Basaltic flows and dikes of Tower Falls in the Yellowstone Caldera.

The first seven photos show details of a entablature/colonnade jointed lava flow similar to the Bend, Oregon and Iceland examples.  It could be that a water flood affected the flow during its solidification.

TWO LEDGES OF BASALT spectacularly exposed in the east wall of the Grand Canyon of the Yellowstone at The Narrows near Tower Falls. Pronounced columnar jointing of the basalt is seen at close range (bottom photograph) at the edge of the road on the opposite (west) side of the canyon. Inset shows the dense character of the black basalt, which consists of microscopic crystals of feldspar, pyroxene, olivine, and magnetite. The light-colored rocks between the basalt flows (last two photographs) are ancient stream gravels deposited about 1-1/2 million years ago, when the channel of the Yellowstone River was farther east and not as deep as it is today. The hill is capped by lake sediments, sand, and gravel deposited when the Yellowstone River was blocked by a glacial dam farther downstream (to the left). The brown rocks at the base of the cliff are Absaroka andesite breccias.  (Text from USGS Bull 1347)