OLDER DRIFTS IN THE ST. CROIX REGION^ 



ROLLIN T. CHAMBERLIN 

 The University of Chicago 



Some of the glacial features of the St. Croix-Dalles quadrangle 

 were described in a previous number of this Journal.^ That paper 

 was based upon field studies during the summer of 1904. Up to 

 that time no certain evidence of any drift-sheet older than the Wiscon- 

 sin stage of glaciation had been recognized, and in consequence that 

 paper confined itself to a discussion of the two drifts of Wisconisn 

 age which occur as surface formations within the quadrangle. But 

 with the aid of a class of students from the University of Chicago, 

 during the field season of 1905, the presence of a considerable amount 

 of drift distinctly older than the Wisconsin red drift was clearly 

 recognized at a number of points along the St. Croix River. Two 

 years later a study of the deep ravines which dissect the river bluffs 

 opposite Osceola, Wis., resulted in finding that there are two very 

 distinct drift-sheets in this region belonging to earlier glacial epochs. 

 The earlier of these is a sheet of grayish-black drift brought in from 

 the northwest by a Keewatin glacier. At some later date a sheet of 

 pinkish-red or reddish-brown drift was spread over the region by a 

 glacier from the Labrador center coming by way of Lake Superior. 



THE GRAYISH-BLACK DRIFT 



The deep ravines opposite Osceola which head back from the 

 river to the Minnesota uplands afford the best opportunities to study 

 the buried Pleistocene deposits of the region. At the bottom of the 

 big ravine in the south part of Section 16, Franconia (about lod yards 

 east of the wagon road), is a gully-bank exposing ten feet of very 

 dark grayish to bluish-black till. This dark drift is highly calcareous 

 and contains relatively few pebbles and these mostly small. It is 

 largely a rock flour derived by glacial grinding from limestone and 



1 Published by permission of the Director of the U.S. Geological Survey. 



2 R. T. Chamberlin, "The Glacial Features of the St. Croix-Dalles Region," 

 Jour, of Ceol., XIII (1905), 238-56. 



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