Geological Papers. Ill 



200 feet deep, at least that below Manhattan, and those of its tribu- 

 taries have been excavated since the ice first reached its maximum 

 limit. 



It has been assumed by some that this main valley was pre- 

 glacial, because large northern erratics are now found in its bot- 

 tom and under heavy alluvium ; but a moment's thought will show 

 that it would not be otherwise if these boulder deposits had been 

 let down by undermining, from terrace to terrace, in erosion which 

 was subsequent to the advent of the ice. Perhaps the idea was at 

 first received because these boulders were so deeply covered with 

 alluvium. Not long ago it was thought that the recent alluvium of 

 a stream could not exceed 20 or 25 feet, but now it is known that 

 it may exceed 100 feet. ( See Bulletin 158, U. S. Geol. Survey, p. 

 150.) 



That the general surface of the region and level of drainage was 

 high at that time is indicated in several ways : (1) Glacial deposits 

 are limited to higher levels. About Kansas City glacial striae have 

 not been found lower than about 125 feet above the Missouri river, 

 or 850 A. T. (2) The till in Kansas lies on the divides but has not 

 been found in the valleys. (3) At Weston, Mo., a cobble-stone 

 stratum, about 20 feet thick, lies upon Carboniferous shales about 

 150 feet above the Missouri. It contains red quartzite and granite 

 boulders, mingled with many of limestone. It seems therefore to 

 show that the river was flowing at that level when glacial erratics 

 entered the region. As already stated, the earliest bouldery strips 

 beyond the edge of the till in Kansas were at high levels. We 

 may imagine the shallow pioneer streams around the front of the 

 ice, as assisted by floe-ice and anchor- ice. 



The erosion of valleys 200 feet deep, and mainly during the 

 Kansan epoch, may seem too much for the credulty of some, but 

 there are several considerations which may make it more plausible. 



1. The probable volume of the stream. The whole western 

 slope of the great ice-sheet of that time probably included half of 

 the Dakotas, much of Minnesota, Iowa, Nebraska, Kansas and Mis- 

 souri. Omitting the last as not affecting our problem, but remem- 

 bering that probably as much in Canada as that enumerated which 

 would drain this way, we may reasonably count the ice-surface 

 contributing as over 200,000 square miles. The amount of ablation 

 or melting of Alpine glaciers has been estimated by DeSor as ten 

 feet a year; by Forbes as twenty feet, and even two and one-half 

 inches a day have been reported. Taking five feet as a conserva- 

 tive average, we should have a little less than 100 cubic miles 



