Yellowstone River Overlook - Yellowstone National Park

In cool weather, wisps of steam rise from Calcite Springs—the pale slope near river level. There the Yellowstone River flows above a volcanic fracture zone that allows geothermal discharge to reach the surface. Chemicals from Calcite Springs’ hot water vents are slowly turning the cliff to whitish, yellowish pulp.

Away from the vast geyser basins, look for subtler patterns of thermal activity. Bare, bleached patches of ground—signs of hot springs and steaming earth—often lie hidden in lodgepole forest and along stream banks.

River canyons expose the landscape’s hidden excesses, its violent past. The orderly columns across canyon are volcanic. From vents in the Yellowstone Plateau lava welled up and flowed—a vast flood of fire 25 feet deep. As the lava cooled and contracted about 1.3 million years ago, it formed contraction cracks, producing hexagonal columns of basalt.

The rock layers are a geologic photo album. Lava flows and other dynamic events have become benign scenic features, frozen in time. Above and below the basalt lies a loose mix of gravel carried here by glacial meltwater.

A similar basalt formation can be seen from a pullout 0.5 miles south of here.