74 GEOLOGY. 



valley might then work back across the former site of the lake into the 

 territory beyond. Valleys might have developed above the lake before 

 it was drained, and after this event, such valleys would make connec- 

 tions with the valley below (Fig. 55). A valley developed in this man- 



FiG. 55. — The stream leading out from the lake (Fig. 54) has drained the lake, and 

 the valleys above and below the site of the former lake have united. 



ner is not simply a gully grown big by head erosion, and the valley would 

 not precede the stream. 



If a surface of land were notably irregular before valleys were devel- 

 oped in it, there might be many lakes, and the flow from a higher lake 

 might pass to a lower. If the lakes were ultimately drained, the several 

 sections of the valley would be joined to one another without intervening 

 basins. In certain regions, especially those which have been affected 

 by continental ice-sheets, this has been a common method of valley 

 development in post-glacial time. In this case also the stream precedes 



