Lewis and Clark Caverns: The Cave That Waited Fifty Million Years to Be Found

Lewis and Clark Caverns State Park in southwestern Montana contains the largest known limestone cave system in the Northern Rockies. The name is an error of geography: Lewis and Clark's expedition route passed several miles away, and neither man entered or apparently knew about the cave. The name attached to it in the early twentieth century because the expedition was the most famous thing associated with this region, and the cave needed a name that would sell tickets.

The cave was known to white settlers in the area by the eighteen nineties, and various groups explored portions of it before it became a state park in nineteen forty-one, Montana's first. The limestone in which the cave formed was deposited roughly three hundred fifty million years ago in a shallow sea that covered much of the region during the Mississippian Period. The cave itself formed through a process of dissolution, as slightly acidic groundwater percolated through cracks in the limestone and dissolved the rock away over millions of years, eventually creating interconnected chambers.

The cave is within the Custer Gallatin National Forest's broader administrative region, though the state park itself is state-managed. The Jefferson River, visible from the road approaching the cave entrance, was one of the rivers Lewis and Clark followed on their westward passage in eighteen oh five. The expedition journals document the Beaverhead Rock landmark just south of here, a distinctive outcropping that Sacagawea recognized as the territory of her birth, confirming to the captains that they were in Shoshone country and on the right path toward the Continental Divide.