10 ON THE FRESH-WATER GLACIAL DRIFT 



On Lake Superior, the clay beds attain their greatest thickness. Following the 

 shores of Lake Michigan towards the south, another change takes place in color, 

 from bright red to purple, and finally to blue. These changes are not sudden, but 

 pass into each other by degrees, generally in the form of wedge-shaped layers, 

 which taper out each way as represented in the sections at and near Milwaukee, 

 Wisconsin. Where the change takes place on the St. Louis river, the ash-colored 

 portions on the north predominate. Lower down, the red layers gradually prevail 

 until finally the purple and ash disappear. The same occurs on the shores of Lake 

 Michigan, where the red is passing into the blue. On Lakes Erie and Huron the 

 blue is almost universal, at some places inclining to yellow, as may be seen on the 

 St. Clair river. 



There is generally a layer of gravel or sand between the drift clay and the rocks 

 on which it reposes. It is this thin bed of porous materials which furnishes the 

 water to Artesian wells. Water is also found in sandy layers in the clay. The 

 clay bed above and the clay or the rock below being impervious to water, retain 

 that which belongs to the porous layers. 



Where the rock bed is limestone, the gravel interposed between it and the drift 

 clay is principally derived from fragments of the limerock, and gives rise to hard 

 water. Artesian wells have been sunk through the clay around Lake Winnebago 

 and Green Bay, and also at Detroit and Toledo. Sections of the strata passed 

 through will be given hereafter. Everywhere in these clays there are to be found 

 small pebbles of the northern rocks and occasional boulders. The pebbles and 

 boulders are marked, polished, and striated, like the rocky basis on which the drift 

 rests. Over all the space through which the section extends, wherever the rocks 

 are hard enough to retain the glacial markings, they are found to be very distinct. 



The trap formations of Isle Royal Point, Kewenaw, and Marquette, show the 

 efiiects of this universal scouring process more perfectly than granite, sienite, or the 

 Azoic slates. On portions of the conglomerate of the trap system, the marks are 

 weU preserved, especially upon the close-grained trap pebbles and boulders, which 

 enter into the conglomerate layers intercalated with the trap. 



Most of the Potsdam sandstone is too soft and too easily weathered to retain 

 impressions made so many ages ago. Where the glacial movement was parallel, 

 or nearly parallel with the strike of the strata, sandstone beds lying between those 

 of trap as at the east end of Isle Royal, have been carried away to a considerable 

 depth, leaving long narrow promontories and ridges which were better able to resist 

 the grinding process. In this way the contour of the shore and the topography 

 of the country depended upon the hardness of the rocks. On Lake Superior the 

 direction of the arrows of the accompanying map shows that the movement was 

 from northeast to southwest. The trap uplifts of both shores have the same general 

 direction, which also determined the strike of the sedimentary rocks. 



This coincidence has had a powerful influence upon the formation of the basin 

 of the lake, which is partly due to the excavating power of the drift forces. The 

 .basins of all the great North American lakes and many of the smaller ones have 

 been modified in this way since the last disturbance of the strata. Further refer- 

 ence to this branch of the subject will be made in this paper. 



