SECTION AND DESCRIPTION 311 



is a section four miles west of Bull Lake, in section 11, township 2 north 

 of the Wind Eiver base line, range 1 west of the Wind River meridian. 



The contact between the Amsden and the overlying Tensleep is one of 

 disconformity, witli a. clear-cut wavy line of demarcation. 



1. Compact, tiiie-^a-aiued. light-j^ray limestone 8 feet 2 inches 



2. Buff, fine-grained, sandy limestone 4 feet 



3. Buff, compact, sandy, thin-bedded limestone much like 1 and 



2 but without the gray. The lower part weathers to a 

 pinkish color while retaining its strength and compactness 14 feet 



4. Dark-gray, compact, hackly limestone 7 feet 6 inches 



5. Dark-gray, friable, coarsely crystalline limestone, inter- 



bedded with thin, discontinuous layers of darker gray, » 



compact limestone, fossiliferous 23 feet 



6. A heterogeneous mixture of limestone, sandstone, shale, and 



conglomerate in a peculiar arched structure. The under 

 part of the arches weathers out, leaving caves. At the 

 bottom occurs a conglomerate which generally contains 

 many fragments of Madison. In many places breccias 

 resulting from cave-filling occur. Contact with the Madi- 

 son sharp, disconformable 20 feet 



Total 76 feet 8 inches 



Measurements were made on the face of a vertical cliff, excepting the last 

 20 feet, which was estimated, because the top could not be reached. 



Many caves occur at the base of the Amsden, and in early Pennsylva- 

 nian time the caves were more extensive than at present. In the Bull 

 Lake region the base consists of a thin, irregular basal conglomerate, above 

 which there is a dolomite containing old caves filled with large, angular 

 fragments of Amsden limestone mixed with greenish shale and lenticular 

 sandstone beds. Only one filled cave was noted near the top of the forma- 

 tion and it was filled with sand, like that of the overlying Tensleep, the 

 sand occurring as filling matter between Amsden boulders. The sand in 

 the caves near the base also resembles the Tensleep and occurs as filling 

 matter, indicating that the caves were formed in pre-Tensleep time and 

 filled during the Tensleep. Where the base of the Amsden is well ex- 

 posed the lower 20 to 30 feet appear as a series of arches with the over- 

 lying beds overhanging. The tops and sides of the old caves form the 

 tops and sides of the present arches and the old cave-fillings have weath- 

 ered back, forming reentrants. A coarsely crystalline bed of limestone, 

 3 to 4 feet thick, that is highly fossiliferous, occurs 35 feet from the top, 

 but fossils were not found in any other bed. 



