552 THE ICE AGE IN NORTH AMERICA. 



tion in the Clinton Shale had been made fifty-five years be- 

 fore. The average of fifteen measurements made at twenty 

 foot intervals, showed that at the line of greatest erosion 

 fourteen feet of the Clinton Shale had fallen away during 

 that period; giving a rate of three inches a year. 



Section II was made 6.317 feet from the tunnel in what was 

 a perpendicular excavation in the Niagara Shale fifty-five 

 years before. The average amount of greatest erosion along 

 this exposure was obtained by eleven measurements through- 

 out a distance of 1.185 feet, and proved to be fourteen and 

 eight tenths feet, or three and one quarter inches per year. 



From these measurements it appears that the rate at which 

 the Clinton and Niagara shales crumble away over the whole 

 surface, through atmospheric agencies alone where unpro- 

 tected, is one and a half inches per year. 



The question as to how much protection has been afforded 

 by the talus and the growth of vegetation cannot be defi- 

 nitely answered, but as our photograph on p. 544 shows the 

 Niagara shale has not been protected to any extent by a talus, 

 and but slightly by vegetation. It therefore seems entirely 

 within the bounds of probability that the erosion of the N ia- 

 gara Shale at the mouth of the gorge has proceeded at one- 

 seventh the rate at the exposures measured, which is about 

 one quarter of an inch per year, or one foot in forty-eight 

 years, which is the rate necessary to accomplish the whole 

 amount in 10,000 years. 



A second typical place for the study of the recession of 

 post-glacial waterfalls is presented in the gorge of the 

 Mississippi River below the Falls of St. Anthony at Min- 

 neapolis. The problem here presented has been carefully 

 studied by Professor N. H. Winchell as follows.* 



From the Falls of St. Anthony to Fort Snelling the gorge 

 between the rock-bluffs is somewhat less than a quarter of a 



♦"Geology of Minnesota," vol. ii, pp. 313-316, 340, 341. 



