The Black Canyon National Monument is located on the Gunnison River about seven miles northeast of Montrose, Colorado. Although the canyon is not actually black, the fact that its depth often exceeds its width often leaves the canyon in shadow, hence the name. The Black Canyon was quickly cut, for the most part because the Gunnison River drops 2150 feet in about 50 miles. The maximum gradient in the monument reaches near a 95 foot drop in a mile. The Black Canyon is not a result of mountain building, but rather that of erosion of preexisting rocks. The accompanying stratigraphic column illustrates the formations that comprise the Black Canyon.

The oldest rock formation located in the Black Canyon area are Precambrian metamorphic rocks. These schists and gneisses serve as the basement rock. Compositionally they range from quartz-mica gneiss and migmatite to micaceous schist and amphibolite.

Overlying the basement rock are several Precambrian igneous formations. From oldest to youngest, they are the Pitts Meadow Granodiorite, the Vernal Mesa Quartz Monzonite, and the Curecanti Quartz Monzonite. Intruded into these three formations is pegmatite, often forming into dikes and sills. This group of igneous rocks forms most of the actual canyon walls.

In the Cambrian or Ordovician Period, diabase intruded the existing rock to form dikes. While diabase is fairly uncommon in the area, the dikes it forms can be as wide as 3 miles.

No rock record exists in the area for the Silurian through the Permian periods. While these rocks were probably deposited, erosion caused by the uplift of the Uncompahgre Highland sometime in the Pennsylvanian removed any trace of their existence in the area. In the Jurassic, the highland began to subside, but no formations where deposited until the very end.

Deposition resumed in the Jurassic with the Entrada Sandstone. This nonmarine is believed to be deposited at the margin of a shallow seaway, and the orange sandstone's uppermost few feet contain ripple marks, indicative of transgressing seas.

The Wanakah Formation was deposited next on top of the Entrada Sandstone. The bottom-most member of the Wanakah , the Pony Express Limestone probably formed as an enclosed arm of the sea. The Junction Creek Sandstone Member is also well represented. Evidence of eolian deposition indicate dunes on the now dry Wanaka sea bed.

Also of Jurassic age is the Morrison Formation. Formed on land, it is represented at the Black Canyon by the Salt Wash Member and the Brushy Basin Member. These two members were deposited as part of a large and fertile alluvial fan.

The Cretaceous Burro Canyon Formation was formed in stream channels and lakes.

On top of this is the more common Dakota Sandstone. Similar to the Burro Canyon Formation, the Dakota is interfingered with the overlying Mancos Shale. This shale was deposited when marine waters flooded the area. As mud was deposited, the sea floor subsided.

After an unconformity, volcanism returned to the area, and the rock record resumes with the igneous Tertiary West Elk Breccia.. A wide variety of material, mainly various sorts of breccia, formed in great quantities. Shortly after, a number of individual times of volcanic activity deposited five formations of tuffs; Blue Mason Tuff, Dillon Mesa Tuff, Sapinero Mesa Tuff, Fish Canyon Tuff, and Carpenter Ridge Tuff. Ending the volcanism in the tertiary was the basalt flows of the Hinsdale Formation.

Hansen, Wallace R. The Black Canyon of the Gunnison; in depth. Southwest Parks & Monuments Association, 1987.