78 



GUIDEBOOK OF THE WESTERN UNITED STATES. 



the work of construction a central shaft was sunk, the top of which 

 was 331 feet above track grade. From the bottom of the shaft 

 headings were started east and west to connect with the end headings* 



Tlie greatest depth reached below the surface is 456 feet; the highest 

 point above sea level 7,296 feet. The tunnel accommodates a single 

 track and is lined with timber and concrete. The new route was 

 completed in 1901, at a cost of $12,000^000; and shortens the line 



10 miles. 



At the point where the road leaves the main branch of Muddy Creek, 



2\ miles south of Leroy^ the traveler may obtain a view, toward the 



left (east), of the edge of the plateau of Bridger beds on which stands 



Bridgor Butte. A mile west of Ragan may be seen, to the right 



(north), a group of derricks where oil wells have been sunk into the 



Aspen shale/ which includes the oil-bearing rocks of this region. A 



small refiner}^ was built at Leroy, but it was not in operation in 1914. 



faulted and iiptux-iied iu ridges, but the 



movement was attested before high 



mountains were formed here. 



Two main gi'oupa of fault lines are 



crossed .by the Union Pacific in this gen- 

 eral region. The Absaroka fault and the 

 Oil Spriugii faults are crossed at the Aspen 

 tunnel and the Almy and Medicine 

 Butte faults at Evanston. The Absarota 

 is a thrust fault bv which the rocks on the 

 west have been pushed eaatward and 

 raised more than 15,000 feet, some of the 

 older sedimentary rocks being brought to 

 altitudes much greater than those of the 

 younger rocks of tliis region. This rela- 

 tion is conspicuous west of the Aspen 

 tnnnel, where rocks of early Tertiarj^ age 

 abut against some of Jurassic age. The 

 Medicine Butte fault, which the road 

 crosses at Evanston, is also an ovcr- 



sraA 



fault 



ma 



being 



Later the rocks wore again 



Erosion, which followed the initial 

 mountain-forming disturbance, carved the 

 older rocks into low hills and shallow val- 

 leys, and these in turn were buried by 

 accumulations of sediment in early 

 Eocene time, 

 upheaved^ erosion was renewed, and other 

 hills and \'alley3 were car^'ed out. These 

 a!?o were buried by the rod sands and 

 gravels of the Wasatch group, which re- 

 cent erosion has removed in some places, 

 exposing again the pre-Wasatch hills, 

 but which still remain as the surface rocks 



over large areas of western Wyoming and 

 eastern Utah. 



^ The Aspen formation consists of shale 

 1,500 to 2,000 feet thick, in which are 

 layers of sandstone that contain oil. Near 

 the top of the formation occurs the 

 *'Sprhig Yalley oil sand," which con- 

 tains the principal oil pools, although 

 some have been fouiid in lower sands. 

 The formation is of marine origin, and the 

 shaly parts contain numerous scales of 

 fishes, from which they have been called 

 the "fish-scale shales." Certain fossils 

 found in the formation prove that it be- 

 longs in the lower part of the Upper Cre- 

 taceous series. 



Although most of the oil of this region 

 has been found in the Aspen formation, 

 some comes from the Bear River forma- 

 tion, which immediately underlies the 

 Aspen, The occurrence of oil in this 

 region was known to James Bridger and 

 other early trappers, but the first pub- 

 lished account of it resulted from a visit 



made by the Mormon pioneers in 1847 to 

 the natural oil spring, known as the 

 Brigham Young oil well, 6 miles south- 

 west of Spring Yalley. Small quan- 

 tities of oil were collected fforg this and 

 other springs, and prospecting was car- 

 ried on intermittently until 1900, when 

 high-grade oil was struck in a well near 

 Spring Valley. Since that time several 

 pools have been found, but the yield is 

 small, the best wells producing only a 

 lew barrels a day. 



