448 



E. K. SOPER 



All four of the formations, each of which constitutes the first 

 rock beneath the drift at different places, originally extended over 

 the entire area of the city and far beyond. Before pre-glacial 

 valleys were carved the rock now topmost was continuous over all 

 others. The amount of this surface erosion has been greatest along 



TABLE I 

 General Geologic Section for Minneapolis 



Era 



System and 

 Series 



Formation 



Character of Strata 



Approximate 

 Alaximum 

 Thickness 



Cenozoic 



Pleistocene 



Glacial drift 



Bowlder clay, sand, 

 gravel clay, allnvium, 

 etc. 



175 feet 



Paleozoic 



Ordovician 



Decorah shale 

 Platteville lime- 

 stone 

 St. Peter sand- 

 stone 



Shakopcc dolo- 

 mite 



New Richmond 

 sandstone 



Oncota dolomite 



Green limey shale 



Blue to gray thin- 

 bedded limestone 



White or yellow fine- 

 grained sandstone 

 with some shale 



Yellow, bulT, or pink do- 

 lomitic limestone 



White sandstone 



Buff to reddish dolomite 



35 

 25-35 



17s 



70 



40 



100 



Cambrian 



Jordan sandstone 



St. LaAvrencc for- 

 mation 



Dresback sand- 

 stone 



Coarse-grained white 

 sandstone 



Dolomite, shale, and a 

 little sandstone 



Fine-grained white 

 sandstone, shaley to- 

 ward base 



80-100 

 200 (?) 

 250 



Proterozoic 



Algonkian 



Red Clastic 

 Series 



Red sandstone and shale 

 and some volcanic 

 rocks 



0-? 



Archean 



Archean 



Granite 



Granite and gneiss 



? 



the courses of the old abandoned river channel and its tributaries. 

 These ancient streams had cut entirely through the Decorah shale, 

 Platteville limestone, and well into the St. Peter sandstone through- 

 out their courses, and, as explained above, the trunk channel was 

 so deeply eroded as to expose the Oneota in its bed along the lower 

 portion of its course through the city. 



