Fossil Falls in eastern California’s Coso Range was formed when glacial meltwater from the Sierra Nevada was diverted across basalt lava flows, carving a waterfall, polished channel, and large potholes into volcanic rock. Lava from the Coso Volcanic Field and Red Hill spread across the area between about 400,000 and 10,000 years ago, and runoff during the last ice age, especially the Tioga glaciation, created much of the erosion visible today. The site also preserves evidence of long human use, from Native American camps and obsidian toolmaking to later desert-adapted lifeways, and it is recognized for both its geological and archaeological importance.