Paleozoic 



GEOLOGY OF ROCK RIVER BASIN. 11 



GEOLOGY OF WISCONSIN AND OF THE UPPER DRAINAGE BASIN 

 OF THE ROCK RIVER. 



The geology of this region has a direct bearing upon the water now, 

 and should be briefly considered before discussing the other factors 

 connected with this problem. 



The following epochs are represented in the geologic formations of 

 Wisconsin: 



Pre-Cambrian An unseparated complex of crystalline rock. 



Cambrian Potsdam sandstone. 



'Lower magnesian limestone. 



St. Peter sandstone. 

 Lower Silurian. J Trenton limestone. 



Galena limestone. 



Cincinnati shales and limestone. 



Upper Silurian Niagara limestone. 



Devonian Hamilton limestone. 



The pre-Cambrian is represented by intricately folded ancient 

 crystalline rocks, forming the base upon which the later rocks, belong- 

 ing to the Paleozoic, rest unconformably and comparatively undis- 

 turbed from the base upward in the order named. A subsequent 

 slight tilting of the entire series toward the south, followed by a cutting 

 away of the upper surfaces, has resulted in the exposure in successive 

 zones of the various kinds of rock represented. The oldest is found 

 in the northern part of the State; the more recent deposits to the 

 southeast, south, and southwest. They have since been covered in 

 great part by the drift and morainic debris of glacial times. 



The alternation of layers of rock of different composition and the 

 variations of structure found within some of these separate formations 

 facilitate the drainage. The geological dip also contributes to this 

 result. It slopes to the southeast and, on the average, is not over 5 

 feet to the mile; } T etthis is sufficient to convey some of the percolating 

 waters from the north down to the southeastern part of the State, where 

 they reappear as springs to feed the glacial lakes of this region. 



The principal source of the water supply within the Rock River 

 drainage basin is the precipitation, and the economy of its distribu- 

 tion depends largely upon the character of the surface upon which it 

 falls. The topography, as already noted, presents no prominent 

 features. The soil conditions vary on different parts of the water- 

 shed, according to the exposures of the different layers of rock. In 

 the west and northwest the headwaters of the river spread over 

 the Potsdam sandstone, the lower magnesian, St. Peter sandstone, 

 and Trenton limestone, and over the glacial drift that has covered 

 these deposits. The main part of the drainage, however, lies over the 

 area of the Galena and Niagara limestones and the Cincinnati shales. 



