The Soda Dam, along NM state road 4, is a large hot spring deposit, made mostly of calcium carbonate. The springs used to empty out onto the dam over the Jemez River, but most of the water now exits from the roadcut across the road. Courtesy of the NM Department of Transportation. The Soda Dam hot spring waters start at the Valles Caldera, heated by the hot rock and magma beneath. They travel as hot ground water through Pennsylvanian limestones and shales, and rise to the surface along a fault at the Soda Dam. Most of the carbonate rock of the Dam bears traces of algal filaments, and Soda Dam rock has been used in studies of how ancient Martian life might be detected.