308 GEOLOGICAL EXCURSION TO THE ROCKY MOUNTAINS. 



that could only have been out by a volume of water many times tJic 

 size of the present river.* 



the recession of the Falls of St. Anthony. 33 - 3 * — This subject is of 

 interest, as it furnishes data for the calculation of the time which lias 

 elapsed since the glacial period. The results practically agree with 

 those obtained from Niagara Falls and elsewhere, and furnish strong 

 evidence that the disappearance of the ice sheet from North America 

 was less than ten thousand years ago. 



From the Falls of St. Anthony to Fort Snelling (see map), a distance 

 of a little over 8 miles (18 km.), the Mississippi Hows in a deep gorge 

 about 1,000 feet (305 in.) wide. The rocks of this region are covered by 

 drift averaging, where the river has cut through it, probably not more 

 than 30 feet (9 in.) in thickness. Immediately below the. drift occur 

 the practically horizontal beds of the Trenton, with a thickness of from 

 20 to :>0 feet (6-9 m). The upper and lower beds are of comparatively 

 soft shales, but the greater partis of hard compact limestone. The 

 Trenton lies conformably on the St. Peter sandstone, which is a pure 

 sandstone of uniformly tine grain and, notwithstanding its age, has 

 undergone very little induration. This very friable sandstone is nearly 

 worn away and allows large blocks of the limestone to fall down, thus 

 materially increasing the rate of recession of the Falls. 



Fig. 12— Section across Mississippi River above Fort Snelling, 



From the Falls to Fort Snelling the rock on the sides of the gorges 

 has a freshly broken appearance, the large fragments thrown down by 

 the action of the water on the easily crumbled sandrock, as the falls 

 receded, still existing in the talus below the bluffs. Throughout this 

 extent the strata are horizontal, the thickness of the drift sheet over- 

 lying them nearly uniform, and all other conditions, so far as they can be 

 seen, that would affect the rate of recession, seem to have exerted an 

 unvarying influence. The inference is that the rate of recession has 

 been practically uniform between the, two points named. 



There is an aspect of age and long weathering presented by the rock 

 in the bluffs of the Mississippi below Port Snelling. It has a deeply 

 changed color, a light yellow oxidized exterior, which marks all old 



* Lake Agaasiz is to be fully described by Mr. Warren Upham in a forthcoming 

 monograph of the United states Geological Survey. 



