Lava Beds National Monument covers 72 square miles within Siskiyou and Modoc counties in northeastern California. The Monument is the site of the largest concentration of lava tube caves in the United States. The monument lies on the northeast flank of the Medicine Lake shield volcano, the largest volcano in the Cascade Range. The region in and around the monument is unique because is lies on the junction of the Sierra-Klamath, Cascade, and Great Basin geological provinces. In addition, the monument is geologically outstanding because of its great variety of "textbook" volcanic formations; i.e., lava tube caves, fumaroles, cinder cones, spatter cones, maar volcanoes, and lava flows. Over 30 separate lava flows located in the park range in age from 2,000,000 years BP to 1,110 years BP. Some of the major Lava Flows within Lava Beds National Monument include: Callahan Flow, Schonchin Flow, Mammoth Crater Flow, Modoc Crater Flow, and Devils Homestead Flow. In addition to these surface geologic resources , the monument has a significant and popular underground resource. Presently over 460 lava tube caves and features have been identified in the monument. The field inventory process for cave resources is not yet complete. These caves are notable in their abundance, length of passage, and excellent preservation of primary volcanic features; several also contain specific features of interest such as pictographs, ice formations, and biological resources of scientific interest.