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From Pliocene to middle Pleistocene time, a large, highaltitude lake occupied most of the San Luis Valley of southern Colorado. This ancient lake accumulated sediments that Siebenthal (1910) designated as the Alamosa Formation, for which it is herein named. The existence of this lake was first postulated in 1822 and proven in 1910 from well logs. It was one of the largest high-altitude lakes in North America, comparable only to historic Lake Texcoco in the Valley of Mexico. Lake Alamosa persisted for about 3 m.y., expanding and contracting and filling the valley with sediment until about 440 ka, when it overtopped a low sill on Oligocene volcanic rocks of the San Luis Hills. The resulting overflow cut a deep gorge, coursed southward, and flowed into the Rio Grande, entering at what is now the mouth of the Red River. The key to this new interpretation is the discovery of stillextant shoreline deposits, including spits, barrier bars, and lagoons nestled among bays and in backwater positions on the northern margin of the San Luis Hills, southeast of Alamosa, Colo. Alluvial and lacustrine sediment nearly filled the basin prior to lake overflow, which happened sometime around 440 ka as estimated from a He surface-exposure age of about 439±6 ka on a boulder of reworked basalt and from strong calcic soils on barrier bars and spits at 2,330–2,340 m (7,645 ft–7,676 ft). Overtopping of the lake’s hydrologic sill may have been stimulated by high lake levels during marine oxygen-isotope stage 12, which was one of the most extensive middle Pleistocene glacial episodes. This integration expanded the Rio Grande’s drainage basin by nearly 18,000 km to include the high-altitude, glaciated San Juan, Sawatch, and Sangre de Cristo Mountains.

u.s. geological survey open file report/open-file report

2007 Rocky Mountain section Friends of the Pleistocene field trip - Quaternary geology of the San Luis basin of Colorado and New Mexico, September 7-9, 2007