0. P. Hay — Deposit of the Glacial Drift. 55 



ing the whole Glacial period. At the beginning of that period 

 the ice with its enclosed bowlders and sand, did undoubtedly 

 pass over the old surface and plane it down and groove it ; but 

 later there may have intervened between the glacial ice and 

 this eroded surface a greater or less thickness of bowlder clay. 

 Of course, some trouble may be experienced in comprehending 

 how great glaciers thousands of feet in thickness and exerting 

 a downward pressure of probably 50,000 pounds to the square 

 foot* and containing sand, gravel and bowlders could move 

 over deposits of bowlder clay, and instead of erodi.ng them, con- 

 tinue to make additions to them. 



This and many other difficulties have arisen from pushing 

 too far the usually good practice of interpreting the events of 

 the past by what we see occurring in our own day. At the 

 present, we are acquainted with no glaciers except such as flow 

 down steep inclines and terminate on such inclines either by 

 melting or breaking off in the sea. On the other hand, the 

 glaciers that deposited the Drift materials of the Western States 

 after descending from the elevated regions of Canada, deployed 

 out on nearly level plains for hundreds of miles. It is quite 

 improbable that such a glacier would erode its bed and deposit 

 its detritus in the same way that a Swiss glacier does. We 

 could, perhaps, determine as correctly the eroding, transporting 

 and depositing effects of running water by studying a roaring 

 Alpine brook, as we can judge of the phenomena that attended 

 the movements of the ancient ice-sheet by studying the Mer de 

 Glace. But we know that the stream that in one part of its 

 descent wears away the hardest rocks and bears along with it 

 in. its impetuous course, gravel and stones, may in its lower 

 reaches, deposit the most impalpable sediments, and move so 

 gently as not to disturb the most delicate leaf that may have 

 fallen on its bed. 



In like manner, as the glaciers of the Ice Age descended 



I O CD ^ 



from the Laurentian Mountains and ploughed through the nar- 

 row channels now occupied by the Great Lakes, their eroding 

 action was incessant and irresistible ; but when those great 

 streams of ice were spread out over Ohio, Indiana and Illinois, 

 they became far less destructive. But even here, and how- 

 ever slow the motion, the underlying deposits were in most 

 places worn away. But the soils and other deposits in de- 

 pressed places of small area and behind cliffs of rocks probably 

 would not suffer much erosion ; since there the upper layers of 

 the ice would flow on over the stationary lower layers. In 

 this way we may account for the preservation of the soils and 

 the contained remains of trees in the Cromer Forest-bed and 

 for the abundant remains of trees and old soils below and even 

 * Newberry, Pop. Sci. Monthly, November, 1886. 



