250 THE GEOLOGY OF MINNESOTA. 



[Trenton rocks. 



Trenton limestone and the shales and shaly limestone which overlie it in 

 western St. Charles and Saratoga, reaching an aggregate thickness of per- 

 haps seventy-five feet. This thickness, however, is estimated. It may be 

 partly made up of glacial clay and loess-loam, which begin to combine in 

 that part of the county in rendering the geology more uncertain. Within 

 the thirteen-hundred foot contour-line there is something, in St. Charles 

 and Saratoga townships, above the Trenton, amounting to sixty or seventy- 

 five feet. It is probably partly made up of Hudson River shales, but it is 

 superficially composed of loam, with an occasional appearance of a little 

 drift. The best exposures of the shales and shaly limestones referred to are 

 found in the high bluffs in the southern part of St. Charles. The greatest 

 observed thickness is not more than twenty-five feet. They may be seen, 

 S. W. J sec. 29, by the side of the road, where they consist of alternate thin 

 beds of limestone and green shales with numerous fossils. The same, or 

 similar, beds are found at W. H. Shelton's, N. W.J sec. 6, Fremont, whose 

 well and cistern are partly excavated in them, revealing numerous fossils. 



The most interesting observation made in Winona county on the rocks 

 of the Trenton period, was in section 29, St. Charles, where there is an 

 apparent unconformity between the Trenton and the underlying St. Peter 

 sandstone. The St. Peter, as exposed, dips about six degrees south-south- 

 west. It is separated from the Trenton by four to ten feet of green shale, 

 which seems to vary, and to lie in a depression in the upper surface of the 

 sandrock. The overlying Trenton is about horizontal. Yet at a point about 

 two miles further north, where the St. Peter rises at least fifty feet higher, 

 the Trenton is still present on the top of it in a thin scalp. 



The area occupied by the Trenton rocks in Winona county is small, 

 but nearly all the peculiar features of topography produced by them, as 

 mentioned in the reports on Houston and Fillmore counties, are well exem- 

 plified. It invariably produces a rather abrupt ascent in the contour and 

 general level of the country, amounting to about a hundred feet, and it is 

 hence distinguishable by the observer for many miles. This plateau-like 

 elevation is not dry, as might be expected, but the shaly character of the 

 rocks, together with some inequalities of surface prior to the deposit of the 

 loam, serve to retain the surface waters as in tight basins, only allowing 

 them to escape slowly in springs about the border of the plateau, by perco- 



