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Heart Mountain   (Explored Sep. 28, 2016) | by Tim Lumley
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Heart Mountain (Explored Sep. 28, 2016)

The eastern view across the Bighorn Basin from just inside Shoshone National Forest. Heart Mountain is the distant peak in the center of the frame. The Absaroka Mountains are behind me, due west, and beyond them is Yellowstone NP.


The Bighorn Basin is an intermontane (surrounded by mountains) plateau in north-central Wyoming approximately 100 miles wide. It is bounded by the Absaroka Mountain Range on the west, the Bighorn Mountains on the east, and the Owl Creek Mountains and Bridger Mountains to the south. The region is semi-arid, receiving only 6–10 in of rain annually, although a bit more than 100,000 acres is sufficiently irrigated from the nearby Buffalo Bill Reservoir to farm sugar beets.


Heart Mountain is an 8,123-foot klippe just north of Cody, Wyoming. The mountain is mostly limestone and dolomite that’s approximately 350 million years old, but it sits on base rock that is only 55 million years old. So, somehow the rock at the summit of Heart Mountain is almost 300 million years older than the rock it sits on???


The rock that formed Heart Mountain was deposited on an ancient seabed of 2.5 billion year-old granite while the area was covered by a large, shallow tropical sea. Up until about 50 million years ago, these rocks lay about 25 miles to the northwest, where the eastern Absaroka Mountain Range now stands. Geologists think that between 50 and 75 million years ago (sometimes geologist are not very precise) a period of tectonic plate activity uplifted the nearby Beartooth Mountain Range to the northwest and caused the adjacent Bighorn and Absaroka Basins to subside. This was followed by volcanic eruptions which over the millennia formed the now extinct volcanoes of the Absaroka Mountain Range just south of the Beartooths. A couple of million years later things started to get interesting when a 500 square mile sheet of rock approximately two to three miles thick detached from a gently sloping plateau along the edge of the newly formed mountains and slid into the Bighorn and Absaroka Basins pushing a large mass of rocks in front of it. Although the slope was less than 2 degrees, the landslide traveled at least 25 miles and the slide ended up covering over 1,300 square miles. This is by far the largest rockslide known to have occurred on land and is comparable in scale to some of the largest known submarine landslides. Geologist still don’t know for sure what caused this huge slab of rocks to start sliding and what allowed it to slide so far on such a gradual slope. However, there seems to be consensus that it happened very rapidly with the front of the sliding mass accelerating to over 100 miles/hour; meaning that the mountain traveled to its present location in approximately 30 minutes.


In the 48 million years since the slide occurred, erosion has removed most of the slide sheet that moved into the Bighorn Basin, leaving just one big block of carbonate rock— Heart Mountain.


Shoshone National Forest, Wyoming, USA. Elevation: 6,674 ft. September 21, 2015.

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Taken on September 21, 2015