180 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



In the Owl Creek and Wind River mountains of central and western Wyoming 

 the Bighorn dolomite occurs with much of the character which it has in the type 

 locality of the northern part of the Bighorn Range. 



K-L 15. MINNESOTA, NORTHEASTERN IOWA, SOUTHWESTERN WISCONSIN, AND NORTH- 

 WESTERN ILLINOIS. 



The Ordovician exposed on both sides of Mississippi River comprises the fol- 

 lowing formations, in descending order: 



Maquoketa shale. 



Galena dolomite. 



Decorah shale ("Green shales'' of Minnesota reports). 



Platteville limestone. 



St. Peter sandstone. 



Prairie du Chien group (of Beekmantown or Lower Ordovician age): 



Shakopee dolomite. 



New Richmond sandstone. 



Oneota dolomite. 



The Prairie du Chien group is discussed in Chapter III (pp. 119-120). The 

 classification of the Ordovician into the formations enumerated above is based on the 

 work of Calvin"^ and of Grant, Burchard, and Ulrich.^*^ In the f oho by the three 

 authors last cited the middle and upper Ordovician rocks are described in detail 

 and hsts of fossils are given. The following is an abstract of their description: 



Unconformably upon the Prairie du Chien formation hes a comparatively thin sandstone 

 long known as the St. Peter from the fact that it occurs along the lower course of St. Peters 

 (now known as Minnesota) River. The thickness of this sandstone varies considerably, aver- 

 aging in this district perhaps 70 feet but ranging from 35 to 175 feet. * * * The variation 

 in the thickness of the St. Peter sandstone is due, at least in the main, to irregularities in the 

 surface of the underlying formation — the Prairie, du Chien — which suffered considerable erosion 

 before the sandstone was deposited. The top of the sandstone likewise may have been somewhat 

 eroded prior to the deposition of the succeeding Platteville limestone, although this is not 

 evident from discordance of stratification. 



* * * Nearly everywhere at the base of the formation there is a foot or two of bluish- 

 green sandy clay shale. Above this basal shale the St. Peter is practically pure quartz sand- 

 stone, the percentage of sihca in its composition being very high and at some places exceeding 

 99 per cent. * * * 



The grains are well waterwom and of medium fineness, the greater part passing a sieve 

 having 40 meshes to 1 inch. 



The transition from the St. Peter sandstone to the overljdng Platteville limestone is marked 

 by a bed of blue sandy shale which ranges in thicloiess from a few inches to 6 feet or more. 

 This shale is not commonly sharply separated from the sandstone, but grades into it. On the 

 other hand, the upper edge of the shale is sharply delimited by the overlying massive beds of 

 the lower part of the limestone. The sand grains in the shale do not usually extend to its upper 

 surface, and very rarely are these grains found in the lower beds of the Platteville. * * * 



The St. Peter can be regarded as a practically nonfossUiferous formation, though a few 

 casts, mainly of pelecypods or bivalve shells, have been reported from it. This scarcity of 

 fossils seems to be due as much to the fact that in so porous a rock solution would destroy 

 calcareous shells as to the fact that few animal remains were entombed in it originally. The 

 St. Peter is a wide-reaching formation in the upper Mississippi Valley, and in the Ozark region 

 it is represented by the sandstone which immediately underlies the Joachim Umestone and 

 which has frequently been termed the "First Saccharoidal" sandstone. 



