HOUSTON COUNTY. 219 



St. Peter sandstone.] 



but also a belt extending in width from the foot of that slope over the 

 more level country surrounding, so that its irregular area is often a mile or 

 two in width. As already remarked, while its upper limit has a very easily 

 recognized location, by reason of the terrace-like topography of the Tren- 

 ton, its lower horizon is often very uncertain on account of the very easy 

 and gradual destruction of its layers, and the prevalence of the loess-loam. 



The characters of the St. Peter sandstone are pretty well known to 

 geologists. It spreads into Iowa, Wisconsin and Illinois. Toward the east, in 

 northern Wisconsin, Prof. T. C. Chamberlin has traced it to the Michigan 

 state boundary, though there it is reduced to a thickness of no more than 

 twenty feet.* It contains but the merest traces of fossil remains. It 

 consists of nearly pure silica, in rounded grains, with so little cement that 

 the rock can generally be crumbled in the hand. It is nearly white; and 

 the soils which are situated near its line of outcrop are apt to be loose 

 and arenaceous from its disintegration. 



It was noticed, however, that for some reason it is more frequently 

 hardened by iron, or lime and iron, in Houston county, into a firm rock, 

 which causes it to sustain a weathered exposure without crumbling rapidly 

 away, than in counties further north or west where the northern drift pre- 

 vails. This, however, is purely an accidental and surface quality, the in- 

 terior of the formation being about the same as at other places. The 

 cement which it possesses in Houston county, in its exposed portions, in ex- 

 cess of the same at other points, is no doubt due to the water by which it 

 has been submerged and stained during the deposition of the loess-loom. 



The thickness of the St. Peter sandstone was very satisfactorily ascer- 

 tained on S. W. ^ sec. 17, Wilmington. The well of Mr. 0. A. Bye is situ- 

 ated near the Trenton bluff, and by uniting the known depth drilled in the 

 sandstone with aneroid measurement of the bluff, the St. Peter was found 

 to be between seventy-five and eighty feet thick, the Shakopee below having 

 a thickness of sixty-four feet. 



The Shakopee limestone. The continuity of this formation from the 

 Minnesota valley to the Mississippi, and its idenity with the limestone at 

 Shakopee, where it was first recognized as a distinct member of the Cam- 

 brian in Minnesota, was fully established in the survey of Houston county. 



Geology of Wisconsin, Vol. II, p. 289. 



