El Gigante Rock Shelter: Archaic Mesoamerica and Transitions to Settled Life

Stratigraphy and Chronology

The strata can be described as nine lithostratigraphic units (I-IX). Each of these units is divided into several sub-strata and described individually. In 2000 we determined that no incontrovertible evidence existed for habitation remains below stratum V. In 2001 a series of six radiocarbon assays were performed on material from the 2000 excavations (see Table 1 and Figure 3). The following general description of the site stratigraphy was consistent across the main block of our excavations. Unless otherwise stated, dates mentioned in the text are 2-sigma calibrated results.

The most recent of the dates pertain to stratum I, which we believe spans the Formative (1760 B.C. to 220 A.D.) in age. Its upper portions are patchily but seriously disturbed by looting and modern activity. This stratum consists of densely packed mostly unburned organic material, including petates, matted grass, wood, leaves and reeds, floral material and bone. It contains sparse amounts of lithics and ceramics as well.

Stratum II begins around 30 cm depth though much deeper in areas. The organics are significantly less in this stratum, and it is much thinner. The intensity of occupation seems to be less, and the stratum’s character is dominated by pit features, as opposed to the upper grass strewn floors of stratum I. It is composed mainly of interspersed fine ashes, charcoal and limited bone fragments. A single date from the base of this stratum placed it in the Middle Archaic (5650-5480 B.C.).

Stratum III is somewhat thicker, ranging in depth from 40 or 50 cm down to 70 cm. This stratum is much more homogenous than the upper ones. We suspected that the bright yellow fine ash originated from volcanic sources, but pedological investigations have since ruled this out and we now think that the layers are composed of anthropogenic wood ash. These yellow layers are interbedded with more gravelly grey ash and very fine sediment, perhaps eroded from the walls and roof of the cave. Stratum IV is very similar in character to III though slightly darker and browner in color. This stratum also contained bone, charcoal and lithics in more dense quantities. We suspect strata III to represent Archaic period occupations as old as 6410 B.C. Stratum IV returned a date of 9220-8750 B.C.

Stratum III is often pinched out of the sequence due to intrusions from above. Similar intrusions or other as yet unknown taphonomic processes may be responsible for the large gap in the dating sequence between I and II, amounting to 3720 years of missing time. However, we have as yet only these six dates and future radiocarbon determinations may resolve the issue.

The last of the cultural material bearing sediments are represented by strata V. This stratum consists of abundant angular gravel, ash and coarse charcoal, organic material, flaked lithics and large mammal bone. This stratum’s date is very close to that of IV, revealing that a large portion of the terminal cultural remains is from this ancient period. The discontinuous boundary between stratum V and the following strata is where the oldest signs of first human occupation lie. The multiple 9000 B.C. dates confirm a human presence at this time and materials found below these stratigraphic contexts will be referred to as Paleoindian.

The remaining, non-cultural strata in the series begin as a sequence of alternating white, indurated and pumicy tephras (VI and VIII) and very carbonaceous loose black soils (VII and IX). A single radiocarbon date reported by the previous investigators is available for the dark soil that overlies the bedrock (which we assumed to match our strata IX). This revealed an uncalibrated date of 39,820 +/- 1100 years B.P. However, the artifacts said to be associated with this very early date were never reported and appear to be lost. It may be that some cultural remains date to occupations preceding to our earliest date, however, the excavations and reprobing of the original 1994 trench, combined with a knowledge of the volcanic history of the region (see below) bear out the conclusion that Paleoindian remains are to be found above the white tephras.

Volcanoes were the source of these tephras. The highly organic laden sediments sandwiched between them contain paleoecological information seldom preserved in the New World Tropics. As the single radiocarbon date available for these lowest soils is essentially infinite (i.e. at the limit of Radiocarbon dating) it must be viewed with a great deal of suspicion and it is my guess that the age is actually much older. I would place the age of these tephras in the Pliocene or early Pleistocene. The character of strata VI and VIII (their pumicy texture, angular gravels and lapilli) suggest they are the result of pyroclastic flow. Pyroclastic flows are fast moving extremely hot mixtures of gas and ash. These deposits are capable of extreme destruction over sometimes long distances, but no active volcanoes near enough to be their source have been active since those distant epochs (Williams and McBirney, 1969).

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