Clear Lake is believed to be the oldest warmwater lake in North America, due to a geological fluke. The lake sits on a huge block of stone which slowly tilts in the northern direction at the same rate as the lake fills in with sediment, thus keeping the water at roughly the same depth. The geology of the county is chaotic, being based on Franciscan Assemblage hills. Numerous small faults are present in the south end of the lake as well as many old volcanoes, the largest being Cobb Mountain. The geologic history of the county shows events of great violence, such as the eruption of Mount Konocti and Mount St. Helena and the collapse of Cow Mountain, which created the hills around the county seat of Lakeport. Blue Lakes, Lake Pillsbury, and Indian Valley Reservoir are the county's other major bodies of water.