CHAPTER XII. 



PREGLACIAL DRAINAGE. 



One of the most marked effects of the Glacial period 

 was its influence upon drainage systems. The changes pro- 

 duced in numerous river-courses of North America by the 

 irregular deposits of till and modified drift, as well as by 

 the existence of temporary barriers of ice during the con- 

 tinuance of the continental ice-sheet, are subjects of unfailing 

 interest to the student of physical geography, and are also of 

 great practical significance in their relation to the economic 

 and hygienic interests of the country. 



As compared with preglacial time, that which has elapsed 

 since the close of the Ice age is admitted by all to be very 

 short. Consequently, post-glacial erosion is much less than 

 preglacial erosion. Before the advent of the continental 

 ice-sheet, all the great valleys of North America had been 

 sculptured by preglacial streams.* The effects are still to 

 be seen even where extensive deposits of the Glacial period 

 have partially obliterated them. The sedimentary rocks, 

 occupying the basin of the Mississippi, and filling it with 

 strata thousands of feet in depth, serve as one index of the 

 extent of preglacial erosion ; for all the material of this class 

 of rocks has been ground up and transported by water. 

 Coming down from the neighborhood of the White Mount- 

 ains, the Adirondacks, and the Archaean highlands of Canada, 

 sediment-laden streams have, from the very earliest geologi- 

 cal ages, been engaged in wearing away the hills, scooping 



* See Chapter X. 



