Valentine Camp lies within the southwestern part of the Long Valley caldera, a volcanic depression that formed during the cataclysmic eruption of the Bishop Tuff about 760,000 years ago. The caldera, which measures about 18 miles (29 km) from east to west and 10 miles (16 km) from north to south, collapsed when about 200 cubic miles (800 km) of tuff was erupted. Because of the removal of a large volume of material from beneath the caldera during the massive eruption, which may have lasted less than a week, the ground surface within the caldera dropped about 2 miles (3 km). However, about two thirds of the caldera depression was subsequently filled in by volcanic rocks, glacial deposits, lake beds, and stream-deposited alluvium. Today, the topographic relief between the lowlands of the caldera and the surrounding highlands is slightly more than 3000 feet (1 km).
Within the boundaries of Valentine Camp are several distinctive volcanic flows and sedimentary deposits. The oldest geologic unit within the camp boundaries is Rhyodacite of Mammoth Mountain (also called Quartz Latite of Mammoth Mountain), which ranges in age from about 220,000 years to 50,000 years. Rhyodacite, which makes up most of Mammoth Mountain, is the volcanic equivalent of quartz monzonite, a close relative of granite. The volcanic rocks that formed Mammoth Mountain were probably extruded as a series of discrete lava flows from at least ten separate source vents. The rhyodacite crops out in three principal areas within and adjacent to Valentine Camp: (1) near the west boundary of the camp along Mammoth Creek; (2) on the south boundary of the camp north of Panorama Dome, and (3) on the east flank of Mammoth Mountain above Lake Mary Road, north of the camp. The rhodacite lava flows that crop out in the western parts of Valentine Camp probably flowed westward from source vents on the east or southeast flank of Mammoth Mountain.
Andesite, a dark gray volcanic rock similar to basalt, is exposed along Mammoth Creek for a distance of about 1000 ft (300 m) within the camp property. The andesite, which has been informally named the Valentine Andesite, has a radiometric age of 86,000 ± 10,000 years, based on potassium-argon dating. The andesite is at least 50 feet (15 m) along the creek, and it most likely flowed from a no-longer-exposed source vent somewhere to the west. The Valentine Andesite is part of a series of basalt and andesite flows that were extruded in the western part of the Long Valley caldera between about 230,000 and 60,000 years ago. The waterfall (Valentine Falls) along Mammoth Creek at Valentine Camp occurs in this geologic unit.
Most of Valentine Camp is blanketed by glacial deposits that were probably left behind by the last major glaciation in this region, the Tioga (late Wisconsin) glacial event. This episode of glaciation began about 25,000 years ago and ended about 12,000 years ago. In the Mammoth area, a glacier formed in the Mammoth Lakes basin below Mammoth Crest and flowed northeastward into the Long Valley caldera through the gap between Gold Mountain ridge and Mammoth Mountain at Twin Lakes. Advancing eastward, it flowed into the valley of Mammoth Creek, now occupied by Old Mammoth, and eventually reached a point about half a mile (0.8 km) east of the junction of Old Mammoth Road with Sherwin Creek Road. The sagebrush-covered knoll near the northeastern corner of Valentine Camp is part of a long lateral moraine (linear ridge of glacial deposits) of Tioga age that extends eastward for about a mile (1.6 km). The till (glacial deposits) at Valentine Camp contains boulders and cobbles that consist mostly of three rock types: granitic, basaltic, and metamorphic. The till here, like most Eastern Sierran tills, comprises all sizes of rock particles from silt to large boulders.
All of the geologic units of Valentine Camp have been blanketed by pumice ash from numerous volcanic eruptions that have occurred during the past several thousand years. Source vents for the pumice eruptions include Mammoth Mountain, the Inyo Craters, and the Mono Craters chain. One of the most recent eruptions occurred in the Inyo Craters chain around A.D. 1240; the eruption produced a layer of ash and lapilli (pebble-sized pumice fragments) that is about ½ to 1 foot (15 to 30 cm) thick in the Mammoth Lakes area. Obsidian flakes, probably transported to the area by indigenous Paiutes, can occasionally be found scattered in the pumice deposits.
Although technically not a geologic unit, “marshy ground” is a distinctive feature of the meadow areas of Valentine Camp. In these areas the glacial soil with its pumice veneer has been modified by the effects of a high water table and the presence of more abundant vegetation. The vegetation has contributed more organic constituents to the “marshy ground” than are found in the other glacially derived soils. So the “marshy ground” is distinguished from the glacial deposits primarily by the degree of saturation and the relatively high content of organic material. A prime example of “marshy ground” at Valentine Camp is Woody’s Meadow, which was named for the late Woody F. Sampson, caretaker of the camp from 1954 to 1979.