Page:The Journal of geology (IA journalofgeology21894univers).pdf/219

The French term col is gradually coming into use to signify low passes or saddles on the watershed between drainage systems. 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 the 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 gradually 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 gradually 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 surroundings. If, in such a case, the valley debris is penetrated by wells at only a few points, and the investigator ascertains thereby only