The Color Is Geology

Thomas Moran had never seen anything like it. He was standing at the rim of a canyon as much as a thousand feet deep, looking at walls that were yellow and orange and red, and he understood immediately that he was looking at something no painting had ever attempted. He sketched for two weeks in September of eighteen seventy-one as part of Ferdinand Hayden's government survey. The paintings he made afterward were hung in the United States Capitol. Congress looked at those paintings and voted to create Yellowstone National Park about six months later.

The colors are geology. The canyon walls are rhyolite, volcanic rock, the same material the Yellowstone hotspot has been extruding for millions of years. But rhyolite in its natural state is gray. What makes these walls yellow and orange is hydrothermal alteration: hot acidic water and steam, rising through the fractured rock for thousands of years, oxidizing the iron minerals inside. The canyon walls are being chemically transformed from the inside. What you see is a staining that goes all the way through.

The Lower Falls that drop into the head of the canyon fall three hundred eight feet, nearly twice the height of Niagara. The Yellowstone River carved the canyon by exploiting that same hydrothermal weakness: the altered rock is softer than unaltered rhyolite, and the river cut through it faster, leaving the harder canyon rim standing above.

Artist Point, where you are standing or will stand, is where Moran set up his easel. He was not painting a scene. He was making an argument that wilderness had value. Congress agreed. The painting is in the collection of the United States Department of the Interior. The original is the canyon.