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I I I I I I I I I I I I I I I I I I I OC169:1 Siebenthal's text in the form ofa basin cross-section (see Figure 3, page 45; a similar cross- section is given in Reclamation, 1987, page 25), Sand dunes are found throughout the San Luis Basin and reported by Siebenthal to be wind deposited materials that often build up behind small brush, and by doing so, may have formed embankments on the north and east shore of San Luis Lake, and around other basin lakes (USGS, 1910, page 47), There appears to be some difference between the sand in the dunes on the basin foothills and that in the valley areas, The sand in the dunes areas is darker and has a high content of auriferous magnetite, while the valley sands are whiter in nature, The Shallow Ground Water System Siebenthal's report deals basically with the deeper artesian wells in the San Luis Valley, because of their much greater productivity, and because of the relatively large number of deep wells in the Valley, His report indicates that there were about 2000 wells in the Valley in 1889 and about 3,500 at the time of his study (between 1889 and 1899 the irrigated acreage in the Valley doubled to 300,000 acres; USGS, 1910, page 20), With the exception of a very few wells that pumped the "sub", there were no irrigation wells reported to be in the shallow aquifer in the Closed Basin in 1910, "Pumping the sub" was a phrase applied to the use of infiltration galleries to collect shallow ground water. Siebenthal provides a sketch of gasoline pump and gallery system used to harvest shallow ground water in the Closed Basin (see Figure 2, USGS 1910, page 21), Siebenthal notes that it is "impossible to correlate well sections" when the Valley wells are at any great distance apart, or to trace layers of sand and clay from well to welL However, the thickness of the shallow zone and the top of the "blue clay" that separates the unconfined from the confined aquifer, seems to be relatively constant (USGS, 1910, page 44), 9