550 F. T. THWAITES 



top. Near the base, particularly in the southwestern quarter of 

 the state, is about 15 feet of micaceous sandy shale; this horizon 

 is very important in wells since under ridges the water is held 

 above it causing a spring line around the sides of bluffs. If wells 

 go through the shale, the water level drops very markedly. Below 

 the shale are a few feet of hard calcareous coarse-grained sandstone, 

 the Ironton member. The thickness of the Franconia varies from 

 90 to 175 feet. 



DRESBACH FORMATION 



Distribution. — The Dresbach sandstone forms the cliffs of the 

 Franconia escarpment in western Wisconsin. It is prominent in 

 the numerous fantastic towers and buttes of the central part of the 

 state. In the valleys within the Lower Magnesian cuesta, it 

 outcrops in cliffs along the streams as far south as near Lone Rock 

 on Wisconsin River. The white cliffs west of Prairie du Sac are 

 Dresbach. 



Character. — The Dresbach consists of medium-grained pure white 

 sandstone with some yellow layers. The cement is mainly silica 

 and varies greatly in amount; locally the formation is quartzitic. 

 The upper and lower contacts are difhcult to make out in some well 

 sections. The writer has excluded from the formation all fine- 

 grained, calcareous, or glauconitic sandstones. In central and 

 northeastern Wisconsin the base cannot be determined, since the 

 only change is in thickness of bedding. Not counting this district, 

 the thickness varies from 40 feet at Union Grove, Wisconsin, to 

 180 feet at Chicago. In consequence of the high porosity and great 

 purity of the formation it yields excellent water. 



The Dresbach sandstone of Illinois was formerly correlated 

 with the Jordan,^ but the writer is convinced that this was an error 

 because of the similarity of the succession of formations in deep 

 wells to that found in outcrops near Sauk City, Wisconsin, and by 

 the fact that the underlying formations are totally unlike any 

 known Trempealeau, Franconia, or Mazomanie. The white 

 sandstone formation has been followed through in well sections 



I C. B. Anderson, "The Artesian Waters of Northeastern Illinois," Illinois State 

 Geol. Survey Bull. 34, (1919), pp. 84, 107, PI. II. 



