These features formed during volcanic activity 3 to 6 million years ago. Erosion Most of the erosion that carved today's landscape occurred after the uplift of the Colorado Plateau sometime within the last 20 million years. Most of the major canyon cutting probably occurred between one and six million years ago. Even in this desert climate, water is the erosional agent most responsible for the carving of the landscape. The pull of gravity, in the form of rock falls or rock creep, plays a major role in the shaping of the cliff lines.
Wind is a minor agent of erosion here.
The landforms are a result of different responses of the various rock layers to the forces of erosion. Hard sandstone layers, like the red Wingate and the white Navajo Sandstones , form cliffs. Softer shale layers, like the Chinle Formation , form slopes and low hills. The barren slopes found in many areas are due in part to the presence of bentonitic clays in the shale which make an inhospitable environment for plants. The black boulders, found scattered throughout the Fremont River valley and along other drainages, are recent geologic arrivals to Capitol Reef.