PRE-GLACIAL SAINT CROIX VALLEY 317 



neath the present famed Chisago lakes, until it emptied into the present 

 Saint Croix valley, in section 7, township 32, range 19 west. This old 

 Saint Croix river channel has been traced and its general course estab- 

 lished through contour features and well-borings.* 



ABSENCE OF LAKES 



Another character of this district is the notable absence of lakes, inas- 

 much as the region on all sides stretches off into a veritable lake park. 

 This physiographic feature, like that of the stream courses, is due to 

 conditions dating from Glacial time. 



To the north and west of the great Kettle moraine f is a wide belt of 

 country extremely monotonous in its physical features. Swamps con- 

 stitute a conspicuous feature. The friable Cambrian sandstones and 

 the more varied elastics of the Upper Keweenawan offered slight yet 

 equal resistance to glacial erosion. 



Relation to the associated Formations 



the underlying formations 



Nowhere in the eastern Minnesota area of the Keweenawan is its rela- 

 tion to the underlying formations seen ; yet from observations along 

 the Saint Louis river between Thomson, Minnesota, and Duluth,! along 

 the Pigeon river, and around Grand Portage bay, § and along the north 

 side of the Penokee range in Wisconsin, || it is clear that the Keweenawan 

 is nonconformable on the earlier rocks. This nonconformity seems 

 locally to be an eruptive one — that is, the separation is the intrusion of 

 the later formation in the form of dikes and sheets thrust into the under- 

 lying strata or in extensive beds lying on the sedimentaries, and in other 

 places the contacts of the older rocks and Keweenawan show a noncon- 

 formity by erosion between them. McKellar, Irving, and Merriam have 

 pointed this out most emphatically for northeastern Minnesota and the 

 Thunder Bay district.^ 



THE OVERLYING CAMBRIAN SANDSTONES 



Along the Saint Croix river, both in Minnesota and Wisconsin, the 



* A. H. Elftman : The Saint Croix River valley, Amer. Geologist, vol. xxii, pp. 58-61. 



t T. C. Chamberlin : Geology of Wisconsin, vol. i, 1883, pp. 275-287. 



X N. H. Winchell : Final Report of Geological and Natural History Survey of Minnesota, vol. iv. 

 pi. B, and pp. 13, 570. 



§ W. S. Bayley : Rocks on Pigeon point, Minnesota, Bulletin 109, U. S. Geol. Survey, 1893, p. 23. 



I Irving and Van Hise : Penokee iron-bearing series of Michigan and Wisconsin, Monograph 

 xix, U. S. Geol. Survey, 1892, p. 470. 



^P. McKellar: Correlation of the Animikie and Huronian rocks of lake Superior, Proceedings 

 and Transactions of the Royal Society of Canada, vol. v, 1887. R. D. Irving : U. S. Geol, Survey, 

 Seventh Ann. Rept. Director, 1888, p. 387. 



