EROSION IN VALLEY OF THE GREAT LAKES 279 



of freshly exposed banks of till, but upon the 5,000-foot section the pres- 

 ent meandering of the stream permits it to touch the till bank only here 

 and there. But it was found on measurement that, of the 10,000 feet 

 of till banks originally exposed to the action of the stream in this 5,000-* 

 foot section, 1,600 feet are still exposed to it in its meandering. On the 

 supposition that in these exposed places the erosion is proceeding at the 

 same rate per foot as in the 500-foot cut-off, we arrive at the conclusion 

 that the annual erosion in the 5,000-foot section is 13,568 cubic feet. 



That the rate of erosion in these exposed places is approximately equal 

 to that in the 500-foot cut-off is evident from the fact that the water in 

 its curves impinges against the banks in substantially the same way and 

 with substantially the same force in the one place as it does in the other; 

 and, furthermore, while we have not definitely measured the amount of 

 erosion in the 5,000-foot section, we do know that great changes have 

 been produced by the erosion there during the last few years. Large 

 trees standing on the top of the bank have in several cases been under- 

 mined and toppled over into the stream and the top of the bank pushed 

 back beyond the area on which they were firmly rooted. 



Dividing 34,000,000 feet of material, the total amount removed from 

 the 5,000-foot section, by 13,568, the number of cubic feet estimated to 

 be annually removed by the stream at the present time, we find that the 

 whole work would be accomplished in 2,505 years — a result so startling 

 that we are compelled to study carefully the modifying conditions under 

 which the erosion has proceeded. Some of these we shall find to be 

 retarding in their effect, while there are others that will be accelerating. 

 The principal retarding conditions are connected with the existence of 

 the forests which prevailed over the area for an indefinite period of time 

 previous to the advent of civilized man a hundred years ago. While we 

 have no definite calculation on which to base an estimate of this retarding 

 influence, I have thought it safe to assume that it would not be more 

 than tenfold, so that the erosion in the forest would accomplish as mucli 

 in 1,000 years as would be accomplished under present conditions in 100 

 years. This would extend the time to 25,000 years. 



But there are various considerations which would cut down this esti- 

 mate, the chief of which is tliat the present exposures to erosion in the 

 5,000-foot section are at a minimum in their extent. Originally the 

 conditions were identical with those produced in the cut-oft' around the 

 reservoir; that is, the creek was a narrow stream eroding from both banks 

 throughout the entire distance. As the stream enlarged its trough and 

 began its meanderings, the exposures to erosion became less and less, but 

 the average would be twice those of the present time, thus reducing tlie 



