386 ■ GLACIAL GRAVELS OF MAIXE. 



irregularly, we can not expect the gravels to be so symmetrical in shape as 

 the drumlins. 



The lenticular eskers or osars, then, have the natural form that a short 

 mass of gravel takes when formed beneath flowing ice. This h3qDothesis 

 assmnes that the present deposits were stationar}- and the ice flowed over 

 them. It is barelj^ possible that a glacial waterfall where a surface stream 

 falls down a crevasse may form a series of enlargements at intervals, and 

 that these enlargements may be pushed bodily forward with their con- 

 tinued gravels. This would imply thick ice and small masses of gravel. 

 At the time the present lenses were formed the ice could not push forward 

 the gravels, but, as at Winslows Mills, flowed over them, only now and 

 then distorting the stratification on the stoss sides. The question whether 

 the continuous ridges began as a series of separated lenses which at last 

 became confluent will be referred to later. 



DECREASE OF GLACIAL GRAVELS TOWARD THE COAST. 



As has already been repeatedl}' stated, the maximum development of 

 the coarser glacial sediments occurs near the highest sea level, which in the 

 coast region is not far from the contour of 230 feet. 



Among the causes of great precipitation at this elevation ma}^ be men- 

 tioned the following : 



1. The length of time the sea stood near that elevation. That the 

 changes of level of the sea with respect to the land were geologically rapid 

 is proved by the fact that the till was only partially eroded over the sub- 

 merged area. I have found no cliffs of erosion in the rock, and thus have 

 no proof of long pauses in either the advance or the retreat, and therefore 

 assume that the rise and fall of the sea were somewhat uniform in rate. If 

 so, it follows that it stood near its highest level for a longer time than at 

 any other — that is, during the last part of the period of advance, the time of 

 stationary IcA^el, and the earlier part of the retreat. During this period the 

 modern rivers began to flow and form deltas off the shore of that time. 

 Thus a vast quantity of sediment was stopped near the highest shore-line. 

 It could not reach farther south because checked in its motion soon after 

 entering the sea. 



2. The effect of steeper land slopes north of the contour of 230 feet. 

 By a coincidence the slopes of the land are steeper to the north of the 



