TEE HEART MOUNTAIN OVERTHRUST, WYOMING 545 



thin lenses of coal. The thickness of the layer ranges from 125 to 

 250 feet. It lies on a surface of low relief cut across the folded 

 Cretaceous rocks, and as the base rises from an elevation of 6,000 

 feet near Owl Creek to 7,200 feet near Wood River, a distance of 

 21 miles, it appears to have been slightly warped since deposition. 

 North of Cottonwood Creek a narrow east-west strip has been 

 down-faulted about 360 feet. 



Throughout this region, these beds are apparently conformably 

 overlain by paper-thin carbonaceous shales that weather white, 

 and these, in turn, by pale greenish volcanic ash and light brown 

 tuff, locally indurated. About 100 feet above the typical Wasatch 

 sediments, the well-stratified fine material is succeeded by coarser, 

 cross-bedded light brown tuff and still higher by heterogeneous 

 fine and coarse brown andesitic breccia that makes up the masses 

 3,000 to 4,000 feet thick in the region east of the Washakie Needles. 

 Except for the reference to lava flows, which are not known between 

 Wood River and Owl Creek, the following statement by Black- 

 welder might be considered an accurate description of conditions 

 in the southwest part of Bighorn Basin (la) : 



At the northwest end of the Wind River Range, where it articulates with 

 the mountains of Yellowstone Park, thick beds of volcanic ash and agglomerate 

 with interbedded glassy lava flows rest upon the pre -Tertiary folded rocks, 

 but are themselves younger than the Wind River Eocene. Traced eastward 

 to Horse Creek, the Washakee Needles, and the valley of Owl Creek, this 

 thick volcanic series is found to rest conformably upon the striped clays of 

 the Wind River formation, with which they intergrade through gray, plant- 

 bearing shales and greenish volcanic sandstones containing petrified logs. 

 A closer examination of the volcanic beds shows that some of them are massive 

 agglomerates, devoid of stratification, whereas other beds are distinctly 

 stratified, cross-bedded, and occasionally interrupted by lenticular sheets of 

 coarse gravel, suggestive of stream channels. The conditions indicated 

 are those which would be found upon low gradient river plains adjacent to 

 active volcanoes. 



The stratified tuffs have yielded several collections of leaves, 

 one collected by Mr. N. H. Darton (3a) on the Middle Fork of Owl 

 Creek and one by the writer south of Sunshine. Both are con- 

 sidered characteristic Fort Union material by F. H. Knowlton. 

 No vertebrate or invertebrate fossils have yet been found in the 

 supposed Wasatch beds, or the overlying tuffs or breccias. 



