In part a product of the world's largest terrestrial landslide, the 8,123-foot Heart Mountain stands about 3,400 hundred feet above the surrounding Bighorn Basin. It came about as the result of a landslide of carbonate rock that was laid down aout 350 to 500 million years ago over 2.5 billion year old granite about 25 miles to the north. When mountain building in that area caused uplifting, accompanied by subsidence of the Bighorn and Absaroka Basins, a large mass of the carbonate rock detached and slid down a slope of less than 2 degrees at speeds up to hundreds of miles per hour.
For over 100 years geologists have been trying to work out, first, how 350 to 500 million year old rock came to be resting upon the 55 million year old rock of the basin, and second, the mechanics of how it arrived at that spot. Heart mountain consists of the 350 to 500 million year old rock which slid over the basin. The rock surrounding it in the Bighorn Basin has been eroded away, leaving Heart Mountain standing alone, like a sentinel, in the basin.
More detail on Heart Mountain and the phenomena which contributed to its existence follows. An excellent spot from which to view Heart Mountain is at the parking coordinates given below, at the
Heart Mountain Relocation Center, about nine miles to the east.
HEART MOUNTAIN
Attraction
Largest terrestrial landslide in the world, memorable summit hike with spectacular views of the Bighorn Basin. This feels like Wyoming with antelope, changing weather, views, geology and solitude. This trip can be combined with a visit to the Heart Mountain Relocation National Historic Landmark, a World War II Japanese-American internment camp.
Geology of Heart Mountain
Heart Mountain is a prominent landmark 10 miles north of Cody. It also happens to be part of the world’s largest terrestrial landslide that geologists call the Heart Mountain Detachment (HMD). It occurred about 49 million years ago.
The white carbonate rocks on the top are blocks of Paleozoic Madison and Bighorn formations. They were originally deposited in a shallow sea about 350 and 450 million years ago, respectfully, about 30 miles away near the northeastern corner of Yellowstone National Park. Before movement, the Heart Mountain block covered an area of about 450 square miles and almost 5,000 feet thick, weighing trillions of tons. It detached and moved catastrophically down a 2 degree slope and over Dead Indian Hill breaking apart as it slid. Heart Mountain is today located along the structural axis of the Bighorn Basin. Other remnants of the HMD are found dispersed in an area over 1,300 square miles (see figure below). With the maximum velocity of the slide estimated to be from one-third to just under the speed of sound (767 mph), the entire process could have been completed in minutes to a few hours.
At the time of the detachment, Wyoming was in the last stages of the Laramide mountain building episode and at the start of Eocene volcanic activity. The Heart Mountain Detachment has been interpreted as a large scale collapse feature from the flank of an active volcanic center. Two ancient extinct volcanoes in the area have been suggested as candidates for triggering the HMD: the Crandall (50.1-49.2 Ma) and/or the Sunlight (49.6-48.1 Ma) vent complexes.
For over a century geologists have been trying to understand the emplacement mechanism for the HMD that allowed it to move on such a flat surface. Recent work has focused on release of gas (CO2) along the detachment surface that allowed it to move like a hovercraft. Whatever the mechanism, it left a true geological enigma.
?Just west of the Buffalo Bill Dam, the mountains either side of the North Fork of the Shoshone River, are capped with Paleozoic carbonates of the HMD. This view of the Sheep Mountain on the south side of the highway to Yellowstone and Logan Mountain on the north side of the highway shows the enormity and power of the earth’s largest subaerial landslide.
From Geo Wyoming