Mountain are testimony to the existence of the former lake, and additional large spits formed close to Sierra del Ojito and at the Brownie Hills. Barrier bars dammed creeks which in turn formed lagoons; such lagoon-bar systems are found for example around Trinchera Creek. Wind-eroded rock structures occur along its former southeastern shore. Climate During the glacial times of Lake Alamosa's existence, the basin was colder than today and possibly also drier. The lake was nourished by glacial meltwater coming from the San Juan, Sawatch, and Sangre de Cristo Mountains. Presently, the climate at Alamosa is cold and dry, with mean annual temperatures of and mean annual precipitation of . Geologic history The San Luis Basin is the largest basin of the Rio Grande Rift and is bordered to the east by a fault. It constitutes an east-tilted half-graben that is subdivided by additional fault systems. Its southern reaches are covered by the volcanic Servilleta Formation. The valley forms the headwaters of the Rio Grande. Lake Alamosa existed for about 3 million years, from the Pliocene to the middle Pleistocene. Rocks in the San Luis horst stretch across the entire Alamosa Basin and together with 4.8-3.7 million years old basaltic lava flows of the Taos Plateau blocked the basin from draining to the south. Tectonic uplift of the Jemez lineament, tectonic subsidence of the Lake Alamosa area and the emplacement of these lava flows may have obstructed former drainages, but evidence of prior southwards drainage has been found only recently. South of Lake Alamosa an even older lake, Lake Sunshine, occupied the Sunshine Valley during the Pliocene. Many lakes in western North America are cyclical, becoming deep and large during stadials and shallow and small during interstadials. Lake Alamosa briefly reached elevations of at Hansen Bluff four times during its history. According to modelling, ongoing sedimentation in the lake basin would have led to gradually increasing water levels during each stadial over time. Aside from climatic influences, the Bishop Tuff eruption of the Long Valley caldera in California and the Huckleberry Ridge eruption of the Yellowstone Caldera deposited tephra in Lake Alamosa. Sedimentation occurred in the basin floor, although slip along the Sangre de Cristo fault to the east created accumulation space. Overflow It overflowed about 440,000 years ago, when it reached a highstand during oxygen isotope stage 12, one of the major glaciations of the Northern Hemisphere. This date