120 GEOLOGY OF THE BLACK HILLS. 



On a small branch of Castle Creek a section of the eastern cliff was 

 obtained, as follows: 



Feet. 



G. Limestone; highly silicious, with no distinct stratification, but breaking irregu- 

 larly; cavities with much calcedony; color white, with pink and red 



blotches 30 



5. Talus, no exposure . 20 



4. Limestone, white and compact, with a few fossils, Productus, a Bryozoan, etc. 30 



3. Talus, with fragments of Bed 4 70 



2. Limestone, white and compact like 4, with Chonetes, Zaphrentis, etc 25 



1. Talus to creek, with the Potsdam indicated by a serk'S of springs flowing 



from a level about 100 feet above the valley 200 



Several miles farther down Castle Creek, which is lined with an escarp- 

 ment of limestone cliffs that render its name very appropriate, a measure- 

 ment showed the silicious limestone to be 150 feet thick, standing in a steep 

 bluff, with its top 750 feet above the bed of the creek. It is separated by 

 a slight talus from a cliff of 100 feet of the white limestone, and from the 

 base of this, evidently concealing a portion of the limestone, a slope extends 

 down 400 feet to the schists, which occupy the lower 100 feet of the sides 

 of the valley. About 75 feet of coarse Potsdam sandstone rest on the 

 slates, and fragments of Potsdam were found on the slope for 200 feet 

 higher, above which are numerous pieces of the purple shaly limestone (1) 

 and rocks from the cliffs above. 



From Castle Creek southward to Red Cation Creek the Carboniferous 

 beds 2 and 3 are continuously seen lining the western border of the schist 

 area. Red Canon Creek runs in a valley a mile or two in width, and 

 for about four miles is bordered by cliffs with the limestones above and the 

 Potsdam below. The increasing dip then brings the limestones in succes- 

 sion down to the stream level and after them the alternating beds, and 

 through the latter the creek runs in a canon to Red Valley. The creek 

 makes its escape through the southeastern corner of the mesa, and is the 

 most southerly of the fifteen streams that carry the drainage of the Archaean 

 district across the encircling belt of Carboniferous. 



At this point the plateau may be said to end and the monoclinal 

 ridge to begin. The ridge runs at first eastward and dips to the south; it 

 then turns toward the north and its dip becomes easterly; and finally to 



