OP THE FALLS 0^ ST. ANTHONY. 889 



it eastward in the valleys. No. 2 (6) sometimes rests on the grey 

 hardpan without the intervention of No. 2 (c) ; and in the same 

 manner No. 3 (b) sometimes passes into the red hardpan without 

 any intervening bed of gravel and boulders. 



In all cases the hardpan is plainly the original deposit, the 

 material from which the other parts have been derived. There is 

 in nearly all cases a greater or less thickness of the red hardpan, 

 the only points at which it is entirely wanting being along the 

 river banks, where it may have been subjected to great erosion and 

 wash, the overlying grey gravel, or grey hardpan, having taken its 

 place. The brick-clay No. 2 (b), is extensively used for making 

 bricks, which are usually of a cream-colour ; it is laminated, 

 generally horizontally, and contains calcareous concretions and 

 shells of the genus Unio. The light colour of the brick is probably 

 due, not to the absence of iron, but to its union with the silica, lime, 

 and magnesia of the clay, forming silicates of those bases. A 

 similarly coloured brick at Milwaukie indicates the presence of the 

 same alkaline bases in the clay, and, a priori, points to the existence 

 of the Cretaceous in northern Wisconsin and Michigan as their 

 source, and not to the presence of the sea during the deposit of the 

 drift. This clay contains sticks and leaves. It is sometimes in 

 strata that undulate over wide intervals, rising five or six feet in 

 twelve or fourteen rods. It lies on a fine sand, with which its 

 strata are not conformable, the transition from the sand to the clay 

 being abrupt. It also sometimes rests on a layer of coarse gravel 

 and sand with boulders. The latter is the case at St. Paul within 

 the valley of the river. This clay prevails in the old valley of the 

 Mississippi river above the Palls of St. Anthony, and especially in 

 the valley of Bassett's Creek at Minneapolis. 



II. Gorge formed hy the recession of the Falls of St. Anthony. 



The gorge formed by the recession of the Palls extends, with 

 pretty nearly the same width and outward characters, to Port 

 Snelling, a distance of about eight miles, where the river enters a 

 gorge of a very different kind. This is an older river-valley, one 

 which probably witnessed, at some more remote period, the reces- 

 sion of the Palls of St. Anthony past the site of the Port, up the 

 Minnesota valley towards Shakopee. Gen. G. K. Warren was the 

 first to call attention to the remarkable character of the Minnesota 

 valley and its great size — so disproportionate to the comparatively 

 small quantity of water which now descends it *. 



The Minnesota occupies the main valley, the external character 

 of which resembles that of the Mississippi valley below Port Snel- 

 ling, the Mississippi river above the union of the two rivers being 

 merely a subordinate tributary. The Minnesota, it is true, is 

 smaller at the present time than the Mississippi ; but it shows 

 evidence of greater age, and of having flowed in larger volume 

 during some late period of its history. The principal points of 



* ' Eeport of the Chief of Engineers for 1875.' 



