EIVER TERRACES. 61 



in local lakes which were coufined between the ice on tlie north and the 

 hills to the south during the final melting of the great glacier. 



And now, after eliminating these more distinctly glacial sediments, how 

 can we account for the remainder of the valley drift? A great part of it 

 is frontal matter, derived from glaciers situated far to tlie nortli. Such 

 sediment would consist mostly of clay derived from the muddy glacial 

 streams, representing work done beneath the ice. In such a case the glacier 

 of that time was so remote from where we now find the sediment that it is 

 difficult to trace the connection. 



RIVER TERRACES. 



Here and there, at Avaterfalls and in the swifter parts of their courses, 

 the streams of Maine have eroded all the supei-ficial drift, and may even 

 flow in channels excavated in the solid rock. A few of these rock channels 

 approach the dignity of canyons, as those of the Kennebec above the 

 Forks and of the Penobscot below Ripogenus Lake. In general, the 

 streams flow in channels lying wholly or chiefly in the till or other super- 

 ficial deposit. In addition to the erosion channels in which the streams 

 flow when at their average height, we find most of the streams bordered by 

 one or more terraces at higher level. The terraces consist of a somewhat 

 horizontal portion, or shelf, ending in a rather steep bank or bluff facing 

 the stream. The material of most of the terraces is some form of water 

 drift, but sometimes it is till. In a few places where the channel proper 

 lies in easily eroded sand, there are no terraces above the banks of the 

 channel of erosion. This occurs when erosion and deposition are nearly 

 equal, and when deposition is the greater. 



River terraces may be divided into two classes. 



1. Terraces of river erosion in drift which was not deposited by the 

 rivers themselves. The till and the marine glacial and lacustral sediments 

 were deposited under conditions independent of the streams which subse- 

 quently began to flow in the valleys of deposition, and the agencies by 

 which they were deposited could not have formed a series of terraces to 

 which the streams bear a causal relation. River terraces in these forma- 

 tions, or in blown sand, must be due to erosion by the rivers. They are as 

 plainly formed by erosion on the land as a beach cliff is caused by waves 

 and currents. The erosion terraces of Maine correspond to the rock bluffs 



