 Sandstone is the dominant rock type, as a remnant of the ancient sea floor.Slopes within the headwaters area commonly range from 15 to 30 percent, and lower reaches have gradients of two to five percent.One of the major soil associations within the upper elevation riparian zone is Goulding cobbly clay loam, which contains roughly 25 percent cobblestones, as well as some basaltic exposures, betraying the volcanic past of the Sonoma Mountains formation.