24 o ROLLIN T. CHAMBERLIN 



THE WORK OF THE SUPERIOR GLACIER 



Whatever older drift there may be in this region has been so deeply 

 buried under the thick Late Wisconsin deposits that no certain 

 evidence of it was seen. The oldest glacial deposit thus far found 

 is the red drift of the Lake Superior glacier. The first characteristic 

 of this drift to catch the eye is its unusually red color, due to the 

 presence of much ferric oxide. The unaltered till is very red; the 

 stratified deposits from which the water has removed part of the 

 coloring matter, less so. Often the surface layer has been partially 

 leached of its ferruginous compounds, which have concentrated below, 

 leaving the soil a brownish color. Another distinguishing char- 

 acteristic of this till is that it is prevailingly sandy, and only locally 

 clayey, signifying that the path of the ice advance was over an exten- 

 sive sandstone and crystalline rock area, and that little limestone 

 or shale was encountered. Of like import is the fact that, while 

 there is the lithological heterogeneity characterizing glacial deposits, 

 the pebbles are confined to a group of rocks chiefly of igneous origin, 

 such as diabase, basalt, serpentine, various porphyries, granite and 

 crystalline schists, together with sandstone. Limestone pebbles are 

 conspicuous by their absence. The basalts predominating, the till 

 may be described as a reddish sandy formation containing chiefly 

 dark- colored igneous pebb es and bowlders. The source of this 

 material is the Lake Superior region, the reddish sands coming from 

 the Lake Superior Potsdam and Keweenawan sandstones, which 

 are similarly tinted. Some of the red coloring-matter was also prob- 

 ably derived from decomposed ferruginous igneous rocks. 



The movements and path of this ice-sheet are known from previous 

 investigations to be these: The great ice advance from the Labrador 

 snow-fields in the last Wisconsin epoch separated, west of the Soo, 

 into two principal parts — one developing into the Michigan and 

 Green Bay glaciers, which deepened the pre-existent valleys they 

 entered, and partly formed and partly molded the troughs for the 

 bodies of water which now bear these names; the other, similarly 

 fashioning the Lake Superior basin, pushed on to the southwest, and 

 formed the Superior and Chippewa glaciers. The Superior glacier, 

 emerging from the Duluth finger of the Superior basin, deployed 

 over much of Minnesota, and, spreading on its left flank in a south- 



