ECOLOGICAL CROSS SP]CTION OF THE MISSLSSIPPL 157 



reasonable to suppose that in earlier days when the volume 

 of water carried by the river was greater^ and the material 

 coarser (the transporting power of a current varies as the 6th 

 power of the velocity)/ and erosion easier (Marbut, p. 95) 



bows 



Under 



this assumption we can interpret the lakes of the American 

 Bottom opposite St. Louis as the results of a single cut-off. 

 A comparison of the St. Louis sheets of the U, S. Geological 

 Survey of November, 1893; and April, 1904, shows the prob- 

 able existence at one time of an ox-bow fully 12 miles across. 

 Thus the most northern part of the cut-off is formed by 

 Horseshoe Lake ; Spring Lake and Grand Marais Lake * form 

 the middle portion, while Cahokia Lake forms the south 



bend. 



surveys 



the two maps (1888-1903), great changes took place, especially 

 as far as the outlines of the lakes are concerned. The 1904 

 map shows that their area has been greatly reduced. Horse- 

 shoe Lake has retreated from its former western boundary; 

 Indian Lake has become a mere swamp, and Spring Lake 

 occupies now but about one-twentieth of its original surface, 

 limited to the most eastern portion, the remainder being 

 swamp. All connection with Pittsburg Lake has been lost. 

 Pittsburg Lake remains practically as it was except 

 that here and there it has retreated and portions of it have 



been 



but 



remams 



mere 



Creek. 



Striking features of the American Bottoms are a number 

 of elevations, known as "Indian Mounds," on which sub- 

 ject it is of interest to quote Worthent at length. 



* Designated Pittsburg Lake in the 1904 map. This is the name 

 now generally given to this lake, the old name being occasionally used by 

 the older of the descendants of French settlers. It may be noted here 

 that a portion of the lake, immediately adjoining the bluff, separated 

 from the main body of water by the bridge of the Illinois and St. Louis 

 railroad and divided into two portions by the bridge of the St. Louis, Alton 

 and Terre Haute railroad, is generally known as Bluff Lake. This name, 

 however, properly belongs to the lake immediately south of Falling Spring. 



* Worthen, A. H. Geol. Surv. III. 1 : 314, 1866. 



