As in many hydrothermally active areas, the rocks at Sulphur Works and Little Hot Springs Valley in Lassen Volcanic National Park have been chemically altered into bright-colored clays. Sulfurous acid and sulfuric acid have broken down hard, gray-green andesite lavas into red, yellow and buff clays and iron oxides. Many visitors pass through Sulphur works on their way north on State Route 89 and sense the rotten egg smell (hydrogen sulfide) when they pass by a hot vent to the east of the road. Sulphur works is said to be the volcanic center of the ancestral Mount Tehama.