Huh. Hadn’t heard of this before now.

https://en.wikipedia.org/wiki/Heart_Mountain_(Wyoming)

Immediately following this period of mountain-building, volcanic eruptions began to form the now extinct volcanoes of the Absaroka Range that lie to the south of the Beartooths and extend into Yellowstone National Park. Between 50 and 48 million years ago a sheet of rock about 500 square miles (1,300 square kilometers) in area detached from the plateau south of the Beartooths and slid tens of kilometers to the southeast and south into the Bighorn and Absaroka Basins.[8] This sheet, consisting of Ordovician through Mississippian carbonate rocks and overlying Absaroka volcanic rocks, was probably originally about 4 to 5 kilometers (2.5 to 3.1 mi) thick. Although the slope was less than 2 degrees, the front of the landslide traveled at least 25 miles (40 km) and the slide mass ended up covering over 1,300 square miles (3,400 km2). This is by far the largest rockslide known on land on the surface of the Earth and is comparable in scale to some of the largest known submarine landslides.[9]

Many models have been proposed to explain what caused this huge slab of rocks to start sliding and what allowed it to slide so far on such a low slope, fragmenting, thinning and extending as it went. Most geologists[who?] who have worked in the area agree that Absaroka volcanism played a role in the sliding and many suggest that a major volcanic or steam explosion initiated movement. Another model involves injection of numerous igneous dikes with the resulting heating of water within pores in rocks causing an increase in pressure which initiated sliding. Some geologists have suggested that hot pressurized water (hydrothermal fluids), derived from a volcano which sat north of Cooke City, Montana, effectively lubricated the sliding surface. Another possibility is that once the slide was moving, friction heated the limestone along the sliding surface, creating pseudotachylite,[10] which then further broke down to calcium oxide and carbon dioxide gas (or supercritical fluid).[9] The gas supported the slide in the way that air pressure supports a hovercraft, allowing the slide to move easily down the very low slope. When the rockslide stopped, the carbon dioxide cooled and recombined with calcium oxide to form the cement-like carbonate rock now found in the fault zone. The consensus favors catastrophic sliding and calculations suggest that the front of the sliding mass may have advanced at a speed of over 100 miles per hour (161 km/h), meaning that the mountain traveled to its present location in approximately 30 minutes.[11]