890 N. H. WINCHELL ON TEE RECESSION 



difference between the Mississippi valley above Fort Snelling aud 

 the greater valley which it enters at that place are as follows : — 

 The gorge above the fort is about a quarter of a mile wide ; below 

 the fort it is a mile wide, the same width continuing up the Min- 

 nesota above the fort. The walls of the gorge above the fort have 

 the appearance of having been freshly broken, the rock lying in 

 uncovered fragments in a talus at their base ; the older valley, on 

 the other hand, is flanked by bluffs that are rounded off, the frag- 

 ments being hidden by a loam, or even by drift gravel, so that they 

 are turfed over or even wooded from the water to the top. The 

 limestone in the bluffs above the fort is visible without interruption 

 from the fort to the Falls ; in the older valley below the fort it is 

 only interruptedly exposed, and is cut out and broken down by 

 other small tributary streams, especially at St. Paul (such valleys 

 now being filled with drift), and above the fort the outcrop of the 

 Trenton is soon lost sight of under a thick covering of drift. 



There is a perpendicular section of the drift running along the 

 top of the limestone in the Mississippi valley, above the fort, as if 

 the drift had fallen when the rock that supported it gave way. 

 This drift section abuts immediately upon the river, and forms a 

 part of the high bluffs that enclose it ; in the old valley which the 

 Mississippi joins, the drift has been deposited ivithin the rock bluffs 

 and hides them, and there is no natural perpendicular section of 

 drift materials running along the tops of the bluffs. The direction 

 of the Mississippi above the fort is towards the south-east; but 

 after entering the old valley it turns at a right angle and runs 

 north-eastwards, that being also the direction of the Minnesota 

 above the fort. 



Near Fort Snelling, but below the union of the two rivers, lies a 

 low, long, alluvial island (Pike Island), which points to the former 

 rapid accumulation of debris at that point in the river-gorge. It is 

 just opposite the point of debouchure of the Mississippi into the old 

 valley, and must have been formed since the excavation of the 

 gorge. On the opposite side of the old valley, from the point of 

 entrance of the Mississippi, the rock-bluff also presents the ex- 

 ceptional character of being freshly broken down by rapid under- 

 mining erosion by the river. It seems as if, at some time later 

 than the covering of the bluffs by the loam, the current had been 

 driven violently and unusually against that bluff, since, both above 

 and below, the bluffs are not thus freshly cut. 



There is one other point in connexion with the description of this 

 gorge to which it is necessary to direct attention. The foregoing 

 facts are alone sufficient to suggest to the reflective observer some 

 difference in the age of these two portions of the great valley. 

 When, however, it is found that above the Falls of St. Anthony, 

 but within the corporate limits of Minneapolis, the rock-bluffs 

 which so closely confine the river below the falls within the width 

 of a quarter of a mile are suddenly diverted from the river, running 

 inland about a mile apart, covered with loam and even with drift 

 like the bluffs below the fort, it becomes evident that here the 



