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THE GEOLOGY OF MINNESOTA. 



[St. Croix Sandstone. 



The fossils that have been gathered from this formation consist very 

 largely of trilobite remains. They were obtained from the quarry of Mr. 

 Whitman at Hokah. 



On section 11, Union township, the sandstone which has been 

 mentioned as having a nearly constant line of exposure, is sculptured, 

 along the north bluffs, into isolated columns and tables, with some rounded 

 buttresses which present a very conspicuous and highly interesting in- 

 stance of atmospheric erosion. There can be no doubt that the bluffs 

 themselves are the result of the erosion of the valley by water by a 

 process that began thousands of years before the glacial epoch, but the 

 present condition jof most of the curious forms, like that of the "sculp- 

 tured bluffs," is certainly due to the effect of wind in conjunction with 

 moisture and frost. There are also cavities and sheltered nooks, and deep, 

 crooked passages and sharp niches, in which the wind could barely enter, 

 and from which there could not have been any wind exit sufficient to have 

 maintained a current capable of producing the most of this sculpture, which, 



moreover, are lichen-covered, and bear an aspect of age and roughness that 

 forbids their reference to any present atmospheric forces. They can be 

 explained only by the solvent action of water in agitation, and are compar- 

 able to the purgatories that are often seen about the rocky shores of lakes 

 or of the ocean. But where the rock shows a recent, fresh erosion, and is 

 soft and crumbling, the present forms are due to more recent causes, and 

 can only be assigned to wind and frost. Table Rock, represented in figure 5 



