EROSION FORMS IN HARNEY PEAK DISTRICT 581 



The second paper was 



EROSION FORMS IN HARNEY PEAK DIS TRICT, SOUTH DAKOTA 

 BY EDMUND OTIS HOVEY 



[Abstract with discussion} 



The pre-Cambrian geology of the Black hills of South Dakota has been ably treated 

 by Van Hise* before this Society, and by Newton, f Crosby, t Carpenter, \ and 

 others elsewhere, and the present paper does not presume to attempt to add to the 

 geological facts brought out by these observers. The surface features, however, 

 of the granitic region near Harney peak are so very peculiar that the verbal de- 

 scriptions of N. H. Winchell || and Newton f convey but an inadequate idea of the 

 relief of the country, and a reproduction of some photographs taken last summer 

 (1899) may not be without some value. 



The so called granite area forming the Harney Peak district and the culminat- 

 ing point of the Black hills is an irregular oval about 16 miles long from north to 

 south and 10 miles wide, but it is by no means all granite. The central portion, 

 including Harney peak, shows nothing but the coarse grained granite, the valleys 

 between the resistant ridges being covered with soil and bearing forests of the 

 Rocky Mountain pine up to the vertical walls of the granite ridges, so that no 

 other rock seems to be exposed. The outer portion of the area, however, consists 

 of numerous lenses or bosses of granite which have forced their way up through 

 the mica-schists of the general ragion. The schists have suffered most from ero- 

 sion and have left the granite standing in high, narrow ridges, the summits of 

 which rise from 200 to 500 feet above the intervening valleys, and are often wholly 

 inaccessible. The granite is intersected by numerous joint planes, and erosion 

 has progressed in such a way as usually to produce sharp pyramidal and needle- 

 like forms in the rock. The ends of the lenses being narrower than the middle, 

 the terminal needles have disintegrated and worn down more rapidly than the 

 others, and the upper portion of a vertical section is elliptical, a form which may 

 or may not correspond with the original shape of the lense. This feature is well 

 shown in figure 1 of plate 55, which represents the end of a ridge descending into 

 the valley between two other ridges. That there has been no glaciation of the 

 region is indicated by these jagged forms and the absence of grooved and polished 

 surfaces and erratics. 



Figure 1 of plate 53 shows a part of the view northwestward from the summit 

 of Harney peak. The ridges of granite are seen projecting above the tree tops. 

 The heavily wooded hills in the near distance are of schist. The outlook in every 

 direction from the peak shows how intricate is the network of these lenses. Figure 



* C. R. Van Hise : The pre-Cambrian rocks of the Black hills. Bull. Geol. Soc. Am., vol. 1, 1890, 

 pp. 203-244. 



t Henry Newton, E. M., and Walter P. Jenny, E. M.: Report on the geology and resources of the 

 Black hills of Dakota, with atlas, 4to, Washington, 1880. 



% W. O. Crosby : Geology of the Black hills of Dakota. Proc. Boston Soc. Nat. Hist., vol. 23, 1888, 

 pp. 488-517. 



§ Franklin R. Carpenter: Preliminary report of the Dakota School of Mines upon the geology, 

 mineral resources, and mills of the Black hills of Dakota, 1888. 



|| William Ludlow : Report of a reconnaissance of the Black hills of Dakota, made in the sum- 

 mer of 1874, 4to, Washington, 1875 ; Geological report by N. H. Winchell, pp. 21-66. Pp. 42-46. 



\ Geology of the Black hills, pp. 65-80. 



