Heart Mountain Landslide (Subaerial)

The largest known subaerial landslide is the Heart Mountain Landslide in northwestern Wyoming. Although this landslide occurred about 50 million years ago, it was so large that weathering, erosion, and volcanic activity have not yet obscured all of the evidence. The most revealing feature of the slide is a klippe known as Heart Mountain, a massive block of Ordovician- to Mississippian-age limestone that rests atop undeformed rocks of the Willwood Formation that are merely Eocene in age.

The slide occurred when a large slab of Madison Limestone, about 1600 feet thick and over 400 square miles in area, became detached and slid down a gradual slope that had an average slope of less than two degrees. As the limestone slab moved, it broke into many smaller pieces. Today over 100 pieces of the slab are scattered across an area of about 1300 square miles. Some of these blocks are up to five miles across, and many of them have been buried by volcanic material.

Geologists are in general agreement that a slide is responsible for scattering these blocks. However, numerous theories have been proposed to explain how such large blocks of rock could be transported up to 30 miles over a surface with such an insignificant slope. Were these blocks moved by volcanic activity, earthquakes, or simply gravity? Did the movement occur in one episode or in events that were repeated over time?

This is the largest subaerial landslide that has been recognized. Much larger landslides may have occurred at earlier times in geologic history. Any evidence for these slides has either been destroyed or has not yet been recognized.