OLMSTEL) COUNTY. 333 



Geological structure. 



ceases rather suddenly in Olmsted county. The streams and all ravines 

 rise, in the western part of this county, to near the surface of the surround- 

 ing country, and flow upon the drift-sheet which grows deeper and deeper 

 as one passes further westward. Tliis material is rather thin in Olmsted 

 county, except in the southwest corner where it is thick enough to conceal 

 the rock features entirely. Eastward it appears only in thin outlines, mark- 

 ing the ragged edge of deposition, or in patches and masses which are rem- 

 nants left by subaqueous erosion. In order to see to the best advantage, 

 the changes either in the drift, the features of erosion, or the stratification 

 of the rocks, one must cross the county obliquely. The drift is lightest, 

 generally speaking, in the northeastern corner, and thickest in the south- 

 western corner. On the other hand the southeastern and northwestern cor- 

 ners are much alike in the very features in which the other two corners 

 differ. In a rough way the lines of change cross the county diagonally in 

 a southeasterly and northwesterly direction. This is due to two facts 

 which may have some relation to each other. In the first place the great 

 river in the vicinity of the county runs in a generally southeast direction. 

 The erosion-valleys extending from it would tend to take a direction per- 

 pendicular to it, and the lines of equal depths of erosion would tend to be 

 parallel to it; again the dip of the rocks of this county is slight toward the 

 southwest; hence the edges of the strata as presented on the surface would 

 tend to be in lines perpendicular to this direction. 



There are no signs of noteworthy upheaval, depression or other changes, 

 in the relations of the strata to each other in this county, as in the whole 

 of this part of the state the strata are in general conformable.* The pecu- 

 liar structure of the bluffs enables one to trace some of the strata at a dis- 

 tance. As far as the eye can follow them their planes occupy the same 

 position with reference to the horizon. 



The strata do not lie in a horizontal plane, but they dip slightly to- 

 ward the southwest perhaps at the rate of ten feet to the mile. 



The stratigraphy of this fine county is easy to read in most cases. The 

 form of the bluffs, the line of springs marking a definite point in the rocks 

 of the Trenton period, the varying solubility of the rock and the conse- 

 quent occurrence of sink-holes and caves in/>neTormation and not in an- 



*See, however, the report on Winona county, p. 250. 



