The Rio Grande has carved Boquillas Canyon, a gorge 1,500 feet deep, directly through the massive cliffs of the Sierra del Carmen escarpment. So narrow that its entrance is almost invisible from a distance, the canyon shows how the river acts as a liquid rasp, grinding the gorge deeper with tons of silt and sand beside towering walls. The river established its present course on basin-filling sediments that once covered the rocks and faults now exposed, then eroded through the surface layers and cut steep-walled canyons in the more resistant Lower Cretaceous limestones, whose ancient formations are visible today in the canyon walls.