WINONA COUNTY. 249 



Geologic 1 structure.] 



THE GEOLOGICAL STRUCTURE OF WINONA COUNTY. 



The bedded rocks of this county are the same as those of Houston 

 county, given in the last chapter; the thickness of the St. Croix sandstone 

 is a little greater, however, than there, owing to the occurrence of an anti- 

 clinal axis which lifts the strata a little higher above the Mississippi in this 

 county than in that. This broad swell extends between Dresbach and 

 Homer, or, more strictly, between Dresbach and Richmond. On the east 

 side of the Mississippi, where the limestones of the Cambrian are wholly 

 broken down and removed, the country is much lower on this anticlinal, 

 and the lower portion of the valley of Black river is located on it. 



Plate 9 represents Winona county. The colors and the characters are 

 the same as for Houston county. Although the indurated rocks only are 

 represented by colors, it should be remarked that some glacial drift is found 

 in St. Charles and the north part of Saratoga townships, and that the actual 

 surface everywhere consists of the loess-loam. These are not expressed 

 because the underlying rocks throughout the county are so well known 

 that they should take precedence in the coloring of the geological map. 

 This minuteness of knowledge of the rocks of the state gradually gives 

 place to doubt, and finally to a mere general knowledge, in going west 

 from Winona county, on account of the increase of the drift ; and hence in 

 Fillmore, Olmsted and Wabasha counties the drift characters are repre- 

 sented on the county maps, and in some counties still further west the drift 

 only is susceptible of such delineation, 



In the coloration of the Cambrian strata on the county map, the St. 

 Peter and St. Croix sandstones, the former the top and the latter the low- 

 est of the Cambrian within the county, are represented by special colors, 

 while the Shakopee, Jordan and St. Lawrence are all colored together as 

 one. This is because the St. Peter and St. Croix are distinctly set off from 

 the rest by certain natural causes, bringing them into bold stratigraphical 

 recognizance, while the three that are associated under one color are also 

 associated in topographic features so closely, that much uncertainty pre- 

 vails not only as to their individual boundaries, but even as to their indi- 

 vidual existence, in many parts of the county. 



The Trenton rocks. Within the Trenton period are placed the known 



