Between Green River, Utah, and the eastern edge of the Wasatch Plateau, 70 miles to the west, Interstate 70 crosses a broad ridge, adorned with spectacular rock formations. This topographic dome, up to 40 miles east-west and 100 miles north-south, is known as the San Rafael Swell.
At the onset of the Eocene Period, almost 60 million years ago, this region was characterized by flat terrain, a subsurface layer cake of Paleozoic and Mesozoic sediments overlying the deep, Precambrian basement. At the surface, early Tertiary deposits lay on successively older rock layers (from top to bottom: Cretaceous, Jurassic, Triassic, Permian and older Paleozoic sediments). Then, during the Eocene, the Precambrian basement folded upward as a broad dome, lifting the overlying layers of rock which have since eroded into the formations that we see today; resistant sandstones and limestones form ridges, domes and pinnacles, separated by valleys of softer shale and mudstone.
Atop the Swell, the Tertiary and Mesozoic layers have been stripped away by erosion, leaving a landscape of Permian sandstone; to either side, the traveller passes through successively younger rock formations as he descends from the crest of the dome to the valleys of the San Rafael River (east) and Muddy Creek (west). Prominent hogbacks (reefs) of Dakota Sandstone, Cretaceous in age, rise along the outer edge of the Swell. By crossing the San Rafael Swell on I-70, we pass through almost 200 million years of geologic history....twice.
