PAPERS ON GEOGRAPHY AND GEOLOGY 189 



Upper Ordovician: Maquoketa shale. 



Middle Ordovician: Galena dolomite. 



Platteville limestone. 

 Grlenwood sandstone. 

 St. Peter sandstone. 



Lower Ordovician: Sliakopee dolomite. 



The oldest rock known by actual observation to under- 

 lie this part of the state is the upper Cambrian, or Pots- 

 dam, sandstone. It does not appear at the surface but 

 has been encountered in a few of the deeper city wells. 



Above the Potsdam sandstone is the Prairie du Chien 

 series, which is commonly divided in this general region 

 into three formations — Oneota limestone at the base, New 

 Richmond sandstone, and the Sliakopee dolomite at the 

 top. The lower two formations do not crop out in this 

 quadrangle. The Sliakopee is exposed in a single re- 

 stricted area, on the crest of a sharp plunging anticline. 

 Its most striking characteristic at this place is the num- 

 ber of sudden changes in lithology in the vertical succes- 

 sion of the beds. This can be illustrated by listing the 

 principal lithologic types in the exposed 13 feet of the sec- 

 tion. They are 1) dense, dull, platy limestone, 2) finely 

 to coarsely crystalline limestone, 3) fucoidal limestone 

 with poorly preserved fossils, 4) magnesian limestone, 5) 

 cherty dolomite, 6) limestone or dolomite with well 

 rounded grains of quartz scattered through it or segra- 

 gated in irregular patches, 7) calcareous sandstone com- 

 posed largely of the same type of sand, and 8) thin, va- 

 riegated shales. 



The well-known St. Peter sandstone is extensively ex- 

 posed along the crests of folds in the vicinity of Oregon, 

 and a few small outcrops are present elsewhere. It has 

 commonly the same lithologic characteristics that are 

 typical of it throughout the general region, but differs in 

 some details, as, for example, the local concentration of 

 iron. 



Almost everywhere in this quadrangle the St. Peter is 

 overlain by the Glenwood sandstone. This is a bluish- 

 green rock composed in the main of fine, angular quartz 

 sand through which is scattered a variable percentage of 

 typical St. Peter sand grains. The persistent color and 



