212 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



SODA BUTTE VALLEY. 



For some distance above the mouth of Amphitheatre Creek the valley 

 of Soda Butte is narrow, the stream flowing through a gorge cut in volcanic 

 rocks. Its upper course is, however, through a broad valley, with flat 

 gravelly bottom and with striking cliffs of limestone appearing on either side. 

 The limestones exposed at the mouth of Pebble Creek extend up the western 

 side of the Soda Butte Valley to near the mouth of Amphitheatre Creek. 

 As no exposures appear on the east side of Soda Butte Creek, it is evident 

 that the sedimentary rocks were cat away before the deposition of the 

 volcanic breccias. 



An examination was also made of the rocks exposed on the south 

 slopes of the high limestone mountain that is capped by breccia and lies 

 north of Soda Butte Creek. The lower slopes of this peak are covered 

 with large blocks of limestone, the talus from the cliffs above. The lowest 

 exposures are of strongly mottled dark-colored limestones, overlain by 

 limestone conglomerates, with light-gray, chert-bearing, massive-bedded 

 limestones above. These rocks show an old surface that is quite irregular. 

 The andesite rests upon a cemented mass of large blocks and fragments of 

 limestone. The heavy limestone belt of Soda Butte Creek above this point 

 is exposed some 500 feet above the stream, the rocks being nearly horizontal. 



The following sections, made by Professor Iddings, show the sequence 

 and character of the sedimentary rocks exposed in this vicinity: 



Section of beds on north side of Soda Butte Greek. 



Feet. 



(■ Gray limestones, carrying fossils and chert 10 



Jefterson ) White limestones, much crackled and brecciated 20 



I Massively bedded, light-gray limestones, forming cliff 200 



f No exposure 50 



Thinly bedded limestones, carrying trilobites 50 



Thinly bedded limestones and limestone conglomerates, carrying abundant fossils 

 near top. The limestones are glauconitic, thinly bedded, and weather with a 



yellowish surface, often studded with fossils in relief 190 



Q a Thinly bedded limestones and much limestone conglomerate. The rock is a 



dense light-gray limestone, and the conglomerate is formed of flat and very thin 

 beach pebbles, and the rocks carry trilobites and a few shell remains at summit 

 of exposure. The conglomerate is intraformational; that is, the pebbles are of 



the same limestones 60 



Mottled dark-colored limestone, forming a massive bed that is generally a cliff 

 100 feet i n height 100 



(Trilobite remaius occur in a black limestone that is oolitic, full of dark glauconitic 

 grains, and is quite characteristic for this horizon. The thickness of limestone 

 probably does not exceed 100 feet. It is underlain by soft laminated shales 400 



