SURFACE GEOLOGY. 29 
the product of its grinding action was a calcareous clay, thickly studded 
with fragments of the excavated material. From the fact that the slope 
over which this glacier moved was very gentle, and a great barrier 
crossed its path, the drainage was never free, except locally, and as a 
consequence, the greater part of the ground material was never washed 
away, nor even suspended in water, and it remains as a sheet of irregu- 
lar thickness, and mostly unstratified bowlder clay. It is mainly com- 
posed of the debris of the shales and limestones which occupied an area 
of many hundreds of square miles, from which they have been removed, 
in and north of the basin of Lake Erie. The few well-worn pebbles 
and bowlders of crystalline rock contained in the bowlder clay are frag- 
ments brought by the glacier from the far-distant Canadian highlands. 
In their long journey, few have resisted the attrition to which they were 
subjected, and these have been worn, scratched, and planed off as we find 
them. As we go northward and approach their place of origin, they 
become more and more abundant, and increase in size. 
Since the glacier that formed our bowlder clay was a broad ice-sheet, 
and passed over a nearly level plain, where there were no summits or 
pinnacles from which stones or earth could fall upon it, all the material 
it carried was pushed along beneath it, or was frozen in to its under sur- 
face. Hence few large, and no angular blocks were brought by it from 
the northern highlands. Some blocks of large size were, as we know, 
torn by it from ledges of limestone and sandstone within or near the 
limits of our State; as we find in the northern counties masses of Cor- 
niferous limestone, evidently taken from the islands in Lake Hrie, or 
from the outcrops of this formation north of the Lake. These were car- 
ried one hundred miles or more south-west, to points several hundred 
feet above their places of origin. 
That the bowlder clay was not deposited beneath the glacier, as some- 
times stated, is apparent from the fact that it covers the glaciated surface 
on which the ice rested, in a sheet sometimes a hundred feet in thick- 
ness. lt must, therefore, have accumulated at the margin of the glacier. As 
the glacier retreated northward, the clay which it pushed out accumu- 
lated year by year, following it till it rose on to the Canadian highlands, 
where, with hard material, and free drainage to wash away the finer 
portions, it is largely replaced by beds of gravel, sand, and bowldezs. In 
the retreat of the great ice-sheet across the lake-basin, at first small 
pools, then larger basins, and, finally, a great inland sea, bordered it on 
the south. In these bodies of water a portion of the material ground up 
was suspended, and then deposited as the laminated portion of the Hrie 
clay. This, as I conceive, is the true and simple history of its formation. 
