Pre-Quaternary, igneous and sedimentary rock compose the Yonna Formation, which crosses much of the region and rises above the surface in large outcroppings of solid rock in many of the ridges. Underlying rocks are generally younger from east to west. The many ridges crossing the upper Klamath Basin divide it into valleys with up to of vertical relief, and drainage patterns generally follow the topography. An extensive geothermal system occurs deep underground within the upper basin, creating hot springs and artesian springs, but is not well understood. Further south, in the Shasta River area in Siskiyou County, much of the underlying rock is composed of lava flows issuing from the Mount Shasta volcanic region.