Orange Spring Mound

Orange Spring Mound is a rounded hot spring mound along the Upper Terrace Drive at Mammoth Hot Springs. Water issues from several openings at its top and sides, leaving pale travertine streaked with orange and brown. This is a brief roadside stop, not a hike: the short boardwalk reaches an interpretive exhibit and gives you a close look at the layered face and water at its base. Wheelchair-accessible parking is provided at the mound. Stay on the boardwalk; hydrothermal water can severely burn.

The 1.5-mile Upper Terrace Drive is one-way, narrow and has limited parking. Buses, motor homes and trailers can't drive it; large vehicles should park at the entrance and visit on foot. In winter the route becomes a groomed cross-country ski trail.

Orange, brown and green patches come from thermophiles living in cooler water; the hottest water supports colorless and yellow forms. Look where flows leave the vents and trace the bands across the mound rather than expecting one uniform color.

Water moving through limestone picks up calcium carbonate. At the surface, carbon dioxide escapes and the mineral is left behind as travertine. The deposits form quickly enough for Mammoth's terraces to change over time. Fractures and fissures carry the hot water upward, and small earthquakes may help keep those pathways open. The heat source is still uncertain. Scientists have proposed either Yellowstone's large magma chamber or a smaller source nearer Mammoth.

For hundreds of years, Shoshone and Bannock people collected minerals from Mammoth's terraces to make white paint. The mound belongs to a landscape with a long human history as well as an active geological process.