Yellowstone is full of volcanic features, including this columnar basalt at Sheepeater Cliff (named for Shoshone Indians who raised bighorn sheep and who sometimes used this area). The columns formed during the cataclysmic volcanic activity of around half a million years ago, which created many of the distinctive features of the park. Yellowstone has several such basalt formations.
Marmots live among the columns:
Columnar basalt is extremely fascinating -- we saw perhaps the most spectacular example a couple of years ago at the Giant's Causeway in Ireland, where the tops of the columns form what appears to be a tiled pavement made by giants: