FILLMORE COUNTY. 281 



St. Croix sandstone. 



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most accurately defined, owing to the terrace which marks it. The bound- 

 ary between the St. Peter and Shakopee it is impossible to ascertain cer- 

 tainly, because of the universality of the loam, which acts, in that respect, 

 just the same as a heavy drift deposit, and also because of the persistency 

 of the Shakopee compared to that of the St. Peter. When the friable rock 

 is below a hard and persistent one, as the St. Peter below the Trenton, the 

 boundary between them can be traced out easily by the resulting topo- 

 graphy; but when the soft one is uppermost it wedges out imperceptibly 

 under the loam, or drift, and one can not say when it is all gone. In the 

 western part of the county the lines are all obscured by the prevalence of 

 the drift. The Maquoketa shales have but little exposure in the county. 

 They are visible in the bluffs of the Upper Iowa river, at Lime Springs, 

 about three miles south of the state line, and continue through Fillmore 

 county, in the strike of the Lower Silurian, appearing at Spring Valley. 



The St. Croix sandstone. The area of the St. Croix sandstone is small. 

 It occupies the lower portion of the river bluffs, and the bottom-land in- 

 cluded between them, from the county line, near Rushford, to near Lanes- 

 boro. This bottom-land is sometimes two miles, or more, in width, but it 

 is an alluvial deposit and never reveals the rock. The only rocky outcrops 

 are in the slopes of the bluffs. This sandstone also enters the county in a 

 similar manner, in the valley of the south branch of Root river, and extends 

 about three miles west of the county line. 



Its general litholoyical character is all that can be learned of this rock 

 from its exposures in Fillmore county. The opportunity for examination 

 is very unfavorable. The bluffs, over the interval occupied by it, are almost 

 universally turfed, and a heavy talus rises nearly or quite to the lower level 

 of the St. Lawrence limestone. It is in general a light-colored sandstone, 

 with alternations of limestone, and some shale, in its upper portions. The 

 sandstone layers crumble easily. Some of the beds are of a very coarse 

 grain, but the quartz is generally white, almost transparent. The lime- 

 stone layers are like that of the St. Lawrence, and contain a few fossils, 

 none of which have been studied yet with care sufficient for reliable specific 

 identification. At Whalen about ninety-five feet of the St. Croix sandstone 

 is included in the lower slopes of the bluffs. This thickness of bedding dis- 

 appears below the river level before reaching Lanesboro. At Rushford the 



