The Geology of Colorado National Monument: Red Cliffs, Spires and a Great Unconformity

Colorado National Monument is one of the oldest National Park Service sites on the Colorado Plateau. President William Howard Taft proclaimed it under the Antiquities Act in 1911 for its remarkable eroded landscapes. It sits at the northeastern end of the Uncompahgre uplift and rises about 2,000 feet (600 m) above the Grand Valley. Mesozoic sedimentary rocks form the walls of canyons cut into the edge of the Uncompahgre Plateau, and much older Precambrian metamorphic rocks lie exposed in the canyon floors.

A small park with the plateau's greatest hits

At 32 square miles (83 km²) the monument is small, but it shares features with other parks of the region:

• miles of red cliffs of Wingate Sandstone and the Kayenta Formation, like those in Canyonlands;
• the Chinle Formation of Petrified Forest, the Entrada Sandstone of Arches and the Morrison Formation of Dinosaur;
• a Great Unconformity like Grand Canyon's;
• uplift along a monocline, as at Capitol Reef;
• dry-land landforms such as Independence Monument.

Wingate Sandstone and the Chinle Formation above the Precambrian basement. National Park Service.

The 23-mile (37-km) Rim Rock Drive follows the canyon rim. Championed by the monument's founder and first superintendent, John Otto, it was built largely with hand tools, partly by the Civilian Conservation Corps, and is on the National Register of Historic Places.

The Great Unconformity

The monument is one of the best places on the Colorado Plateau to see Precambrian basement rock, like that in the Inner Gorge of the Grand Canyon and in Black Canyon of the Gunnison. It is schist, gneiss, pegmatite and igneous dikes. The metamorphosed igneous and sedimentary rocks formed between 1,741 and 1,721 million years ago, and lamprophyre dikes — a mafic intrusive rock very high in potassium — cut through them about 1,400 million years ago.

Directly on top of this basement lies the Chinle Formation. The gap between them — an unconformity, a missing stretch of the rock record — spans 1,500 million years, about a third of Earth's history. John Wesley Powell named the equivalent surface in the Grand Canyon the Great Unconformity. Here it covers more time than there (1,200 million years or less), and the park road passes it in several places, though it is poorly exposed because the Chinle wears away easily.

The contact between the Precambrian basement and the sedimentary rocks above. National Park Service.

There are no Paleozoic rocks here. In the Late Paleozoic (320–280 million years ago) the Uncompahgre Plateau was the westernmost of the Ancestral Rocky Mountains, shedding its sediment into the Paradox Basin and the Central Colorado Trough. Only after those highlands had been worn down did the area become a basin again and begin collecting the Chinle.

The layer cake

All of the monument's sedimentary rocks are Mesozoic, from the Late Triassic through the Cretaceous. From oldest to youngest:

Two names are disputed. Some geologists assign the Wanakah to the lower Summerville Formation, and the Tidwell Member is placed in the Morrison by some and the Summerville by others. And because the Dakota Sandstone was first described along the Missouri River, on the far shore of the Western Interior Seaway, the North American Stratigraphic Code does not allow these unconnected rocks the same name; some stratigraphers use "Naturita Formation" instead.

The Mancos Shale is not exposed in the monument but is in the neighboring Grand Valley and the Book Cliffs to the north: marine shale from the Western Interior Seaway, 95–85 million years old and more than 4,000 feet thick in western Colorado.

Fossils

Almost every sedimentary unit in the monument holds fossils; only the Dakota (Naturita) and the Entrada have none recorded inside the boundary, though the Dakota may and the Entrada shows bioturbation. The Morrison's record is particularly significant: within the monument it has yielded dinosaur bones and tracks, other vertebrates, petrified wood and other plants, and invertebrates. The Chinle has root traces and rare vertebrates, the Wingate has tracks of dinosaurs and other vertebrates, and the Burro Canyon has dinosaur bone and petrified wood. All fossils in national parks are protected under the Paleontological Resources Preservation Act of 2009.

Uplift

The monument's sedimentary rocks were laid down at or near sea level, but now stand as high as 7,000 feet (2,100 m). The Colorado Plateau has been raised several times:

• The Laramide orogeny, 70–40 million years ago, did most of it. A thick oceanic plateau within the subducting Farallon plate made it descend at a shallow angle, so mountain-building reached far inland. The Laramide built the Rocky Mountains and the Uncompahgre uplift but left the plateau itself largely undeformed. A monocline along the Redlands Fault lifted the northeastern Uncompahgre Plateau in the monument.
• The mid-Cenozoic uplift, 35–25 million years ago, when hot mantle flowing in above the sinking Farallon slab raised the surface.
• The last 10 million years, from complex mantle processes after Farallon subduction — with signs of up to 2,100 feet (640 m) of further differential uplift of the Uncompahgre Plateau in roughly the last 3 to 4 million years.

The Redlands Fault Complex — three faults and two monoclines at the northeastern end of the uplift — is in the U.S. Geological Survey's Quaternary Fault and Fold Database; the uplift's faults are generally thought to have moved in the Quaternary.

Carving the canyons

The landscape is geologically very young, shaped within the last few million years. Across the plateau, up to 5,000 to 13,000 feet (1,500 to 4,000 m) of rock has been stripped from the center, less toward the edges.

Differential erosion did the sculpting. Hard layers such as the Wingate and Kayenta wear away slowly, soft ones such as the Chinle quickly, so the hard layers cap mesas, buttes and spires and shield the rock beneath. As the cliffs retreat, side canyons eat headward into the nearly flat-lying plateau, leaving ridges between them, and in time pieces of those ridges are cut off as free-standing towers.

Independence Monument. NPS photo.

Independence Monument, about 450 feet (140 m) tall, is the largest spire in the park. John Otto climbed it on July 4, 1911, beginning a tradition that continues every year.

Water, crusts and varnish

• Biological soil crusts — communities of cyanobacteria, mosses, lichens, algae, fungi and bacteria — fix nitrogen and hold soil in place. A footprint can destroy them, and repair can take decades. Stay on roads and trails; off trail, walk on bare slickrock or in dry washes.
• Springs and seeps, and the hanging gardens they feed, are rare, reliable water. They emerge along canyon walls near the base of the porous Wingate, or from fractured Precambrian rock and alluvium in canyon bottoms.
• Desert varnish, clay minerals with iron and manganese oxides, coats stable sandstone; it shows best as brown-to-black streaks down the Entrada and Wingate cliffs.

Hazards

Rockfall and other slope failures, and flash floods during heavy rain, are the main geologic hazards. Colorado National Monument has a moderate seismic hazard: the USGS 2014 hazard map gives a 2% chance in 50 years of ground shaking above 12–14% of gravity, about intensity VI on the Modified Mercalli scale. The Park Service has recorded nine abandoned mineral land sites — hard-rock mines or borrow pits — with hazards from radon to unstable old explosives; at the Kodel Gold Mine it backfilled the steep shafts while keeping the historic site.

Part of the monument's digital geologic map. National Park Service, Geologic Resources Inventory.

The Geologic Resources Inventory's map and report, and a soil survey, are in the NPS DataStore.

Sources

• National Park Service: "NPS Geodiversity Atlas—Colorado National Monument, Colorado." The atlas gives the Chinle Formation's age as about 220 million years in one place and about 205 million years in another, and the Burro Canyon Formation's thickness as 130 ft (29 m), which do not agree, so none of these figures is given here. A sentence about earthquake shaking near Capitol Reef National Park, apparently carried over from another park's atlas, is left out, as is a garbled remark about which mining features need mitigation. "Independent Monument," "burrow pits," "Uncompaghre" and "Civilian Conservation Corp" are corrected.