THE CRETACEOUS RIM OF THE BLACK HILLS/ 



As IS well known, the Black Hills District was surveyed by 

 the party in charge of Professor W. P. Jenney in 1875, the geo- 

 logical report being written by Professor Henry Newton, whose 

 death occurred two years later. The report, edited by Mr. G. 

 K. Gilbert, was published in 1880 by the U. S. Geographical and 

 Geological Survey of the Rocky Mountain Region in charge of 

 Major J. W. Powell. The peculiar and interesting geological 

 features of this remarkable outlier of the Rocky Mountain range 

 need not be set forth here further than to say, that on all sides 

 after leaving the central nucleus of eruptive rocks sedimentary 

 deposits occur with diminishing dip in an ascending geological 

 order. First, there is a narrow ring of Potsdam sandstone ; then 

 a wide belt of Carboniferous limestone ; next an encircling 

 trough, aptly compared by Professor Newton to a moat, of red 

 sandy gypsiferous clays, in which is included a purple limestone 

 terrace, all of which is supposed to be Triassic and to be the 

 equivalent of the " Red Beds" of more southern regions ; skirting 

 this is a very narrow border of highly fossiliferous light colored 

 Jurassic clays or marls ; then come the foot-hills, which consist 

 of Cretaceous sandstones and shales referred by Professor New- 

 ton to the Dakota, No. i, of Meek and Hayden's section; these 

 slope back to the dark shales of the Fort Benton group, which 

 are succeeded by higher Cretaceous beds that extend to the 

 plains and pass under the Bad Lands of the White River forma- 

 tion. 



The belt of Cretaceous, which lies outside the Red Beds and 

 Jurassic and forms the foot-hills, constitutes an elevated rim 

 with an escarpment at its inner margin rising abruptly above 

 the Triassic trough, the Jurassic exposures being often confined 

 to the lower part of the escarpment. This cannot be better rep- 



' Published with the permission of the Director of the U. S. Geological Survey. 



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