548 JOHN L. RICH 



centage have an earlier origin. In some of these cases all that can be 

 said concerning the time of their formation is that they antedate the 

 final retreat of the last ice-sheet. In others, notably Cayuta Gorge 

 channel, the development of tributary gorges subsequent to the 

 channel cutting points unquestionably to one or more interglacial 

 intervals at least several times longer than post-glacial time. 



Clear evidence of the burial and subsequent melting of large ice 

 masses, with consequent settling of the overlying drift, has been found 

 in one of the channels. Under ordinary conditions it would be diffi- 

 cult to obtain conclusive evidence of settling caused by melting ice 

 because of the difficulty of distinguishing the resulting depressions 

 from normal morainic topography. When, however, such a sink- 

 ing lowers a portion of the bottom of a large stream channel, the 

 evidence is clear. 



One of the most important features of these channels is that they 

 show the direction of the ice flow, and in this are of great value in the 

 mapping of the glacial geology of a region. In favorable cases, 

 especially in connection with lateral channels, it is possible to ascer- 

 tain the slope of the ice with a very fair degree of accuracy. 



Glacial channels are easily recognized. They are characteristi- 

 cally flat bottomed and are usually swampy. The banks, in almost all 

 cases, are distinct and, except where disturbed by cultivation, show 

 little effect of post-glacial degradation. 



