The McCullough Range preserves a unique record of Miocene volcanism in the western Lake Mead area of Nevada. The basal part of the volcanic section is composed of interbedded basalt and dacite of the McClanahan Spring, Cactus Hill, and McCullough Wash volcanoes (Eldorado Valley volcanic section), and the Colony volcano, which is age-equivalent to, but does not crop out within, the Eldorado Valley volcanic section (18.5–15.2 Ma). These units lie on Precambrian basement and locally on the Peach Springs Tuff (18.5 Ma). Over 400 m of andesite lava, agglomerate, and breccia of the Farmer Canyon volcanic section forms the McCullough stratovolcano. Eruptions occurring after 15.2 Ma were lower in volume and are mainly present on the flanks of the McCullough stratovolcano. These include the eruption of (1) the McCullough Pass caldera and outflow tuff (14.1 Ma), (2) Hidden Valley andesite, including 300 m of andesite lavas erupted from local centers (mainly cinder cones), (3) four Sloan volcanoes on the west flank of the McCullough stratovolcano (Mount Ian, Mount Sutor, Center Mountain, and Mount Hanna) (13.1 Ma), and (4) the Hender son dome complex on the northern flank of the McCullough stratovolcano. The volcanic rocks in the McCullough Range are calc-alkaline and vary in composition from rhyolite to basalt. Intermediate compositions (andesite and dacite) prevail, while basalt and rhyolite are rare. The trace-element signature (low Nb, Ti, Zr, and P compared to primitive mantle) is an indication of either a magma source in the continental lithosphere or lithospheric contamination. Rhyolite and dacite probably formed by partial melting of crust, while mafic magmas (basalt and andesite) either originated by melting of lithospheric mantle or reflect asthenospheric magmas contaminated in the lithosphere.

The eastern flanks of Mount Elgon, an early Miocene stratovolcano, host caves (∼150 m long, ∼60 m wide, ∼10 m high) of debatable origin. Many animals, primarily elephants, “mine” the pyroclastic bedrock for sodium-rich salts. Speleogenesis has been argued to be primarily zoogeomorphic, or primarily dissolutional with only minor zoogeomorphic modification. This report provides the first detailed mapping and geomorphological study of the caves. Speleogenesis is polygenetic and strongly related to lithology. Geological units are, from the top down, ∼2 m of dense pyroclastic agglomerate cap rock over which water falls, ∼10 m of more permeable agglomerate, up to ∼0.2 m of discontinuous impermeable lava, ∼2 m of very soft and permeable agglomerate, and >2 m of impermeable swelling-clay tuff. Caves develop behind waterfalls under surface stream valleys by sapping of the incompetent agglomerate above the clay, and failure of the clay (aquiclude and base level for speleogenesis), followed by collapse of harder agglomerate layers above. The dominant passage shape is breakdown dome, with abundant fresh collapse. Geophagy by elephants and other species, and human mining significantly modify and enlarge the caves and remove collapse debris. These activities, focused on accessible and salt-rich units, create quasi-horizontal undercuts (up to ∼4 m tall and deep), the loci of which move upward as collapse raises the floor. Significant erosion also occurs by incongruent dissolution, corrosion, pressure release, efflorescence flaking, and biogeochemical activity from huge bat colonies. No evidence was found of channeled flow, or of phreatic or vadose activity. These caves are probably no older than Holocene.