The Unaweep/Tabeguache Scenic and Historic Byway runs through the heart of the Uncompahgre Plateau. About 300 million years ago the area that is now the Plateau was uplifted into a mountain range known as the ancestral Rocky Mountains or "Uncompahgria." Over time the mountain range was eroded down to its roots, furnishing the sand, gravel and mud for the redrock formations of southwest Colorado. This process of erosion removed over one half billion years worth of material from the rock record, causing the Chinle Formation of the Triassic Period to lie directly on the Precambrian rocks. You can best see this "Great Unconformity" at the Divide Road turnoff.

Unaweep Canyon cuts like a knife through a layer cake down to redbeds and eventually deep into the granite and metamorphic "basement" rocks. The canyon is truly unique as it is the only one in the world with a divide in the middle and streams running out of both ends. The Ute Indians name "Unaweep" means "Canyon With Two Mouths." The small streams that now wind along its floor seem much too small to have cut the canyon. Based on the types of gravel left throughout the area, geologists believe that it is the former course of the Gunnison River. A second uplift of the Plateau about 1 million years ago caused the Gunnison to cut Unaweep Canyon and to later change its course.

You can take a trip back in geologic time by following 4.2 Road to the north from Gateway. This dead-end road follows the Dolores River along the base of The Palisade, a part of the BLM's Palisade Outstanding Natural Area and a favorite of geologists. Here extended erosion allows you to view layers of rock which are not normally visible and to imagine the landscape's appearance in ancient times.

Between Gateway and Placerville the Byway traverses the northeastern edge of the Paradox Basin known for its redrock canyonlands and salt anticline valleys. Thick beds of salt and gypsum were deposited here when an ancient sea evaporated over 300 million years ago. Both salt and gypsum flow in a solid state like silly putty. Weighed down by overlying layers of rock they try to escape upward, pushing against the layers and arching them into domes. Ground water seeping into the faults along dome edges caused the salt to be dissolved away. The domes collapsed leaving deep valleys edged with sandstone cliffs. Paradox, Sinbad and Gypsum valleys are examples of this process.

The sandstone cliffs along the byway from Gateway towards Placerville are actually made up of a number of different layers. This diagram describes their composition.

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