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From Pliocene to middle Pleistocene time, a large lake occupied most of the San Luis Valley above 2300 m elevation (7550 ft) in southern Colorado. This ancient lake accumulated sediments of the Alamosa Formation (Siebenthal, 1910), for which the lake is herein named. The existence of this lake was first postulated in 1822 and proven in 1910 from well logs. At its maximum extent of nearly 4000 km2, it was one of the largest high-altitude lakes in North America, similar to but larger than Lake Texcoco in the Valley of Mexico. Lake Alamosa persisted for ~3 m.y., expanding...

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