A paleontological analysis of the Alamosa Formation (south-central Colorado--Pleistocene: Irvingtonian)
Karel L. Rogers


The floor of the San Luis Valley of south-central Colorado is made up of Alamosa Formation sediments. The Alamosa Formation overlies Miocene Santa Fe Formation (Siebenthal, 1910). In all places where the glaciers of the Sangre de Cristo Mountains extended far enough, they deposited their moraines on top of the Alamosa Formation. The type locality of the Alamosa Formation (Siebenthal, 1910) is an outcrop called Hansen Bluff (Fig. 1), a feature created by the Hansen Bluff fault, a northern segment of the La Sauses fault where it is offset by movement of the Manassa fault. The bluff has receded somewhat eastward because of lateral erosion by the Rio Grande, which has since moved its course westward from the bluff. All wells west of the Hansen Bluff fault are artesian (C. A. Repenning, written comm. 1984).

At the type locality, fossils of fresh-water snails, vertebrate fragments, pollen, and plant remains have been reported (Lochman-Balk and Bruning, 1971). Some of the vertebrate fragments were identified as Camelops sp. (R. C. Peterson, oral comm.), and Price (1971) published a short list of pollen types.

Extensive fossil collection at the Hansen Bluff locality was not carried out until the summers of 1979, 1980, and 1981, when the author led crews of 12 or more to recover fossils using techniques of Hibbard (1949) and refinements of those techniques described by Waters (1978). Matrix was labeled as it was taken from the bluff, transported to the washing site in burlap bags, dried in large frames (as the matrix dried, much of the dirt dropped through the screening in the frame), transferred to a small washing box with its label, gently washed, allowed to dry, transferred to a paper bag with the label, transported to the lab, and then sorted under a dissecting microscope into small plastic boxes labeled as to location and bag number. This technique allowed for control on fossil breakage, location, and on crew-member performance. All but the smallest of fossils, such as pollen and ostracodes, were recovered in this manner.

Fossils were recovered from a 2-km-long section of Hansen Bluff (Fig. 2). Five measured sections were dug and sampled for fossils. Only one unit could be correlated with certainty along the length of the bluff. In addition, concentrations of fossils were dug from seven localities, some of which were made up of more than one unit.

The fossil assemblage has been given the name Alamosa Local Fauna after the county and town of its origin. Fossils will reside at the University of Colorado Natural History Museum, Boulder.

Recent climate of the San Luis Valley is arid to semiarid, with prevalent sunshine, cool temperatures, and frequent southwest winds. Mean annual temperature is 4 I .6°F (5.3°C), with temperatures ranging from -50°F ( -45.5°C) to 93°F (33.9°C). Mean annual precipitation is 17.35 cm. Temperatures at the valley floor are frequently cooler than those in the surrounding mountains because of temperature inversions. Cloudy days tend to moderate temperatures when they occur, although the modern climate has only about 80 days/year classified as cloudy by the National Weather Service.


  1. Rogers, Karel L., 1984, A paleontological analysis of the Alamosa Formation (south-central Colorado--Pleistocene: Irvingtonian), in: Rio Grande rift--Northern New Mexico, Baldridge, W. S.; Dickerson, P. W.; Riecker, R. E.; Zidek, J., New Mexico Geological Society, Guidebook, 35th Field Conference, pp. 151-155.

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