14 W. UPHAM — ICE AND RIVER EROSION IN SAINT CROIX VALLEY 



Berkey * and Mr A. H. Elftman,t who each made extensive field studies 

 of this district. They differ from me, however, in referring the erosion 

 of the new part of the valley to late Glacial and postglacial time, in- 

 stead of which it seems to me to belong to an interglacial stage or epoch. 

 Probably its date was the same as that of the erosion of large water- 

 courses in the drift-sheet of Martin county, on the southern border of 

 Minnesota, which, having become in part and irreguhirly filled by later 

 drift, are marked by three ver}^ remarkable chains of lakes.J 



Subsequently I have again several times examined the portions of this 

 valley next below and above the Dalles, confirming the view that the 

 new course of the river and much of its gorge erosion are referable to a 

 long interglacial stage which in the Mississippi basin followed the Kan- 

 san stage of maximum glaciation. 



During that time of extensive recession of the border of the ice-sheet, 

 the Saint Croix river appears to have channeled this part of its valley 

 and its gorges in the Dalles, thereby uniting two hydrographic basins, 

 which previous to the Ice age had been each separately tributar}^ to the 

 Mississippi. After the readvancing ice-sheet had again reached south- 

 ward nearly to its previous limit west of the Mississippi, and even be- 

 yond it in Illinois and eastward, this river during the final departure of 

 the ice, in the closing Wisconsin stage of the Glacial period, carried for 

 some time a much greater volume of water than now, being temporarily 

 the outlet of a great lake dammed by the barrier of the northeastwardly 

 receding ice border in the western part of the Lake Superior basin. The 

 beds of the Saint Croix and Mississippi, southward from the Dalles, were 

 then channeled considerably below their present depths, and by later 

 partial refilling with sand and gravel they have come to be occupied in 

 the unfilled portions by lake Saint Croix and lake Pepin. 



The Saint Croix River and Basin 



Measured along the general course of the valley, and without includ- 

 ing the minor bends and meandering of the river, its length from source 

 to mouth is about 150 miles. At first it runs south westward about 75 

 miles; then east and southeast about 25 miles, to Taylors Falls, and, 

 lastly, south about 50 miles. In the southern half of the distance last 

 noted it is expanded to the width of a half mile to one mile, forming 



* *' Geology of the Saint Croix Dalles," part I, Am. Geologist, vol. 20, pp. 345-383, with maps and 

 sections, Dec, 1897. This paper is continued by parts II and III, on the mineralogy and paleon- 

 tology of the rock formations, in vol. 21, pp. 130-1.35 and 270-204, with maps and plates, March and 

 May, 1898. 



t " The Saint Croix River valley," Am. Geologist, vol. 22, pp. 58-61, July, 1898. 



t Geology of Minnesota, vol. 1, 1884, pp. 479-485, with map at page 472. 



