Pg. 745; USGS Water Supply Paper 240, p. 40. Alamosa formation. Blue clays and interstratified sands; some gravel, and occassionally, in depth boulders. Thickness 725 to 1,250 feet. Of late Pliocene or early Pleistocene (pre-glacial) age. Unconformably overlies Santa Fe formation. Occurs at foot of Conejos Range, at foot of Blanca Peak, and at other places. Occupies bottom of San Luis Valley proper. Named from Alamosa, Conejos Co., near center of valley.

Source: US geologic names lexicon (USGS Bull. 896, p. 22).

Pg. 20-24. Alamosa formation. Underlies large part of San Luis Valley but is well exposed only in southern part, where Rio Grande and its tributaries have cut into upper part of formation. Thickness 41.5 feet at Hansen Bluff on east bank of Rio Grande southeast of Alamosa. Recent deposits overlie Alamosa, but similarity of materials make it difficult to determine contact between the two; in this report, Recent materials are included in Alamosa except in some well logs. Most reliable data on formation comes from well logs. [Age is late Pliocene or early Pleistocene, and locally, Recent.]

Source: US geologic names lexicon (USGS Bull. 1200, p. 37).