Geography of Ohio 201 



We know very little about the actual methods of glacial erosion. 

 A valley may to-day bear a glacier, and in the part of the valley 

 from which the glacier has retreated the evidence of erosion may be 

 obvious. But no one can tell just what is going on beneath the 

 tongue of ice. All infer that the basal part of the ice is shod with 

 stones which, by the weight of the ice above, are held against the 

 rock beneath, and, as the glacier slowly moves, this bed rock is 

 worn. Probably our chief stumbling block in appreciating the 

 erosion accomplished by glaciers is our difficulty in grasping the 

 time involved. The history of mankind is so short that man's 

 imagination fails him in conceiving geologic time. Primitive man 

 shaped his tools by wearing a softer rock against a harder. Even 

 two rocks of the same hardness may be made to wear each other 

 out. A tiny scratch in a slab of rock removes a measurable amount 

 of material. The entire mass of the rock is only a multiple of that 

 quantity removed. Given time enough for sufficient repetitions 

 of the scratch, and the rock may be worn away. No one hesitates 

 to grant a glacial period tens of thousands of years duration. 

 Alany do not hesitate to make it hundreds of thousands of years. 

 Even the more conservative estimate would account for such 

 glacial erosion as we have studied, provided the ice were contin- 

 uously laden with tools. It is the tongue of ice leading ahead of the 

 main mass, through valleys, that does the great erosive work. 

 Towards the axes of such valleys the weight of the ice trends. 

 The depth of the valley insures this weight. The valley walls 

 supply tools which, added to the rocks already in the ice, insure 

 erosion. Consequently, the rock relief of the region must be in- 

 creased, if the region preglacially had valleys trending in the direc- 

 tion of the ice motion. 



When relief is decreased. The removal of minor irregularities 

 by the general erosion of an ice sheet, to that degree decreases 

 relief. The basins and vallej^s existing preglacially in the area 

 measure its maximum relief. If these depressions are transverse 

 to the direction of ice motion, they will not be deepened; more of- 

 ten they will be shallowed through the accumulation of glacial 

 debris. The orientation of the lines of greatest relief decides 

 whether that relief will be increased or decreased through glacia- 

 tion. It is possible that the irregularity of glacial deposits, as 

 discussed in the next section, may have greater relief than the 

 area had originally. 



