PSEUDO-COLS/ 



The French term col is gradually coming into use to signify 

 low passes or saddles on the watershed between drainage sys- 

 tems. Its use is very convenient in the discussion of reversed 

 or diverted drainage, particularly that caused by the intrusion of 

 glacial or igneous obstacles. It not infrequently happens that, 

 when a glacier enters the lower part of a drainage basin, it ponds 

 back the waters, and causes them to pass over such a col into a 

 neighboring basin. Sometimes the valley becomes permanently 

 filled with glacial wash and morainic debris to such an extent as 

 to cause the diverted stream to retain its new course after the 

 retreat of the ice. In such cases the stream, in subsequently 

 deepening its valley, forms a trench across the col, which grad- 

 ually takes on the form of an ordinary valley. In time the col 

 is only represented by a constriction of the new valley and by 

 certain residual features of the old topographic configuration. 

 The floor of the trench across the col obviously assumes the 

 slope of the new stream that caused it, and has its highest part 

 on the up-stream side of the former col. As the trench is cut 

 deeper and deeper, the highest point in the rock floor is gradu- 

 ally carried up stream. It may, in this way, be transferred some 

 distance from the original col and may thus become entirely 

 disassociated from its peculiar topographic relations. If, after 

 this has been done, another glacial invasion takes place and the 

 valley becomes again filled with glacial wash to some considerable 

 height, the transferred summit of the rock floor is liable to lose 

 its obvious connection with the old col and may perhaps seem 

 to be associated with new and misleading topographic surround- 

 ings. If, in such a case, the valley debris is penetrated by wells 

 at only a few points, and the investigator ascertains thereby only 



' Presented in substance before the Geological Society of America at Boston, Dec. 

 31, 1893. 



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