THE GLACIAL, IMRIOD AND ITS RKCORDS 



its own weight, just as a mass of clay or dough piled on a 

 board spreads outward from its thick central mass. Once 

 started to move, tiie mass of snow-ice became a glacier, which 

 is estimated to have been from two to six miles thick and 

 which reached a maximum extent of approximately 1600 

 miles from its center to the Ohio Valley. Lour times during 

 that million years the ice advanced over and retreated from the 

 continent. Each advance had some effect upon the lake re- 

 gion, but it is only with the last advance — the so-called Wis- 

 consin glaciation — that we are particularly interested. This 

 last advance obliterated or buried much of the record of the 

 earlier invasions, although it did not quite reach to the Ohio 

 valley, as the preceding Illinoian invasion had done. 



Once started, the ice moved slowly, relentlessly southward 

 from the Hudson Bay area. Its freezing grip clutched the 

 loosened rock ledges and plucked them away; it crushed and 

 scooped up the surface debris, absorbing boulders, sand and 

 clay into its mass, and slowly carried forward its load of rock 

 materials to deposit them far from their source. -It used peb- 

 bles, cobbles and boulders frozen in its base as great files, rasps 

 and plough points to scratch and groove the surfaces of the 

 bed rock over which it passed; or with fine rock and clay it 

 scoured and polished these surfaces and rounded and polished 

 the boulders and pebbles within its mass. By the scratches, 

 grooves and polishings on rock ledges, we know in what di- 

 rections the ice moved. At times this last great glacier, like 

 the others before it, melted faster than it pushed forward, so 

 that the southward movement was somewhat pulsating; but in 

 the main, for thousands of years it moved steadily southward, 

 carting along within its mass unmeasured quantities of rock 

 debris which it had taken from the lands to the north. When 

 it reached the area of the Great Lakes, tongues of ice entered 

 the river valleys in the ancient rocky moats and there became 

 thicker and moved more rapidly. These river valleys were in 

 the soft shales, hence the ice gouged and furrowed them; it 



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