A graben is a collapsed or down-dropped block of rock that is bordered on its long sides by faults. Grabens are normally associated with horsts, which are the up-thrown blocks of rock in between. (Both words are of German origin: graben meaning ditch or grave and horst meaning aerie, referring to the high nesting sites of predatory birds.) The Grabens in The Needles district of Canyonlands is a system of linear collapsed valleys caused by the movement of underlying salt layers toward the Colorado River canyon. The grabens begin near the Confluence of the Green and Colorado rivers and run roughly parallel to Cataract Canyon for 25 km, veering slightly westward before they end.
The processes that led to the development of the grabens began approximately 300 million years ago in the Pennsylvanian period with the deposition of evaporates (salts) in a shallow inland sea. These deposits, known as the Paradox member of the Hermosa Formation, were later covered by the limestone layers of the upper Hermosa and Rico formations. In The Needles, the Paradox layer can be 3,000 to 5,000 feet thick.
Sea levels eventually dropped, and white sands blew in from the west, forming large sand dunes. At the same time, red mud and silt was deposited by rain and snow melt from the Uncompahgre Mountain to the east. The resulting red and white beds alternated, forming the lower beds of the Cutler Formation, or the Cedar Mesa Sandstone that is dominant in The Needles today.
Sediment from a variety of environments continued to accumulate on top of these layers for millions of years. Approximately 60 million years ago, a tectonic plate collision called the Laramide Orogeny created the Rocky Mountains. Shortly after, a regional upwarp called the Monument Uplift caused the sedimentary layers in The Needles to tilt gradually westward. This event also formed joints, or long parallel fractures in the rock, throughout The Needles. In the vicinity of The Grabens there are two joint sets: one trending roughly northeast to southwest, and one trending northwest to southeast. Some of these joints became the faults that border The Grabens.
Around 10 million years ago, the uplift of the Colorado Plateau gave rise to the Colorado River and its tributaries. As the Colorado River cut its way downward through the rock layers, it carried away millions of tons of sediment towards the Pacific Ocean.
Finally, about 55,000 years ago, the ingredients were in place and the grabens began to form. Four factors have been identified as critical to the formation of grabens:
- The ability of evaporates to flow plastically. The evaporates in The Needles district are slowly flowing westward towards Cataract Canyon due to the pressure exerted by the rock layers above.
- The erosion of the Colorado River down to the Paradox Formation, creating a low pressure zone the evaporates are drawn to.
- The gradual tilt created by the Monument Uplift, which allows gravity to act on the evaporates.
- The interaction of water with the evaporates, dissolving the salts and facilitating their ability to flow.
The grabens are a very young geologic feature. Graben growth is thought to be a slow process where small, seismically undetectable movement occurs: as little as one inch per year. The grabens continue to drop and slide toward the river today, and are a fascinating feature of The Needles district.