3J2 MAN AND THE GLACIAL PERIOD. 



Fort Snelling up is not old enough to have permitted 

 much enlargement by the gradual undermining of the 

 superficial strata on either side, which slowly but constantly 

 goes on. 2. From the abruptness with which it merges 

 into the preglacial valley of the Minnesota-Mississippi. 

 The opening at Fort Snelling is not Y-shaped, as in gorges 

 where there has been indefinite time for the operation of 

 erosive agencies. 3. Furthermore, the precipices lining 

 the post-glacial gorge above Fort Snelling are far more 

 abrupt than those in the preglacial valley below, and they 

 give far less evidence of weathering. 4. Still, again, the 

 tributary streams, like the Minnehaha River, which empty 

 into the Mississippi between Fort Snelling and Minne- 

 apolis, flow upon the surface, and have eroded gorges of 

 very limited extent ; whereas, below Fort Snelling, the 

 small streams have usually either found underground 

 access to the river or occupy gorges of indefinite extent. 



The above estimates, setting such narrow limits to 

 post-glacial time in America, will seem surprising only to 

 those who have not carefully considered the glacial phe- 

 nomena of various kinds to be observed all over the glaci- 

 ated area. As already said, the glaciated portion of North 

 America is a region of waterfalls, caused by the filling up 

 of old channels with glacial debris, and the consequent 

 diversion of the w^ater-courses. By this means the streams 

 in countless places have been forced to fall over preci- 

 pices, and to begin anew their work of erosion. Water- 

 falls abound in the glaciated region because post-glacial 

 time is so short. Give these streams time enough, and 

 they will wear their way back to their*sources, as the pre- 

 glacial streams had done over the same area, and as simi- 

 lar streams have done outside the glaciated region. Upon 

 close observation, it will be found that the waterfalls in 

 America are nearly all post-glacial, and that their work of 

 erosion has been confined to a very limited time. A fair 

 example is to be seen at Elyria, Ohio, in the falls of Black 



