Forbidden Archeology_ The Full Unabridged Edition - Michael A. Cremo [255]
5.4.4.2 Other Methods Used for Dating Hueyatlaco and El Horno
In addition to the uranium series method, the team of geologists used fission track counting, tephra hydration dating, and mineral weathering analysis to assign dates to the implement-bearing layers at Hueyatlaco and El Horno.
Fission track dating is based on the accumulation of radioactive decay tracks in volcanic mineral crystals as a function of time. The more tracks, the older the crystals. When the age of crystals in a volcanic deposit has thus been determined, it is possible to assign appropriate dates to implements or fossils found beneath the volcanic layer in question. The fission track method was applied to two volcanic layers (the Tetela mud and the Hueyatlaco ash), situated above the most recent Hueyatlaco artifacts. The fission track dates for these layers should give a minimum age for all of the Hueyatlaco tools. The fission track dates were 260,000 to 940,000 years for the Tetela mud and 170,000 to 570,000 years for the Hueyatlaco ash. The considerable ranges in the dates were attributed to statistical effects due to the small number of fission tracks that were counted (Malde and Steen-McIntyre 1981, p. 419). The date ranges for the two volcanic layers overlap in the interval from 260,000 years to 570,000 years b.p. Tephra hydration dating is a relatively new technique. It relies on the fact that volcanic glass, or tephra, slowly absorbs water. For this method to be feasible, it is necessary to have independently dated control samples of volcanic glass with the same chemical properties and geological situation as the samples to be dated. In this case, control samples were taken from the nearby La Malinche volcano. The method gave a date of about 250,000 years b.p. for tephra deposits associated with the Hueyatlaco artifacts (Steen-McIntyre et al. 1981, p. 13).
The final method of dating, the study of the weathering of a volcanic mineral, hypersthene, gives only a relative measure of age. As time passes, exposed crystals of this mineral are slowly etched, leaving a “picket fence” profile when viewed under a microscope. At the nearby human occupation site of Tlapacoya, this etching was rare and incipient in volcanic deposits dated by carbon 14 to about 23,000 years b.p. In contrast, the etching was pronounced in volcanic deposits associated with the Hueyatlaco artifacts. This suggested that the Hueyatlaco artifacts must have an age considerably greater than 23,000 years (Steen-McIntyre et al. 1981, p. 11).
A final consideration in the dating of the Hueyatlaco artifacts is that they were found buried beneath at least 10 meters (33 feet) of sediment. Geological study showed that these strata had to accumulate before being cut by the nearby Atoyac River, which has carved a valley 50 meters (164 feet) in depth (SteenMcIntyre et al. 1981, p. 10).
In other words, the geological history of the site would go something like this. The artifacts were left on an ancient land surface. Layers of sediment were deposited over them. Then the river began to cut through the layers of sediment.
Given this sequence, it is possible to estimate the age of the tools. Two elements are required. The first is the time required to deposit at least 10 meters of sediments over the tools. The second is the time that the river took to cut its valley, which is now 50 meters deep. If one could estimate the time it took the river to cut its valley and then add the time it took to deposit at least 10 meters of sediments over the tools, then one would have a rough date for the tools.
Since the valley and its side channels have gentle slopes, it is not likely that the river has exhibited an unusually high rate of erosion. But even if we assume a rather high rate of erosion, as in the Colorado River valley, the river Atoyac would have required around 150,000 years to carve out