Cultural remains were recovered in close association with extinct animal remains including mammoth, mastodon, antelope, horse, and camel at two sites (Stations 1 and 2) at the Hueyatlaco locality. Stratigraphically, Unit C appears to represent a broad but shallow channel with internally bedded sediments. Unit E is described as a series of alluvial sediments in a shallow channel bed directly underlying Unit C; eight different subdivisions were visible within Unit E. Cultural remains recovered from Unit C at Station 1 included artifacts made of nonlocal material closely associated with (within 1 cm) a horse mandible. In Unit E (1E2), a semi-articulated horse skeleton was documented with a small number of associated artifacts, including a broken bi-pointed projectile point recovered near ribs and vertebrae and the fragment of another well-made biface lying under an associated vertebra. At Station 2, Unit E also was shown to contain cultural remains associated with extinct animal fauna. The base of 2E2 contained remains of mastodon, horse, camel, and mammoth. Evidence for cultural activity here included bone fragments from a mastodon mandible and maxilla that may have been purposefully split or broken. A thin burinated flake was recovered between the cusps of one of the molar teeth and a chert “wedge” (IrwinWilliams 1978:17) was found driven into another mandible fragment. Taken together, Units C and E represent stratified Pleistocene deposits containing extinct fauna with associated artifacts reflecting hunting and processing of several different taxa. No reliable dates are available for this section, but freshwater mollusks from a stratum believed to be correlative from the Barranca Caulapan site, located about 5 km distant, produced a date of 21,850±850 14C bp (W-1850) in direct association with a flake tool (Irwin-Williams 1978:15). Establishing a reliable chronology for the cultural sequence at Hueyatlaco and nearby sites has proven to be a considerable a challenge, with claims of ages exceeding 200,000 years based on uraniumseries dates on bone and fission-track dates on ash (Steen-McIntrye et al. 1981). The differences between the expected ages of deposits based on archaeological and paleontological data and the estimates produced by project geologists resulted in considerable controversy. Recent geoarchaeological and tephrachronological work by Gonzalez et al. (2006a) has produced estimates ranging from ca. 23,000 to as old as 29,000 years for the cultural sequences in the region, dates that broadly correlate with the general age range of >9000 to