102 GEOLOGY OF THE BLACK HILLS. 



Tlio 8teins usually divide into threes or bifurcate, and the smaller branches 

 IVequuntly terminate abruptly in rounded ends. 



E.xcellent exposures of" the Potsdam were found on Spring Creek, and 

 a section of tlie strata there found has been given on a preceding page. 

 On Rapid, the next creek north, the Potsdam presents some of the most 

 interesting features observed in the Hills — the occurrence of large amounts 

 of calcareous matter; the presence of great quantities of greensand or 

 glauconite; and the rapid changes in character and thickness found within 

 short distances in the canon. A highly arenaceous series with a heavy 

 bowlder conglomerate at the base changes in three miles to the highh' 

 calcareous series represented in Figure 14, and the .section increases in thick- 

 ness in the same distance fully 100 feet. The Potsdam was evidently 

 deposited on an uneven bottom of slate, and a local sorting of its material 

 seems to have been determined thereby. The calcite and glauconite, or the 

 chemical and organic material, found there way into the depression, while 

 the purely fragmental material remained in the shallower water. From the 

 canon of Box Elder Creek northward for ten miles the Potsdam is exposed 

 at the base of the limestone cliffs which follow the creek. The creek valley 

 then bends westward and the limestone cliffs run more directly northward 

 to Bear Butte Creek. From the base of the cliffs there is usually a broad 

 slope or talus, broken now and then, by outcrops and small cliffs of the 

 Potsdam sandstone, but rarely revealing any well-exposed section of the 

 forniation. Several outcrops of the sandstone exhibit intercalated strata of 

 quartzite, and the thickness of the formation was found to vary from 200 

 to 275 feet. 



The width of the Archeean area, which near Sf>ring Creek separates 

 the east and west outcrops of the Potsdam by more than twenty miles, has 

 narrowed down, until near Custer Peak the outcrops on either side of the 

 Hills are separated by only five or six miles, while some ten miles directly 

 north froui the same peak, near Black Butte, the eastern outcrop swings 

 around and joins the exposures of the Potsdam on the western side of the 

 Hills. In the area north and northeast of Custer Peak there are so many 

 »>utl}ing tracts of Potsdam and even of Carboniferous strata that the imagina- 

 tion is but lightly taxed to restore them over the whole, and their former 



