Geography of Ohio 223 



We have absolute proof that ice lobes occupied each of these 

 basin areas for a long time. The concentric moraines about the 

 basins give us the pattern of each lobe at several recessional 

 stages. Knowing that heavy bands of drift in recessional mo- 

 raines usually contain a high percentage of local material, we 

 infer that these concentric moraines are witnesses of long con- 

 tinued ice-erosion of the basins. A similar arrangement of mo- 

 raines south of Lake Erie shows that its basin was also subject to 

 the same mechanical work. 



The orientation of the Huron basin does not expose it so directly 

 to the erosion of ice from the Labrador dispersion center, as is 

 the case with either Michigan or Erie. The direction of striae, 

 and the arrangement of moraines, afford the evidences on which it 

 is concluded that this part of the Great Lakes' area was under the 

 influence of the Labrador center during at least the Wisconsin 

 epoch of the glacial period. Whether the Labrador center exer- 

 cised the same control during all the preceding glacial epochs 

 is not known. The direction of Huron's basin indicates possible 

 erosion from the Keewatin center. There is no reason to infer 

 that, during the earlier epochs of the Pleistocene, these two dis- 

 persion centers controlled the same regions southward that they 

 did during the last epoch; but at present we have no evidence 

 that suggests a different ice control for the earlier epochs. Possi- 

 bly, in time, we may have sufficient knowledge of the kinds of 

 rocks found in the drift, and their particular sources in the north, 

 to demonstrate whether there was always identity in the ice move- 

 ments of the several epochs from the two dispersion centers. 



In the absence of a more convincing explanation, it is probable 

 that students will continue to regard these lake basins as largely 

 the product of glacial erosion. We cannot infer the amount of 

 over-deepening by ice, as we have no measure of the depth of 

 these valleys before the ice entered. The most reasonable way of 

 approximating the amount of ice erosion in any particular one, 

 say the Michigan basin, is to note its depression below a line 

 which probably represents the gradient of the former valle}-. The 

 lake basin is rock-bound at either end. Allowing something for 

 the erosion at these termini, in case it is thought that one end has 

 suffered more erosion than the other, the line connecting their 

 levels will be parallel to the gradient of the preglacial stream. A 

 line drawn along the lowest bed rock of the present basin, there- 



