois, mark the retreat of the Wisconsin 

 glaciation in the Ohio valley region. But when the ice had 

 retreated to the Great Lakes area the valleys of the ancient 

 river systems again determined the shape of its front; again 

 the ice assumed lobate form, the lobes acting as independent 

 glaciers in the Lake Michigan, the Huron-Erie and the 

 Superior basins. The moraines which mark the halts of 

 these glacial lobes are festooned about the basins in lines of 

 hummocky hills which roughly parallel the borders of the 

 present lakes. In time, as melting progressed, the lobes came 

 to be more completely separated, so that distinct glaciers 

 occupied the basins of Lake Michigan, Green Bay, Lake Erie, 

 Lake Huron, Saginaw Bay, and finally the Ontario basin. In 

 the Lake Superior region the ice probably divided into lobes 



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