274 THE GEOLOGY OF MINNESOTA. 



I Topography. 



Trenton limestone. 



St. Peter sandstone. 



Shakopee limestone. 



Jordan sandstone. 



St. Lawrence limestone. 



St. Croix sandstone. 



The limestones form the prominent features in the topography. They 

 have the most frequent outcrops. They project along the summits of the 

 bluffs and constitute the brows of benches or terraces that diversify the 

 county. The sandstones never, or very seldom, appear in the tops of the bluffs. 

 They outcrop in sheltered nooks, or below the line of the limestone exposure. 

 They are more likely to be hid by soil and turf. The Lower Trenton is 

 overlain by a layer of about twenty feet of easily eroded green shale, 

 which, outcropping by roadsides, introduces a series of springs and muddy 

 spots, being impervious to water, that invariably follows that boundary line 

 wherever it goes. It withstands the disintegrating action of the elements 

 even more successfully than the limestones themselves. For that reason 

 it protects the Trenton which lies below it, long after the Galena limestone 

 which lies above it has been entirely denuded. 



(Jrc/tfun&rrace) 



FIG. 15. SHOWING THK EFFECT OF THE TUENTON AND GEEEN SHALES ON THE TOPOGRAPHY. 



