THE DATE OF THE GLACIAL PERIOD. 567 



would be removed in 2,505 years; a result so incredible that 

 we are called to examine more closely into the various con- 

 ditions affecting the problem, some of which would tend to 

 retard the eroding action of the stream, and some to accel- 

 erate it. 



Of the retarding influences the most conspicuous is the 

 former existence of a dense forest of large trees covering the 

 whole basin. It is scarcely possible, however, that the rate 

 would be reduced from this cause lower than to one-tenth of 

 that of the present time, which, if there were no counteract- 

 ing causes, would extend the time to 25,000 years. 



But, on the other hand, it is evident that the rate of erosion 

 in the main trough is, at its present width, at a minimum. 

 For, as the width of the trough has enlarged, it has taken the 

 stream a longer and longer time to swing from side to side in 

 its meanderings. At the outset the stream acted through the 

 entire length on both sides as it now does in the cut-off, and 

 when the width of the trough was half what it is now, the 

 erosion was twice as fast. 



It is safe to say, therefore, that the average rate during 

 the forested condition would be twice what we have allowed. 

 This would reduce the time to 12,500 years, which cannot be 

 far from a correct estimate. For, in addition to the early 

 constriction of the channel in increasing the rate, it should 

 be kept in mind that on the first withdrawal of the ice there 

 was no forest to retard the action of the stream. Further- 

 more, it is altogether probable that there was a much greater 

 precipitation over the basin of the creek while the ice lingered 

 over the area immediately to the northward, and this would 

 increase the rate of erosion. 



It has been necessarj' to enter thus fully into details con- 

 cerning one instance in order to get the force of the cumula- 

 tive argument from the innumerable similar instances which 

 present themselves all over the area in which the natural 

 drainage is towards the front of the ice-sheet. Present 



