Heart Mountain is a dramatic, 8,123-foot (2,476-meter) peak just north of Cody, Wyoming, on the floor of the Bighorn Basin. The mountain is composed of limestone formed several hundred million years ago, but it rests on the “Willwood Formation,” rocks that are only about 55 million years old. For older rocks to rest on top of younger rocks is rare; in general, the top layers of a rock formation formed most recently, and the rocks of deeper layers are older. So why is the rock on the summit of Heart Mountain at least 250 million years older than the rock at the base?
The turbulent geologic history of northwestern Wyoming hints at the solution to this geologic mystery. The Madison Limestone that now forms Heart Mountain was laid down on top of layers of dolomite and ancient granite more than 300 million years ago when the area was covered by a large tropical sea. More than 50 million years ago, the Madison Limestone—more than 1,500 feet (460 meters) thick in places—lay about 25 miles (40 kilometers) to the northwest, where the Beartooth Mountains and eastern Absaroka Range now stand.
Around 50 million years ago, a period of mountain-building began along the western edge of the Bighorn Basin, raising the Beartooth Mountains. This mountain-building coincided with a series of eruptions that formed the now extinct volcanoes of the Absaroka Range. The rocks beneath the Madison Limestone were lifted and tilted from west to east. At some point a giant sheet of limestone—perhaps 500 square miles (1,300 square kilometers)—detached and slid southeast towards Bighorn Basin. Scattered remnants of the limestone, such as Beartooth Butte and White Mountain, remained in place. Most of the sheet, such as Sheep Mountain and Heart Mountain, fractured into blocks and came to rest on the younger rocks on the floor of the Bighorn Basin.
Although the general story of the movement of Heart Mountain is known, the specifics remain a mystery. Was the slide triggered by an earthquake or volcanic eruption, or just the steady pull of gravity? Did it occur catastrophically with a sudden collapse, or was it a slow slide over hundreds of thousands of years? Why do the displaced blocks of limestone appear almost unscathed by the effects of their slide? Geologists continue to debate these questions and to develop new theories about how the Earth of the past became the Earth of the present.
This true-color image of Heart Mountain and the surrounding area was acquired on July 24, 2000, by the Enhanced Thematic Mapper plus aboard NASA’s Landsat Satellite.
The Colorado Plateau of Arizona, Colorado, New Mexico, and Utah is made of mostly flat-lying layers of sedimentary rock that record paleoclimate extremes ranging from oceans to widespread deserts over the last 1.8 billion years. Navajo Mountain in southeastern Utah is a dome-shaped chunk of igneous rock that intruded into the sedimentary layers and lifted up the overlying layer. Navajo Mountain is one of several of these rock formations, called laccolith by geologists, in southeastern Utah’s portion of the Plateau. This oblique (from-the-side) astronaut photograph highlights Navajo Mountain in the center of the image, surrounded by light red-brown Navajo Sandstone (also visible in the canyon at bottom of the image). The igneous rock at the core of the mountain is wrapped in sedimentary layers. The peak of Navajo Mountain, at approximately 3,148 meters (10,388 feet) elevation, is comprised of uplifted Dakota Sandstone deposited during the Cretaceous Period (approximately 66-138 million years ago).
Adjoining Galway Bay to the north, the Burren Plateau (Burren is Gaelic for “stony place”) is an example of karst terrain. Karst terrain is generally formed when sedimentary rocks are dissolved by groundwater. This astronaut photograph illustrates the northwestern-most portion of the Burren Plateau, which is characterized by the distinctive bare exposures of almost horizontal, layered Paleozoic-age limestone rocks that form Gleninagh Mountain.