Fort Rock–Christmas Lake Valley is a basin of a former inland sea that existed in that region from Pliocene through late Pleistocene time.

[3] The Fort Rock basin maar field includes over 30 hydrovolcanic landforms spread over an area of 1,500 square miles (4,000 km2).

The highest shoreline at 1,384 m (4,540 ft) is best seen at Fandango Canyon, but traces of beach sediments and wave-washed scarps have also been identified at Cougar Mountain and the southern end of the Connley Hills.

The 1,353–1356 m (4,440–4450 ft) shoreline is the most strongly developed, as it carved the notches in Fort Rock and the caves in the Connley Hills and Cougar Mountain.

Pine charcoal below the Mazama layer is an additional environmental indicator, as the tree only grows at cooler, wetter elevations.

The assemblage consists of obsidian and fine-grain-basalt projectile points, knives, scrapers, drills, milling stones, cordage, and a variety of other items.

Three test pits near the houses encountered three floors, which contained fish, rodent, rabbit, and large mammal bone were dated to 2040, 1890, and 1780 BP.

Moffitt Butte is not associated with a lake basin, as is the case for Fort Rock and Hole-in-the-Ground, but rising magma probably encountered permeable aquifers beneath the cone.

A smaller vent and small tuff ring, 1,700 feet (520 m) in diameter, are located on Moffitt Butte's southwestern flank.

Near the summit, the uppermost palagonites are overlain by massive cinders and bombs from fire-fountaining that preceded the filling of the crater with lava.