DRAINAGE SYSTEMS AND THE GLACIAL PERIOD. 199 



have receded to the very head-waters, and no cataracts 

 exist in them at the present time. Nor are there in the 

 unglaciated region any lakes of importance, such as char- 

 acterize the glaciated region. If there have been lakes, 

 the lapse of time has been sufficient for their outlets to 

 lower their beds sufficiently to drain the basins dry. 



On entering the glaciated area all this is changed. 

 The ice-movement has everywhere done much to wear 

 down the hills and fill the valleys, and, where there was 

 debris enough at command, it has obliterated the narrow 

 gorges originally occupied by the preglacial streams. 

 Thus it has completely changed the minor lines of super- 

 ficial drainage, and in many instances has produced most 

 extensive and radical changes in the whole drainage sys- 

 tem of the region. In the glaciated area, channels buried 

 beneath glaciated debris are of frequent occurrence, while 

 many of the streams which occupy their preglacial chan- 

 nels are flowing at a very much higher level than formerly, 

 the lower part of the channel having been silted up by the 

 superabundant debris accessible since the glacial move- 

 ment began. 



Buried Outlets and Channels. 



It is easy to see how the great number of shallow lakes 

 which frequent the glaciated region were formed by the 

 irregular deposition of glacial debris, but it is somewhat 

 more difficult to trace out the connection between the 

 Glacial period and the Great Lakes of North America, 

 several of which are of such depth that their bottoms are 

 some hundreds of feet below the sea-level, Lake Erie 

 furnishing the only exception. This lake is so shallow 

 that it is easy to see how its basin may have been princi- 

 pally formed by river erosion, while it is evident that 

 such must have been the mode of its formation, since it is 

 surrounded by sedimentary strata lying nearly in a hori- 

 zontal position. 



