The sediments of a late glacial sag pond in Huerfano Park, south-central Colorado, have yielded a varied biota consisting of vertebrates, terrestrial and fresh-water mollusks, sponges, and pollen. Wood from the sediments has a radiocarbon age of 9600 ± 200 years. The vertebrate fauna contains the tooth of a prairie dog and the remains of frog, salamander, snake, and fish—all characteristic of present-day montane lakes and ponds and their margins in Colorado. The mollusks are all living species, and the fauna is dominated by gastropods. With the exception of one form (Vitrina alaskana Dall), whose known range does not extend as far south as Huerfano Park, the fauna is not significantly different from the mollusks living in the area today. The sediments also contain an abundant and unusually diversified sponge fauna in which as many as six species may be represented. A pollen spectrum from the sediments is composed of arboreal pollen characteristic of the present-day Transition Zone flora which, in Colorado, grows on the upper edge of the Great Plains and all of the foothills belt.
Vertical variation in the sediment types and the enclosed biota makes it possible to recreate the changing depositional environment and local climate throughout the life of the pond. The basal sediments, interbedded marl and clastic materials, are interpreted as indicating deposition during alternating wet and dry periods. A bed of nearly pure calcium carbonate marl, which reaches a thickness of 2 feet, forms the middle stratigraphic unit. It is highly fossiliferous and is thought to have been deposited during a warm dry period with relatively mild winters. The uppermost stratigraphic unit is a gray carbonaceous clay which filled the basin as vegetation encroached upon the lake and the lake itself was reduced to a marsh.
Because of the radiocarbon age and the reconstruction of the climate based upon faunal and sedimentological changes, the lower two stratigraphic units are thought to correlate with the Lubbock Subpluvial, and the uppermost unit, the gray carbonaceous clay, with the early part of the Yellow House Interval as described in the Llano Estacado, New Mexico.