THE OLD AND NEW VALLEYS 33 



tc 



old." In fact, the walls of the larger valley from 2' miles above the 

 confluence of rivers to 6 miles below it are "young." That which N. H. 

 WinchelP and others have written about the Minnesota and Mississippi 

 course being "old" is true above and below the limits just mentioned. 

 There the glacial drift lies down over the walls of the valleys. 



The old valleys and new valleys referred to here are respectively pre- 

 Glaeial and post-Glacial valleys. Understanding of the distinction be- 

 tween them involves some general knowledge of the geologic formations 

 of this region. Not including alluvial deposits, dune sands, talus, etcet- 

 era, the formations are the following : 



(1) The glacial drift, consisting of loose boulder-clay, sand, and gravel from 

 to 100 feet or more thick, is tlie topmost geologic formation. 



(2) Next is the base of the Galena (Trenton) series, comprising practically 

 horizontal strata with eroded upper surface. From to 80 feet of shale over- 

 lies 35 feet of limestone. For the greater part the shales are absent and the 

 glacial drift rests on the limestone, which consists of two beds, namely, about 

 15 feet of massive limestone on 18 feet of laminated limestone. At the base 

 of the latter the strata are slialy for 2 or 3 feet. 



(3) The next subjacent formation is the Saint Peter sandstone, which Is 

 over 150 feet thick and passes below the rivers' levels. It is friable, easily 

 eroded, and the removal of it by streams causes the several cascades of the 

 region, which plunge over the persisting ledge of limestone. The top of the 

 Saint Peter sandstone and the limestones is practically level, so that the 

 height of any waterfall here is determined by the depth to which the sand- 

 stone is excavated by the stream. The steep sides of the valleys also are 

 generally due to the easily eroded Saint Peter sandstone and the lateral cut- 

 ting by the streams, by which the more firm limestone ledge is brought out, 

 often as a cliff skirted by a talus slope. 



The valleys in the neighboring region about the confluence of the 

 Mississippi and Minnesota rivers may be classified into three kinds: 



A. Old valleys, with cliffs or slopes covered by undisturbed boulder-clay or 

 other glacial drift. Some of the old valleys are quite or entirely buried. 

 Lakes Minnetonka, Calhoun, and others lie in siich drift-covered valleys. 



B. Reexcavated old valleys, which were partly buried by glacial drift, but 

 later are occupied by streams again. 



C. New valleys, which have been made since the glacial deposits were made 

 and whose cliffs are not covered by glacial drift. 



In short, the last, or Wisconsin, glacial drift was deposited or filled 

 over all preexisting valleys in this vicinity, leaving it for later streams to 

 either reexcavate the old valleys or cut new ones. The valley and gorge 

 from Saint Anthony falls to the Junction at Fort Snelling is clearly a 

 new course made by the Glacial and Recent Mississippi. The valley of 



* Geological Siu'vey of Minnesota, Fifth Annual Rppoit, 1S77, p. 170. 



