560 THE ICE AGE IN NORTH AMERICA. 



of the ice-margin from the vicinity of Fort Snelling to that 

 place where the discharge of Lake Agassiz was opened to- 

 ward the north, reducing the Minnesota to nearly its present 

 size. This change must have given prominence and erosive 

 effect to the waterfall at Fort Snelling, if it did not give it 

 birth. 



These calculations concerning the age of Niagara and 

 the Falls of St. Anthony are amply sustained by the study 

 of various minor waterfalls and gorges in Ohio to which I 

 have myself given special attention. For example, at Elyria, 

 twenty -five miles west of Cleveland, Black River plunges 

 over the outcropping Waverly sandstone, and flows onward 

 to the lake through a wide valley in the Erie shale, which 

 was doubtless preglacial, though no buried channel above 

 has yet been discovered. The gorge below the falls, which 

 has been eroded since glacial times, and which approximately 

 represents the work done by Black River during that time, 

 is only a trifle over two thousand feet long. The water 

 flowing over the falls represents the drainage of about four 

 hundred square miles, and the sandstone which forms the 

 precipice over which the water plunges is underlaid by soft 

 shale very favorable to rapid erosion. In March, 1871, a 

 mass of rock fell which was so large that the concussion 

 shook the whole town and produced the semblance of an 

 earthquake. With the present forces in operation at this 

 point, it would seem incredible that the average rate of re- 

 cession should not be considerably more than one foot in 

 fifty years. Yet thus infinitesimal would be the rate if one 

 hundred thousand years must be allowed for the time separat- 

 ing us from the birth of the present waterfall at Elyria. 

 The shortness of this and other similar gorges in that region 

 points to a great reduction of the prevalent estimates of 

 glacial chronology. 



Another interesting confirmation of this moderate esti- 

 mate is to be found in Paint Creek Valley, in the southern 

 part of Ohio, to which attention was directed in a previous 

 chapter. As was discovered by Professor Orton several years 



