Fort Rock–Christmas Lake Valley basin
Fort Rock–Christmas Lake Valley is a basin of a former inland sea that existed in that region from Pliocene through late Pleistocene time. The Fort Rock basin maar field includes over 30 hydrovolcanic landforms spread over an area of . During the Pleistocene and early Holocene, Paleolake Fort Rock encompassed the entire basin. Paleolake Fort Rock The Fort Rock basin is an endorheic basin. Increased moisture in this semiarid environment during the Pleistocene through increased cloud cover and cooler temperatures resulted in higher lake levels. At its maximum, the lake filled the basin up to 75 m deep (245 ft), with a surface area of over 2,300 km2 (900 mi2). While this is larger than any modern lakes in Oregon, Paleolake Fort Rock was smaller than any of the other paleolakes to the south. There are four major shorelines of Fort Rock basin from the paleolake. These shorelines are identified through recording wave-cut and depositional terraces, lagoons, and convex sedimentary deposits such as bars, spits, and beach ridges. The floor of the Fort Rock basin is relatively flat at around 1,315 m (4,315 ft) throughout much of the basin, so the depths of the lake at each level were fairly