Geodiversity is the full variety of a park's natural geologic and soil resources and processes — rocks, minerals, sediments, fossils, landforms and the processes that shape them. The NPS Geodiversity Atlas, a product of the Geologic Resources Inventory, supports education, geoconservation, and management of a park's living and non-living parts together. This is its page for Black Canyon of the Gunnison National Park, Colorado.
The Geologic Resources Inventory report for the park.
The canyon
The Black Canyon, the park's namesake, dominates the landscape. Unlike most canyons in the western United States, it is in many places deeper than it is wide; its average depth is 610 m (2,000 ft).
The Gunnison River is unusually steep through the gorge. It falls about 655 m (2,150 ft) from the canyon head at Sapinero to its mouth at the North Fork, just 80 km (50 miles) away — an average of 13 m (43 ft) per mile (Hansen, 1965). The Colorado River in the Grand Canyon drops an average of only 2.3 m (7.5 ft) per mile.
How it formed
Erosion dug the canyon, but it took a coincidence of other factors to remove more than 25 cubic miles of rock along so narrow a course through very hard metamorphic rock:
- Before the Gunnison Uplift, the river ran through relatively soft Mesozoic and Cenozoic volcanic and sedimentary rock.
- Its present path follows that old course: because the Gunnison Uplift came relatively fast, the course was impressed onto the hard Precambrian rock beneath.
The river still works today: it carries mud and debris, rockfalls come off the high cliffs, and landslides move relentlessly into the depths. Subtler processes — gullying, frost action and chemical weathering (acid rain) — loosen more material for the river to wash away.
What makes it special
- One of the best exposures in the world of ancient Precambrian rock — nearly 2 billion years old.
- Spectacular dark volcanic and metamorphic rock, deeply cut by the Gunnison.
- Pink pegmatite dikes, with crystals up to 6 feet long, criss-cross the walls.
- One of Colorado's most spectacularly exposed faults, the Ute Indian fault, formed during the Laramide Orogeny, a period of great mountain-building.
Chasm View, Black Canyon of the Gunnison. NPS photo by Lisa Lynch.
The rocks, oldest to youngest
| Age | Rocks |
|---|---|
| Precambrian | the steep "black" walls: granites, gneisses, gabbros, diorites, pegmatites and schists — the gneiss and schist 1.7 billion years old |
| Paleozoic | none: this gap, an unconformity, means about 370 million years of Earth's history are missing here |
| Jurassic | Entrada Sandstone, Wanakah Formation, Morrison Formation — their fossils and structures record a wide range of Jurassic climates and environments |
| Cretaceous | Dakota Sandstone and Burro Canyon Formation (sandstones, shales, coal beds); Mancos Shale muds; Cimarron Ridge Formation volcanic breccia; Fruitland Formation and Pictured Cliffs Sandstone |
| Tertiary | at the end of the Laramide orogeny, eruptions laid down ash, tuff and breccia that now cap the mesas — West Elk Breccia, Blue Mesa Tuff, Dillon Mesa Tuff, Carpenter Ridge Tuff |
| Quaternary | silt, sand and gravel as terraces, alluvium and colluvium — a fraction of what once covered the area before erosion stripped it away |
Fossils
The park has Jurassic and Cretaceous fossil plants, invertebrates, fragmentary vertebrates and trace fossils. The fossil-rich Morrison Formation is exposed in the Black Canyon–Curecanti area, and dinosaur fossils are still found in it locally; the Mancos Shale and Dakota Sandstone also hold fossils that should be catalogued. All NPS fossils are protected by the Paleontological Resources Preservation Act of 2009.
Caves
There are no known caves, but small caves and shelters could exist in the Jurassic and Cretaceous sandstones, siltstones and shale, which also have minor karst potential. NPS caves are protected by the Federal Cave Resources Protection Act of 1988.
Region, maps and reports
The park lies in the Colorado Plateaus province, near its indistinct boundary with the Southern Rocky Mountains to the east.
The Geologic Resources Inventory makes scoping summaries, digital geologic maps, reports and posters for more than 270 natural resource parks; this park's are in the NPS DataStore, along with its completed Soil Resources Inventory. More: the park's geologic story, minerals and dimensions, and the Colorado Geological Survey.
Sources
Based on "NPS Geodiversity Atlas—Black Canyon of The Gunnison National Park, Colorado," National Park Service, citing Hansen (1965); a work of the United States government in the public domain. The source gives the canyon's length, narrowest width, greatest depth and the river's gradient in figures that disagree with one another (50 or 53 miles; 1,100 or 1,750 ft; 43 or 96 ft per mile), and appears to confuse width with depth; only the Sapinero-to-North-Fork figures, which it ties to a measured stretch and a source, are kept.
In these publicationsBlack Canyon Of The Gunnison National Park
Licence: CC0 1.0 (public domain) · Adapted from www.nps.gov
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