The Saint Peter Sandstone. 65 



THICKNESS. 



The Saint Peter sandstone was supposed by James Hall 

 (9)* to be remarkably uniform in thickness, ranging from 80 

 feet in its northern (Minnesota) exposures to over 100 feet 

 in the southern (Illinois) ones. In a sense, it is remarkably 

 uniform, for the thickness in one area of its great extent 

 averages nearly the same as in any other. But Worthen 

 (12) found this formation to be variable in depth in Illinois, 

 andChamberlin(2l and 26) has clearly and concisely shown 

 that it is in fact not uniform, but very variable in thickness, 

 even within short distances. This variability is due to the 

 undulated surface of the dolomite upon which it rests, the 

 undulation being so great as to cause the depth to vary 

 from less than one to over 200 feet. The mean thickness has 

 been estimated by Chamberlin (26) to be between 80 and 

 100 feet. It is more than 100 feet thick in the north. 



These estimates are for the Saint Peter itself, exclusive of 

 the Shakopee dolomite and New Richmond sandstone, which 

 two formations McGee (31) Keyes (32) and Norton (33) 

 have, for Iowa, included in the Saint Peter. Such a course 

 is not warranted; when followed, care should be taken to 

 add to the Saint Peter the thickness of the never-failing 

 Shakopee beneath, lest a false meaning be given to already 

 established data, as it has never been shown that Saint Peter 

 and Shakopee taken together are ever reduced to marked 

 thinness. No regularit3^ for the undulations in the underly- 

 ing dolomite has been observed, hence no explanation for the 

 variability in the thickness of the Saint Peter sandstone is 

 yet known. 



GEOGRAPHICAL DISTRIBUTION. 



The Saint Peter sandstone is seen in numerous exposures 

 in southeastern Minnesota, in northeastern Iowa, in western 

 southern and eastern Wisconsin, and in northern Illinois. 

 In this wide area it is repeatedly interrupted or reduced to 

 isolated areas by river valleys. 



The original extent of the formation we do not know. 

 The general northern edge of its area, presents an eroded, 

 torn and glaciated margin which is very far from being the 



• The numbers refer to the bibliography at the end of the paper. 



