Columns of basaltic lava at Sheepeaters Cliff, Yellowstone
Columns of the Sheepeaters Cliff represent cooling fractures that commonly develop in thick lava flows. As hot lava cools, it contracts, often forming a regular set of joints perpendicular to the cooling surfaces (top, bottom, and sides). The joints may produce regular polygonal columns, plates, or irregularly shaped fragments. Such cooling features also develop in welded tuffs.