MOOK, STUDY OF THE MORRISON FORMATION 99 



Feet 



Nodular limestone 1-2 



Blue shale , ' 50 



Limestone 2 



Concealed, probably blue shale 30+ 



135+ 



On the east side of the Laramie Mountains Darton (1908, 1) describes 

 the Morrison in the first canyon south of South Fork of Horse Creek as 

 consisting of the following : pale gTcen and maroon massive shale on 30 

 feet of light colored massive shale which contains several limestone layers, 

 one being 6 feet thick. "On South fork of Horse creek, the 6-foot lime- 

 stone member is conspicuous, underlain by 20 feet of gray shale lying on 

 a 1-foot limestone bed at supposed base of formation. The total thick- 

 ness here is about 200 feet, which appears to be the average amount, ex- 

 cept on the southernmost prong of Horse creek, where it is less than 150 

 feet." 



MOKEISON FOKMATION IN THE BlACK HiLLS ArEA 



The Morrison formation occurs in the Black Hills area in eastern 

 Wyoming and western South Dakota. It is present in the hog-backs sur- 

 rounding the central area of the Black Hills. Outcrops are present 

 around almost the entire circumference. For a short space on the south- 

 eastern side it is absent, however, the Lakota sandstone lying directly 

 upon the Unkpapa sandstone. The significance of the absence of the 

 formation at this point will be discussed later. The Black Hills Morrison 

 is usually underlain by a reddish, banded, porous sandstone, known as the 

 Unkpapa; in some areas, however, it rests directly upon the Sundance 

 beds. The thickness of the formation in the various sections that have 

 been measured are as a rule less than the thickness in the various central 

 Wyoming areas and much less than the areas in western Colorado. 



The following general description of the Morrison and Unkpapa for- 

 mations in the Black Hills area is from Darton (1909, 5). Detailed de- 

 scriptions of the formation in various quadrangles will be given later. 



The Unkpapa sandstone has been fully described by Darton. This 

 formation is a characteristic one in the Black Hills region. It is more 

 extensively developed in the southern than in the northern part of the 

 area. In the northwestern and western part of the region it consists of 

 a thin yellowish sandstone. In the southern and southeastern part of the 

 area it is represented by uniform-textured, fine-grained sandstone of vary- 

 ing colors. The following thicknesses are given by Darton : near Sturgis, 



