Fossil Springs, the source of about 80 percent of the water in Fossil Creek, issues from several vents in close proximity to one another at the base of the Mogollon Rim. The rim is the southern edge of the Colorado Plateau. Water percolating through limestone layers in the rim becomes laden with dissolved carbon dioxide and calcium carbonate. When the water emerges at the springs, it begins to release carbon dioxide as a gas and calcium carbonate as travertine, a porous form of calcite often found around hot springs such as Mammoth Hot Springs in Yellowstone National Park. The release, affected by stream turbulence and other factors, does not occur all at once but continues downstream.