PROCEEDINGS OF THE CALIFORNIA ACADEMY OF SCIENCES 
Series 4, Volume 64, Supplement 1 
Figure 4. A caver rappels into Fossil Cave, Lava Beds National Monument. The lava tube has a moist microclimate that 
sustains ferns in the entrance. Jean Krejca and Steve Taylor. 
About 90% of the lava in LABE is basaltic, a common extrusive igneous rock formed from the 
rapid cooling of lava at or very near the surface (Lynn 2014; Bunnell 2013), although some GIS 
geological coverage specifies dacite in many areas. 
Lava tubes related to Lassen Peak (Mount Lassen) occur south of LABE in eastern Shasta, 
Tehama, northern Plumas, and Lassen counties in Lassen National Volcanic Park, Lassen 
National Forest and adjacent lands. Lassen Peak, southeastern Shasta County, is quite large, and 
it is the southernmost active volcano in the Cascade Range. The Hat Creek Lava Area in eastern 
Shasta County is an important focus of lava tubes (map, Fig. 5). Subway Cave is less than 2,000 
years old. 
We have data on 358 lava tubes in this region. Table 1 lists 165 lava tubes and 1 nonlava cave 
in the region that were biologically sampled. The region has 5 single-site endemics. Other, more 
isolated lava tubes exist in southeastern Sierra Nevada South and the Mojave Desert. 
Cave Region 3 — Sierra Nevada North 
We define this important region as southern Plumas County in the north to the Tuolumne- 
Mariposa county line in the south. Caves are developed in the Calaveras Formation or Assemblage, 
a highly metamorphosed marble of Permian age dating to between (299 and 252 Ma) in its upper 
part; it is strung out in many outcrops along the west flank of the Sierra. The Calaveras is often 
referred to as a “melange” because it contains many types of rock. The oldest carbonate rocks can 
be found along the easternmost portions of this region, closer to the Sierra Nevada summits. These 
consist of carbonate rocks from ancient islands, accreted onto the western edge of North America 
during the late Permian about 275 Ma. The western carbonate rocks of the region are significantly 
younger, from the late Jurassic Period about 150 Ma. 
