338 THE GEOLOGY OF MINNESOTA. 



[St. Peter sandstons. 



counties.* Such outliers are not common, and are generally small. Streams 

 of considerable size usually leap from the Trenton to the Shakopee in 

 very short intervals, the intervening St. Peter sandstone having been washed 

 completely away at an early period. Sometimes, however, streams of small 

 size remain in a bed of St. Peter sandstone; in which case the vally is 

 sandy, covered with small oaks and worth little for agriculture. This is 

 seen in the valley of Bear creek and its branches. 



The surf ace features caused by the presence of this sandstone are inter- 

 esting, and have already been referred to in reports on other counties. As 

 the incoherency of this rock deprives it of the power of resisting erosive 

 forces, it is usually carried away cleanly wherever exposed. The conse- 

 quence is a precipitous descent from the Trenton to the Shakopee. This 

 appears in lines of remarkable, level, bluffs. The hight of these bluffs is 

 usually the thickness of the formation, with fifteen or more feet of lime- 

 stone on the top. These bluffs are especially noticeable around Rochester. 

 To the east their top is reached by a rugged ascent, to the west by a gradual 

 slope of the surface. The erosive forces have left many small and isolated 

 bluffs, which can be properly described under this head, though the lower 

 layers of Trenton limestone assist in their formation. They appear as rug- 

 ged mounds rising from the Shakopee floor, and form a striking feature in 

 the aspect of the neighborhood. They are most abundant in southwest 

 Farmington and in Elmira. A few are seen along the railroad, just east 

 of Rochester. Perhaps the most remarkable is "Sugar-loaf mound," about 

 two miles east of the city and close to the railroad. Its shape and relative 

 proportions are those of a sugar-loaf. Another remarkable one is " Lone 

 mound", of sec. 11, Farmington. It is about three miles north from the 

 line of Trenton bluffs. Two or three miles northwest are two similar 

 mounds, called" Twin mounds". They are in Wabasha county, but have 

 no limestone capping. 



The thickness of the St. Peter was ascertained with an aneroid barom- 

 eter, near Rochester. The upper layers of the Shakopee were found on 

 Bear creek, near the woolen mills. The upper surface of the St. Peter was 

 ascertained as carefully as might be near Whitcomb's quarry, and near 

 Jenkins' quarry. Three comparisons were made. The proper allowance 



*dce reports of Wabasha and Dakota counties; also p. 231, 



