Presa de la Mula

In northeast Mexico, beyond the northern frontiers of the great Mesoamer-ican civilizations, hunter-gatherer groups still roamed at the time of the Spanish conquest, as they had done for more than ten thousand years. By the end of the eighteenth century they had been annihilated, but several hundred of their rock art sites are known in this region. The dating is generally uncertain within wide bounds, but many are clustered in certain locations, usually where there are wide views of the surrounding landscape, horizon, and sky. These petroglyphs contain numerous depictions of animals and weapons but also, intriguingly, abstract designs consisting of short lines organized in rectangular arrays, and groups of dots organized in sets of parallel rows or columns, crosses, concentric circles, and meandering lines. Anthropologist Breen Murray has examined the numerical character istics of these designs in an attempt to identify patterns of numerical order that might give some insight into their meaning.

The vertical face of a rock at Presa de la Mula, near Monterrey, contains one of the most extensive arrays of lines. This petroglyph, which measures some three meters (ten feet) across by one meter (three feet) high, contains 207 short vertical lines organized in a grid pattern with six rows and four columns, which divides them into twenty-four cells. Reading horizontally from left to right, and from the top row downwards, it is possible to discern sequences of 29, 27, 29, 28, 27, 7, 30, and 28 days. Each of the larger sequences is subdivided into subsequences such as 11+11+4+3 and 7+8+7+6.

Murray suggests that the lines are "tally marks" that reckon the days in relation to the phases of the moon. The number 207 corresponds exactly to the number of days in seven synodic (lunar-phase-cycle) months, and there are seven subtotals of between twenty-seven and thirty days. The fact that these are generally shorter than the true length of the synodic month (29.5 days) could be attributed to the vagaries of observing the moon (especially the thin crescent before and after new moon), and the additional seven days after the fifth month could be a correction, required to bring the "calendar" back into line at this point. The fact that this cell of seven days is marked by two otherwise unexplained arrow-shaped lines backs up this speculation. The whole design, then, appears to be a record of actual day-by-day observations of the phases of the moon lasting over half a year. Perhaps they had a seasonal camp near here, and after this period they moved on.

This interpretation has been arrived at by post hoc reasoning. Reading the count from left to right gives the best fit to a lunar interpretation, so that is what is suggested. A skeptic might argue that the various cells could be conjoined in different ways, or read in a different sequence, and to some extent this is undeniable. However, the interpretation is inherently plausible and is backed up by the presence of circles that seem to mark the completion of the months. Furthermore, the number 207 occurs in at least one nearby petro-glyph—an array of dots found forty kilometers away at a site called Boca de Potrerillos. Finally, the total numbers of dots in several smaller dot counts on other petroglyphs in the area seem to have possible lunar significance.

See also:

Abri Blanchard Bone.

Lunar Phase Cycle.

References and further reading

Aveni, Anthony F., ed. Archaeoastronomy in the New World, 195-204. Cambridge: Cambridge University Press, 1982.

Koleva, Vesselina, and Dimiter Kolev, eds. Astronomical Traditions in Past Cultures, 14-24. Sofia: Bulgarian Academy of Sciences, 1996.

Ruggles, Clive, ed. Archaeoastronomy in the 1990s, 264-269. Loughborough, UK: Group D Publications, 1993.

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