350 HALL AND SARDESON — PALEOZOIC FORMATIONS OV MINNESOTA. 



THE SAINT PETER SANDSTONE. 



Localities. — There are no exposures of this formation in the Minnesota 

 river valley except within two or three miles of the mouth of thai stream 

 and beneath the walls of fort Snelling, where the name was originally 

 given (see ante, page 333) ; along the Mississippi from Minneapolis to 

 Newport, on both sides of the river : along Straight river at and near 

 Faribault and northward from that city in the hanks of Cannon river; 

 at Castle rock. Farmington, Hampton and New Trier in several outliers; 

 near Cannon falls; around Pine island; at Saint Charles and vicinity; 

 in many bluffs along the streams in Houston, Fillmore and Olmsted 

 counties, particularly at Preston and Fountain. 



Structural Characters. — This formation is throughout so extremely 

 friable that it owes its preservation to the protection of the overlying 

 Trenton limestone. As a consequence it plays quite an important part 

 in moulding the topographic features of those counties where it occurs : 

 streams and underground waters erode it with great rapidity. The rock 

 is so friable that blocks will not sustain their own weight in handling, 

 except those taken from the very edge of the exposure, where an infil- 

 trated cement of calcium carbonate hinds the rounded and smooth quartz 

 grains together. In such places considerable use can be made of it for 

 building purposes, bridge construction, etc, as lias been done at fort 

 Snelling. There is considerable diversity in texture, considering the 

 formation as a whole, yet more uniformity is seen here than in the Pots- 

 dam sandstone or in the interbedded sandstones of the Magnesian series. 

 In Olmsted and Fillmore counties the texture is much coarser than in 

 Hennepin and Ramsey counties, as well as more uneven. 



In much of its thickness the bedding of this sandstone is very obscure. 

 Frequently bluffs show many feet where a close inspection is needed to 

 distinguish the lamination. Cross-bedding and slight color alterations 

 are seen. Here and there bright colors are shown in hands and tortuous 

 streaks, as at Minnehaha falls, hut no such strong color contrasts have 

 been noted as are displayed in the sandstones of this formation south- 

 ward in Iowa* Locally, some tendency to a shaly condition appeal's, 

 particularly at Highland park and near south Saint Paul. At the last- 

 named place the lamination is so distinct that, where the layers have 

 been undermined in securing moulding sand, sheets ten feet or more in 

 length can lie split off from the overhanging sandstone roof. The posi- 

 tion of the lamina' here, as everywhere in the state where observed, is 



* W J McGee, Pleistocene History of Northeastern [owa: 11th Ann. Rep. V. S. Geol. Survey, 1892, 

 p. 330. 



