Denali's Geology: A Jigsaw of Terranes, a 1,300-Mile Fault and Dinosaur Tracks

Mount McKinley — whose traditional name, Denali, means "The High One" — has the greatest vertical relief of any mountain on Earth. And it is still rising. The tectonic forces of southern Alaska keep pushing the Alaska Range up along the Denali fault system, while wind and water wear the heights down. Denali National Park and Preserve protects more than 6 million acres of this restless, varied geology.

Mountaineering rangers on backcountry patrol on Ruth Glacier. Credit: Tucker Chenoweth, National Park Service.

A jigsaw puzzle of terranes

Like much of Alaska, the Denali area is made of accreted terranes. For hundreds of millions of years, the Pacific plate has worked like a conveyor belt, carrying bits of islands, ocean floor and slivers of other continents north and fitting them together like a jigsaw puzzle. Each terrane stands apart from its neighbors, separated by distinct faults, in its composition, structure, degree of metamorphism and fossils. Younger sediments cover them, and igneous intrusions stud them.

The Denali fault system

This great arc of north-bulging faults sweeping across Alaska is one of the state's most distinctive geologic features.

Ice and erosion

Glaciers cover much of the park. Flowing more than 40 km (25 miles) and descending more than 4,500 meters (14,800 feet) from the highest peaks to the lowland hills, they cut through thousands of meters of sedimentary, metamorphic and igneous rock, carving wide U-shaped valleys and carrying off vast amounts of sediment. They leave cirques, hanging valleys, arêtes, braided rivers, glacial lakes, glacial deposits and bergschrunds.

Streams, rivers and runoff carve the land too. Where loose material — slope deposits, glacial till and moraines, river sediments, or altered and deformed bedrock — lies on moderate slopes, erosion and mass wasting become more likely. Some volcanic and sedimentary rocks quickly alter into shrink-and-swell clays that expand when wet and contract as they dry, weakening the rock.

Fossils from warm seas and a polar forest

Many of Denali's rock units hold fossils, mostly from the warm, shallow seas of the late Paleozoic and Mesozoic; some terranes traveled north from lower latitudes where marine life thrived. Invertebrates dominate, along with marine mollusks, insects, plant debris and abundant trace fossils.

The star is the Cantwell Formation, with thousands of trace fossils — tracks and traces of fish, pterosaurs, theropods, hadrosaurs, birds and invertebrates on land and in water. It is one of the best-preserved Late Cretaceous polar continental ecosystems in the global fossil record. All fossils in national parks are protected under the Paleontological Resources Preservation Act of 2009.

Mining's leftovers

Gold was discovered in the area in 1903. Mining focused on the Dunkle Mine area in the Chulitna Terrane, the Mount Eielson/Copper Mountain district in the 1940s, and the placer gold of the Kantishna Hills in the northern foothills, seeking gold and silver, base metals (copper, lead, antimony, zinc, tungsten, arsenic) and coal. Abandoned cabins, debris, tailings, trenches, adits and small shafts remain; most old underground workings have been closed or have collapsed. Such sites can be valuable cultural resources and habitat, but many are dangerous and harm water quality and landscapes — stay out and stay alive.

Maps and inventories

The Park Service's Geologic Resources Inventory, which produces digital geologic maps and reports for more than 270 natural resource parks, has a geologic map and report for Denali, and a Soil Resources Inventory has been completed; both are on the NPS DataStore.

Sources

Based on "NPS Geodiversity Atlas—Denali National Park & Preserve, Alaska," National Park Service Geologic Resources Inventory, which cites Moore et al. (1994), Brease et al. (2009), Norris (1998), Van Maanen and Solin (1988) and Metz et al. (1989); a work of the United States government in the public domain.