Volcanism thumb|Columnar basalt near Tower Fall; large floods of basalt and other lava types preceded mega-eruptions of superheated ash and pumice. thumb|Caldera rim on the horizon south of Yellowstone Lake Yellowstone is at the northeastern end of the Snake River Plain, a great bow-shaped arc through the mountains that extends roughly from the park to the Idaho-Oregon border.
volcanism thumb|columnar basalt near tower fall; large floods of basalt and other lava types preceded mega-eruptions of superheated ash and pumice thumb|caldera rim on the horizon south of yellowstone lake yellowstone is at the northeastern end of the snake river plain a great bow-shaped arc through the mountains that extends roughly from the park to the idaho-oregon border