PALEOZOIC ROCKS FOUND IN DEEP WELLS 553 



In the region around Milwaukee the greatest difficulty was 

 experienced in correlating the "red marl zone" as it is known to 

 well drillers. The gray shales are almost entirely lacking in that 

 district and the red layers are so erratic in both vertical and hori- 

 zontal distribution that at first the problem seemed hopeless. 

 The red marl at Milwaukee was formerly believed to be part of the 

 Lower Magnesian.^ However, by means of sections parallel to 

 the lake shore it was determined that the red marls pass below 

 the Dresbach sandstone of Illinois, and grade laterally into the 

 gray and red marls of the Chicago district. This conclusion was 

 verified by fossils which were shot out of a well at Waukesha and 

 determined by Dr. Ulrich. 



The thickness of the Eau Claire, as it can be distinguished in 

 well records, varies from 70 feet at Rockford, Illinois, to 410 feet 

 at St. Charles, Illinois; the average is not far from 350 feet. 



MT. SIMON FORMATION 



Distribution. — Outcrops of the Mt. Simon sandstone have thus 

 far been studied only near Eau Claire, Wisconsin, its type locality. 

 At that place it forms an escarpment which is capped by Eau Claire 

 shaly sandstone. The Mt. Simon must be the basal member of the 

 Cambrian over a wide area of drift-covered country in central 

 Wisconsin. 



Character. — The upper limit of the Mt. Simon sandstone is 

 difficult to determine on account of the variable character of the 

 overlying Eau Claire. In central Wisconsin the formation is 

 mainly coarse to medium grained, gray or yellow sandstone with 

 a few layers of green, blue, and red shale. Farther south less of the 

 fonhation is coarse grained; locally there are pink layers, the color 

 being deepest in the finer-grained sands. The record (Table VIII) 

 is a fair example of the Mt. Simon rather close to the outcrop where 

 there are more beds of coarse grain than farther south. The greatest 

 recorded thickness is 778 feet at Platteville, Wisconsin. 



I W. C. Alden, " Geol. Atlas of U.S.," Milwaukee Folio No. 140, (1906), p. i; "The 

 Quaternary Geology of Southeastern Wisconsin," U.S. Geol. Survey Prof. Paper 106, 

 (1918), pp. 78-82. Samuel Weidman and A. R. Schultz, "The Underground and 

 Surface Water Supplies of Wisconsin," Wisconsin Geol. atid Nat. Hist. Survey Bull. 35, 

 (1915), p. 457- 



