140 



GEOLOGY OF THE BLACK HILLS 



tioii will represent the typical synclinal character of the Red \^illey at the 



northwest angle of the Hills: 



The Red Beds in the northern and northwest- 

 ern part of the Hills contain large quantities of 

 gypsum in strata of considerable thickness as well 

 as in disseminated seams and veins of small size. 

 The larger seams are usually G or 8 feet in thick- 

 ness, and though frequent opportunities were taken 

 to measure them they were not found to exceed 10 

 feet. 



The causes which prevent close measurement 

 of the red clay also preclude an accurate placing 

 of the gypsum strata in their true position in the 

 formation. There appear, however, to be several 

 well-marked horizons occupied by thick beds. 

 The first is about 75 or 100 feet above the purple 

 limestone and the second 40 or 50 feet higher. 

 A third is at the ver}'' summit of the red clay, and 

 a fourth, less persistent, is a few feet lower. Over 

 a large area between Redwater and Inyan Kara 

 Creeks the red clays lie with little or no dip, and 

 in them a continuous seam of gypsum is found 

 wdiich frequently has but a thin stratum of clay 

 covering it. This thin covering is occasionally 

 w^anting, and then the gypsum is exposed. Under 

 such circumstances it is never a finn rock, but its 

 surface is decomposed and undermined by atmos- 

 pheric action. As one rides over it the feet of his 

 horse at every step break a thin crust and then 

 sink a few inches through the pulverulent product 

 of decomposition before striking the more solid bed 

 The latter gives a hollow sound, caused probably by an under- 



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There are numerous sink holes in the red clay, and these on close 



