316 C. W. HALL — KEWEENAWAN AREA OF EASTERN MINNESOTA 



which seem so superior to the amount now carried by the Saint Croix river. Chief 

 among these phenomena are the enormous potholes worn in these rocks (diabases) 

 at Saint Croix falls and Taylors falls." * 



A PRE-GLACIAL SAINT CROIX VALLEY 



It seems that during the later Tertiary epochs the Upper Saint Croix 

 valley was drained by a system of rivers somewhat different in direction 

 from the existing streams and probably of quite different volume. 



A glance at the topographic features of the map. will show that the 

 whole district was leveled to a remarkable degree before the Glacial 

 epoch. In all the views bearing on the effectiveness of glacial erosion 

 the admission is made that the gross topography of a region is only 

 moderately changed ; hence it is assumed that the broader topographic 

 features now existing were imposed preceding the Glacial epoch by the 

 usual processes of weathering and denudation. 



Between the present I^ake Superior basin and the Dalles of the Saint 

 Croix is a notable Keweenawan syncline, stretching from the south 

 shore of lake Superior southwestward for 150 miles, with a maximum 

 breadth of not less than 75 miles, which probably drained southwestward 

 into the Mississippi river at some point between Anoka and Saint Paul.f 



Between the present drainage valleys of the Saint Croix apd Missis- 

 sippi to the southwest of Sunrise creek is a broad and nearly level sand 

 plain, scarcely more than 900 feet above tide, while the Saint Croix at 

 present runs in a narrow valley past the mouth of Sunrise creek at 760 

 feet above tide. These figures show, as Upham points out, a difference 

 in elevation of not more than 150 feet. 



The elevated ridge which forms the so-called " Copper range " of 

 northwestern Wisconsin is a natural divide to the north and northwest 

 between the waters of the Saint Croix valley and those flowing north- 

 ward, while the continuous ridge of diabasic rock stretching from Ke- 

 weenaw point to the Dalles of the Saint Croix forms an equally natural 

 divide to the southeast. The region included between these bounds, 

 formed by the ujiturned edges of the Lower Keweenawan eruptives 

 and reaching to the Mississippi river, com])rises a well defined drainage 

 area of some 8,000 to 10,000 square miles, whose limits were readjusted 

 at the time of the glacial invasions. 



Doctor A. H. Elftman, who, as well as Mr Upham, has given some 

 attention to the Saint Croix valley, aims to show that the Saint Croix 

 river in pre-Glacial time flowed in a nearly straight course south from 

 the mouth of Sunrise creek, section 32, township 36, range 20 west, be- 



*C. p. Berkey : Geology of the Saint Croix Dalles. Amer. Geologist, vol. xx. p. 3G7. 

 t Warren Upham : Pleistocene ice and river erosion in the Saint Croix valley of Minnesota and 

 Wisconsin, this volume, pp. 16-19. 



