KOCKS OF SAINT LOUIS RIVER DISTRICT 347 



The Saint Louis river between Cloquet and Duluth has accomplished 

 an enormous amount of erosion. Several high and precipitous knobs 

 are isolated from the surroundino- rock masses through river erosion. 

 Those in the northeast quarter of section 15, township 48, range 16 west, 

 and extending over the adjoining sections are relatively the highest. 

 They disclose interesting phases in the history of the stream. 



The gorge is chiefly pre-Glacial. Lacustrine deposits, quite extensive 

 both in area and thickness, lie along the slopes of the ancient Saint 

 Ijouis valley. They are evidently the shoreline accumulations of lake 

 Duluth * laid down in a valley already cut by pre-Glacial streams. Ero- 

 sion was suspended during the life period of lake Duluth, but was re- 

 sumed when lake Superior began to settle to its present level. 



The descent of the land surface causes the rapids ; the tremendous 

 eroding power given the water by its descent has carved out the gorge 

 called the Dalles of the Saint Louis river. Several diabase dikes cut- 

 ting the slates produce the falls several times met within the length of 

 the "dalles." 



The Keewatin slates and associated schists and quartzites lie below 

 all other known rock formations in this region. This group is of unde- 

 termined thickness, but is estimated at 25,000 feet. Above it are seen 

 in the Saint Louis valley the following series, in ascending order : 



1. The Keweenawan series, consisting of (a) the gabbro in typical 

 development, regarded as the basal number of the Keweenawan ; f (6) a 

 conglomerate bed, 100 feet or more in thickness, and (c) the extensive 

 series of lava flows which is found in many portions of the Lake Supe- 

 rior basin. 



2. The horizontal Cambrian sandstones from red to white in color, 

 known as the Western sandstone. 



3. Pleistocene deposits, largely lacustrine sands and clays, and unmod- 

 ified material. 



EXTERNAL CHARACTERS 



The external characters of the Keewatin of the Saint Louis river may 

 be briefly summarized : In color the rocks are dark. The fine slaty 

 phases are black, and as the quartzite bands appear this color gives 

 place to a gray which varies between black and light or greenish gray. 

 As the rock weathers, a lighter color appears, due to the removal of car- 

 bonaceous material which locally produces the black color. Toward the 

 north the clastic character of the rocks is clearly expressed. There are 



* B. F. Taj^lor : A Short History of the Great Lakes, 1897, p. 10. 



fThe copper-bearing series of lake Superior, Monograph v, U. S. Geol. Survey, 1883, p. 156. 



N. H. Winchell : Geol. and Nat. Hist. Survey of Minnesota, vol. iv, 1899, p. 13. 



