 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.Typical soil depths in the headwater area range from 14 to 20 inches (35 to 50 centimeters).Much of the soil type in the Yulupa Creek riparian zone consists of Laniger loam, with rhyolite outcrops, another relic of the igneous history.