OKKIIN OV I.AKKS SAINT CIJOIX AND TKIMN . Z.) 



(Iriftless area ami during the long interglacial stage for the more northern 

 valle3'S. Epeirogenic depression in the lowan stage, and the consequent 

 melting of the ice-sheet, continuing in the Wisconsin stage and setting 

 free the ahundant englacial drift, caused these valleys to he deepl}' filled 

 b}' the deposits of the river floods. But soon these de[)osits were in turn 

 deeply eroded by the rivers, especially where the\^ were outlets of glacial 

 lakes; and during the postglacial period they have been partly refilled, 

 forming the lakes through which the Minnesota, Saint Croix, and Missis- 

 sippi rivers flow.* 



Summary of the Geologic History of the Saint Croix Dalles 



1. The earliest geologic events directly concerned in making the grand 

 scenery of the Saint Croix Dalles were the eruption of lavas and their 

 subsequent erosion to form steep ridges. 



2. Against and on these trap formations, submerged under the Cam- 

 brian ocean, were deposited beds of sandstone and shale, which still have 

 nearly their original horizontal position, and which contain shells and 

 impressions of brachiopods and trilobites. The ver}^ valuable early geo- 

 logic explorations and report of David Dale Owen erred in assigning the 

 traj) rocks to eruptions bursting through the Cambrian strata. Instead, 

 the more thorough investigations of the Wisconsin and Minnesota geo- 

 logical surveys have ascertained that the trap rocks are the older. Both 

 belong in the far distant geologic past, variously estimated by Dana, 

 Walcott, and others to be some fifty or more million years old. 



3. Many geologic periods rolled awa}^, until, after having been long a 

 land area, the western three-fourths or perhaps more nearly all of Minne- 

 sota was depressed beneath the sea during the later half of the Cretaceous 

 period. 



4. Through the next ensuing Tertiary era, probably comprising some 

 three to five million years, this region was again a land surface, and has 

 continued so onward through the comparativeh'' short Quaternary era, 

 of probably 200,000 years, to the present time, excepting that during the 

 Glacial period it was covered by the ice-sheet. Two entirely distinct 

 Tertiary rivers drained the present Saint Croix basin. 



5. The obstructions of the ice-sheet during the Buchanan and Wis- 

 consin stages of the Glacial period caused the Tertiary or preglacial 



♦Compare my paper, " The Minnesota valley in the Ice age," Proc. Am. Assoc. Adv. Sci., vol. 32, 

 for the year 1883, pp. 213-231, noting alluvial deposits in a well at Belle Plaine, 150 feet below the 

 present river, and the record of a well at Lake City, Minn., on lake Pepin, which penetrated beds 

 of stratified clay and of gravel and sand, the modified drift swept into the Mississippi valley dur- 

 ing the final recession of the ice-sheet, to the bed-rock about 170 feet below the level of the lake 

 (Minn. Geol. Survey, Thirteenth Annual Report, for 1884, p. 58 ; Final Report, vol. 2, 1888, p. 17). 



