340 MAN AND THE GLACIAL PERIOD. 



A second noteworthy glacial chronometer is found in 

 the gorge of the Mississippi River, extending from the 

 Falls of St. Anthony, at Minneapolis, to its junction with 

 the preglacial trough of the old Mississippi, at Fort Snel- 

 ling, a distance likewise of about seven miles. 



Above Fort Snelling the preglacial gorge is occupied 

 by the Minnesota Eiver, and, as we have before stated, 

 extends to the very sources of this river, and is continuous 

 with the southern portion of the valley of the trough of 

 the Eed Eiver of the North. Before the Glacial period 

 the drainage of the present basin of the upper Mississippi 

 joined this main preglacial valley, not at Fort Snelling, 

 but some little distance above, as shown upon our map.* 

 This part of the preglacial gorge became partially filled 

 up with glacial deposits, but it can be still traced by the 

 lakelets occupying portions of the old depression, and by 

 the records of wells which have been sunk along the line. 

 When the ice-front had receded beyond the site of Min- 

 neapolis, the only line of drainage left open for the water 

 was along the course of the present gorge from Minne- 

 apolis to Fort Snelling. 



Here, as at Niagara, the problem is comparatively 

 simple. The upper strata of rock consist of hard lime- 

 stone, which is underlaid by a soft sandstone, which, like 

 the underlying shale at Niagara, is eroded faster than the 

 upper strata, and so a perpendicular fall is maintained. 

 The strata are so uniform in texture and thickness that, 

 with the present amount of water in the river, the rate of 

 recession of the falls must have been, from the beginning, 

 very constant. If, therefore, the rate can be determined, 

 the problem can be solved with a good degree of con- 

 fidence. 



Fortunately, the first discoverer of the cataract — the 

 Catholic missionary Hennepin — was an accurate observer, 



* See above, p. 209. 



