Just south of the Colorado-Wyoming boundary, the mountain masses of the Southern Rocky Mountains divide into three prongs that extend northward into Wyoming. These three prongs are known, from east to west, as the Laramie Mountains, the Medicine Bow Mountains, and the Sierra Madre. The Laramie Mountains are most closely related to, and are an extension of, the Colorado Front Range. In Wyoming, they separate the Denver Basin and Hartville uplift to the east from the Laramie and Shirley basins to the west. Casper Mountain, a northern salient of the Laramie Mountains, borders the Casper arch on the south. The northeastern Laramie Mountains borders the Powder River Basin on the south.
Near the border between Wyoming and Colorado, the Laramie Mountains have been reduced by erosion to low relief. The mountains are rugged to the north, capped by Laramie Peak (elevation 10,274 feet), a famous landmark well known to emigrants traveling westward along the Oregon Trail. The basic geologic structure of the range is of a large asymmetric arch, steepest on the east. Precambrian rocks are extensively exposed in its core, which is flanked by sedimentary strata. South of Laramie on both sides of the state line, diamonds were discovered in some very unusual and rare rocks called kimberlites. The diamonds are thought to be at least one billion years old, and formed under high temperature and pressure in the Earth’s mantle. Somehow, the diamonds found their way to the surface in kimberlite “pipes” that erupted through the crust, bringing the diamonds from the upper mantle, possibly from a depth of 100 miles. The mechanism by which these kimberlite pipes reached the surface from such depths is not exactly known, but the kimberlites in them represent the deepest rocks known from the Earth’s interior.
Crossing the Laramie Mountains between Cheyenne and Laramie, Interstate 80 and the Union Pacific Railroad first traverse a relatively flat surface of late Tertiary sedimentary rocks called the “Gangplank,” finally reaching the Precambrian core of the mountains and a rolling upland of low relief at about 8,000 feet above sea level called the Sherman surface. The Gangplank is the only place along the entire eastern mountain front where the Tertiary rocks that once buried the Laramie Mountains are still preserved and are in contact with the Precambrian core of the mountains. Further erosion during later Tertiary time left a few higher granite boulder knobs such as the spectacular area at Vedauwoo. Sherman Pass, at the summit of the Laramie Mountains on Interstate 80, is the highest point (8,640 feet above sea level) on this transcontinental highway.