This low rise, barren except for this north facing slope, is the surface manifestation of what geologists call an "anticline" -- an upwarping of bedrock layers. (This is quite visible in the satelite imagery from the MSR Maps link below.) These dome-like structures often capture petroleum seeping from buried oil-bearing layers of rock. Oil Mountain, true to its name, is an example of this. The trail passed just to the east edge of the mountain, and oil seeps existed nearby along the mountain flanks. Some enterprising frontiersmen (reportedly including Kit Carson) were involved during the 1850s in selling the oil (at one dollar per quart) to trail travelers, who used it for everything from axle grease to medicinal ointment. (See page 27 of the 1986 Ball State University thesis by William Metz titled "The Historical Archaeology of the Oil and Gas Industry in Wyoming". Thanks to reader Allan Fraser of Casper, Wyoming, who pointed out this reference and fascinating bit of trail history!)