GLACIAL EROSION AND TRANSPORTATION 257 



glacial origin, these may at some time give us a partial clew 

 to the length of the period. 



Another method of estimating the amount of glacial ero- 

 sion is by calculating the extent of the deposits over the 

 glaciated region. Professor Newberry has estimated that 

 the area south and west of the Canadian highlands, covered 

 with glacial debris, is not much less than 1,000,000 square 

 miles, and that the depth of the deposit over this broad 

 marginal area can not be less, on the average, than thirty 

 feet, and is probably twice that amount.* Professor Ciaypole 

 found, upon comparing the estimates independently made 

 in different counties of Ohio, Indiana, and Illinois, that the 

 average depth for these three States was sixty-two feet, and 



Fig. 76.— Ideal section showing how the till overlies the stratified rocks. 



for Ohio alone fifty-six feet.f Recent extended experi- 

 ments in boring for gas in western Ohio and eastern Indi- 

 ana tend greatly to increase the estimate of Professor Clay- 

 pole. There are whole counties in southwestern Ohio so 

 deeply covered with glacial debris, that few of the citizens 

 have ever seen the underlying rock. In other counties, 

 where the rock occasionally crops out at the surface, the ex- 

 tensive spaces between are found to be valleys of immense 

 depth, filled with the unstratitied material of the ground 

 moraine. In Professor Orton's recent report on the subject, 

 giving the depth in fifty-three of the counties of Ohio as 

 determined by the borings in 122 wells, the average is found 

 to be upward of 90 feet (93 feet +). In some of the wells 

 the depth was truly phenomenal, as in St. Paris, Champaign 

 county, where, in one well, rock was not reached short of 



* "School of Mines Quarterly," January, 1885, p. 7. 



+ "Glacial Erosion," by William M. Davis, "Proceedings of the Boston 

 Society of Natural History," vol. xxii, pp. 19-58. 



