it represents the deep water deposits of Lake Alamosa. Impermeable deposits of the lake such as the "blue clay" generate groundwater accumulations that are exploited for irrigation purposes. The Verdos Alluvium of the Denver Basin may correlate to certain deposits on the shores of Lake Alamosa. While it was formerly thought that the Great Sand Dunes were formed from sediments on the floor of Lake Alamosa but later research indicated that these sediments probably played an only minor role. The development of the dunes however certainly post-dated the disappearance of Lake Alamosa as the lake would have impeded the transport of sand across the San Luis Valley. Research history Decades before the region was settled and earlier than other geologic expeditions such as those of John Wesley Powell, in 1811-1812 Jacob Fowler recorded the following: The lake is named after the Alamosa Formation, which in turn received its name from Siebenthal 1910 who in that year postulated the existence of a former lake in the Alamosa Basin. In the same year proof of the existence of the lake was found in well logs. Later, the geologist Ferdinand Vandeveer Hayden in 1875 wrote a more detailed account of the former lake, although he got some details wrong, and proposed the name "Coronado's lakes" for Lake Alamosa and another lake he believed to have existed in the Costilla Plain. It took almost a century after 1910 until a more detailed analysis of the deposits of the former lake - including identifying its former maximum elevation - was possible, however. Most information on the history of Lake Alamosa's water levels has been obtained at the "Bachus pit" gravel pit in Alamosa County, as elsewhere lake deposits have been altered. Both beach and deep-water deposits (from the highstand before overflow) are encountered there. References Sources External links Former lakes of the United States Alamosa County, Colorado