FILLMORE COUNTY. 307 



Devonian limestones.] 



brachiopod, resembling that of Atn/int reticuluris. These were regarded 

 with great curiosity by many as "little turtles" petrified. 



The outcrops of rock about Spring Valley have unusual geological in- 

 terest, since they seem to demonstrate the entire absence of the Upper 

 Silurian, and the immediate superposition of the foregoing limestones of the 

 Devonian upon the Lower Silurian. The exposure of rock about one mile 

 east of Spring Valley by the railroad grade, having Hudson River fossils, is 

 overlain by four feet of a carious, evenly-bedded, argillo-magnesian lime- 

 stone, like the argillo-magnesian limestones seen at Spring Valley on the 

 north side of the creek at about the same actual elevation, where they have 

 been somewhat quarried but abandoned as unfit for masonry. They can 

 be seen by the topography, and by occasional outcrops, to extend from 

 one place to the other. At the latter place they exhibit fossils that indi- 

 cate their Lower Silurian age, such as Strophomena alternata and fluctuosa, 

 and another strophomenoid brachiopod. They probably belong to the 

 upper portion of the Lower Silurian, and perhaps represent, with the under- 

 lying shaly beds at the railroad cut. the Maquoketa shales of Iowa. They 

 have an exposed thickness of about fifteen feet. Directly across the creek, 

 on the south side, are the nearly horizontal beds of magnesian limestone, 

 containing the coarse casts of Atrypa reticularis, which is supposed to 

 belong to the lower portion of the Devonian limestones, having such a topo- 

 graphic relation to the foregoing that no considerable thickness of beds can 

 intervene between them. The strata are all nearly horizontal, the dip 

 being so gentle toward the southwest that it can not be observed in the 

 short lines exposed. 



N. W. | sec, 16, Jordan. In ascending the south bluff of Lost creek 

 large loose pieces of Devonian limestone are seen in the road, but the 

 Galena is in outcrop at the creek. Similar pieces appear on sec. 31, Jordan. 

 These are on the most eastern limits of the Devonian area, and belong to 

 the lowest layers of the formation. 



The Cretaceous. No attempt is made to map out the Cretaceous area in 

 Fillmore county, inasmuch as it is all embraced in the drift-covered portion, 

 and but one or two localities of its existence are known. It pi-obably 

 extends no further east, however, at any point than the east side of the first 

 tier of towns along the western border of the county. Its area is most 



