STEAMBOAT MESA 325 



the highest cone Essex mountain. At that time it was simply referred 

 to as basalt and was map])ed as being about ten times its real size. 

 From the great sand dune countr}^ on its south it rises very abruptly for 

 1,100 feet to the edge of the escarpment, which is 150 feet below the 

 highest cone (see plate 43, figure 3). In climbing it from the south and 

 the most inaccessible side, one passes over a thin band of Laramie, above 

 which may be a thin bed of Wasatch, for the latter is well exposed only 

 a few miles to the north. Along the mesa it has been probably covered 

 with a heavy talus of sedimentary rocks. Above the horizon where the 

 Wasatch should be found there are well developed beds of Green River 

 shale, and above these a thin bed of Bridger. From the latter beds we 

 obtained typical, siliceous oolitic rock and also an ostracod limestone) 

 which occurs in the Bridger wherever found. From the base of the lava 

 escarpment the talus slopes are exceedingly steep and in places read 

 from 30 to 35 degrees, just about at the limiting angle of repose. On the 

 north of the mesa there are great tablelands that have been developed 

 out of the Green River shales, and occasionally there are remnants of 

 the Bridger beds, and only 6 or 8 miles farther north the Bridger beds 

 reach a thickness of about 1,000 feet. 



From east to west this mesa has a length of between 2 and 3 miles, 

 and its greatest width is approximately a mile. The southern face is a 

 vertical escarpment rising from 25 to 60 feet, and it has a circular out- 

 line. On the north it is deeply dissected and there are gulches extend- 

 ing into the lava mass for a distance of over a half mile, a.nd between 

 these there are long arms of the flow^ projecting to the northw^ard out on 

 the Tertiary beds. From the northern side the mesa is quite accessible 

 and often affords pasturage for flocks of sheep or herds of cattle and 

 horses. The surface is quite irregular, more so than any other of the 

 mesas except Orenda. There are broad gulches with a northern trend 

 which head near the southern side, and along these there are small 

 groves of spruce and pines interspersed with quaking asp. 



From the surface of the mesa three apparent cones arise. They are 

 not, however, in any case pumice or fragmental aggregates, but are hard 

 sheets of lava, either marking the source of vents with huge blisters above 

 the point of eruption, or else they are the unweathered nuclei of thick 

 sheets, elsewhere reduced b}^ erosion to lower levels. The cones were 

 numbered as follows. South cone number 1, middle cone number 2, 

 northwest cone number 3. 



The lava occurs in two distinct and easily recognizable flows, each of 

 which is about 30 feet thick. The contact is marked by a black scoria- 

 ceous crust on the lower surface of the upper flow, which gives all desira- 

 ble evidence of a quick chill. Irridescent hues play over the dark scoria 



