454 ^- K- SOPER 



weight. In either case the shrinkage in a mass 200 to 250 feet 

 thick might be suthcient to produce kiw undrained areas on the 

 surface. Where these areas of depression are sliallow and unim- 

 portant, settling couki account for their existence. \Miere tlie sur- 

 face basins are kirge and deep, as in the areas occupied by Lake 

 Callioun. Lake Harriet. Lake Xokomis. and that portion of the 

 ^Mississippi Ri^"er gorge north of the mouth of Bassett's Creek. 

 their origin may be due to the mcomplete lilHng of the pre-gkicial 

 gorge. 



Because of the rekitively even erosion over the flat areas between 

 the pre-glacial valleys, tliere are few prominent peaks or high 

 points of rock beneath tlie drift, and hence few outcrops are to be 

 found in the city. The prmcipal rock exposures are found along 

 the post-glacial valley of the ^Mississippi from St. Anthony Falls to 

 Fort Snelling and along the gorge of ^linnehaha Creek, which is also 

 post-glacial m age, from its mouth to ^linnehalia Falls. Also at a 

 few localities in Xortli ]Mmneapohs. where tlie drift is relatively 

 thin, post-glacial erosion or artiticial excavation has exposed the 

 bed rock. There are also a few exposures near water-level on 

 Xicollet Island and northward along the ri^-er to tlie mouth of 

 Bassett's Creek. 



V. THE BURIED RIVEE CHAXXELS 



The history of the buried river channels of Minneapolis and 

 vicinity, witli an account of the origin and recession of St. Anthony 

 Falls, has been worked out by X. H. Wmchell.' U. S. Grant.- F. W. 

 Sardeson,^ and otliers. The general course of the main pre-glacial 

 channel of tlie river across ^Mmneapolis has been knowm for many 

 vears. but there has been no attempt to determine accurately the 

 boundaries of the river and its tributaries and to construct a topo- 

 graphic map of the old rock surface beneath the drift that would 



^G(oi. Sunxy of Minn., Fifth Aunuiil Report (1S77), 175; also Final Report, II 

 (iSSS), 313. 



' "An Account of a Deserted Gorge of the Mississippi River near Minnehaha 

 Falls," American Geologist, V US90), i. 



3 "The B^inning and Recession of St. Antliony Falls," BuU. GcoL Society of 

 America, XIX UooS\ 29-52. 



