Boar's Tusk and the Killpecker Dunes

Boar's Tusk rises about four hundred feet out of the Killpecker Valley north of Rock Springs, the eroded throat of a volcano long since worn away. It belongs to the Leucite Hills, a field of some twenty-two volcanic necks, mesas, cones and dikes that geologists count among the most important lamproite fields in the world. Lamproite is a rare rock that comes up from the earth's mantle, in some parts of the world carrying diamonds, and the Tusk holds broken pieces of everything it pushed through on the way: mantle rock, granite from the crust, and the sandstones of the Wasatch and Green River formations.

At its foot begin the Killpecker Sand Dunes, one of the largest active dune fields in North America, running about fifty-five miles east across the Continental Divide into the Great Divide Basin and covering some 109,000 acres. The sand was washed out of the Wind River Range by the Big and Little Sandy rivers as the glaciers melted and has been driven east by the prevailing wind ever since. The dunes soak up snowmelt and rain, and in spring ponds form between them, drawing shorebirds, wild horses and the Red Desert's elk, the largest herd of desert-dwelling elk in the world.

The dunes are reached by gravel county roads off US-191 about ten miles north of Rock Springs. Part of the field is open to off-road vehicles, the base of the Tusk is best reached with four-wheel drive, and there are no services.