I-70 exits the valley through De Beque Canyon, a path carved by the Colorado River that separates the Book Cliffs from Battlement Mesa. The river and its tributaries provide the course for the ascent up the Rocky Mountains. In the canyon, I-70 enters the Beavertail Mountain Tunnel, the first of several tunnels built to route the freeway across the Rockies. This tunnel design features a curved sidewall, unusual for tunnels in the United States, where most tunnels feature a curved roof and flat side-walls. Engineers borrowed a European design to give the tunnel added strength. After the canyon winds past the Book Cliffs, the highway follows the Colorado River through a valley containing the communities of Parachute and Rifle.