GEOLOGICAL SCALE AND STRUCTURE. 39 



attacked it, and it gave way to such an extent that it disappeared 

 entirely from large areas which it must have formerly covered. This is 

 attested by the vast amount of shale which can be recognized in the 

 Bowlder clay. As the ice advanced, it exerted great disturbing force 

 upon what was then the surface of the state. A sheet weighing scores 

 of tons to the square foot and moving forward with irresistible force, 

 holding in its foot fragments of the hardest rocks of the continent, could 

 not fail to leave the traces of its advance by smoothed and striated rocks, 

 which occur everywhere along its path. 



There is a question among geologists as to whether the Glacial 

 deposit can be best explained by a single ice period, or by recurrent 

 ice periods. The question is still under discussion, but the weight of 

 opinion on the part of those best qualified to judge seems to be in favor 

 of the latter theory at the present time. But all of the questions "per- 

 taining to the origin and history of this formation must be counted far 

 from being definitely settled. The average thickness of the drift 

 deposits cannot be given without providing at least for very wide 

 departures from any general figure. The greatest thickness yet observed 

 in the state is five hundred and thirty, feet, which was reached in a boring 

 that was begun near St. Paris, Champaign county, several years ago. The 

 five hundred and thirty feet of Drift there found, did not exhaust the Drift 

 deposits, but the driller was obliged to abandon the work before he had 

 reached bedded rock. The estimates of average thickness that are occas- 

 ionally made are entitled to but small consideration. The valleys of the 

 old surface of the state are packed full to a depth of several hundred feet, 

 while the original up-lands that bound the valleys may be covered by but 

 a few feet or by a few scores of feet. It is sufficient to say that the amount of 

 material thus brought down and distributed over the state is vast in 

 amount and that its deposit has added immensely to the value and 

 resources of the state. The soils of three quarters of Ohio are derived 

 from the Drift and are consequently much more varied in composition 

 and, at the same time, of more uniform excellence, than they could have 

 been if derived solely from the underlying bedded rocks. The water 

 supply of the same portion of the state is almost wholly dependent up- 

 on the Drift formation. There are entire counties in which the bedded 

 rocks do not once come to the surface, and in which they have, conse- 

 quently, no effect upon the character of the surface. 



GEOLOGICAL STRUCTURE. 



The chief formations of the state have now been passed in brief 

 review. As to the geological structure of Ohio, or the mode of arrange- 

 ment of these several formations, as far as their departures from the posi- 

 tion in which they were formed is concerned, a few general statements 

 must suffice. 



