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

Appendix II: Pre-human Site Formation and Geology

El Gigante was formed in an ignimbrite cliff by running water scouring the welded tuff. This bedrock consists of rock belonging to the Grupo Padre Miguel a geological formation of volcanic tephras lain in the Miocene and Oligocene that blankets the entire south-western portion of Honduras (Kozuch, 1991).

Based on the survey results and the observations made in the field, it can be said that most of the rock shelters and caves of the La Paz Highlands are geomorphologically similar. Many are in a stage of "active formation" due to the action of water. Unlike El Gigante, the majority of rock shelters which exist in the bedrock tuffs of the region have water percolating through planes of weakness and fractures in the bedrock, thus scouring the ignimbrite from the inside out. Though dry and habitable during the months of October through May (the approximate dry season), they become flooded when rains arrive. El Gigante is a unique instance in this regard, as it remains dry all year round regardless of rainfall.

El Gigante was only to become a sediment "sink" more recently in its evolution. Water has long since stopped flowing through El Gigante. The abundant "potholes" still visible in parts of the floor are geological features. These were previously identified as looters pits. While it is true that the sediments that had accumulated in them has been disturbed, their recognition as geological features is key to our interpretation. Perhaps as long ago as the Pliocene, two watercourses directly adjacent to the cave that may have been responsible for carving out the original form of the shelter, have since split or changed course. At this time exogenous volcanic inputs (the white tephras discussed above) became the dominant force in the evolution of the shelters sediments, until the Middle to Late Pleistocene. Throughout the Holocene the shelters formation was influenced by anthropomorphic inputs and very slow colluvial erosion from the roof and walls of the shelter itself.

A grain size distribution analysis may be able to wrest paleoenvironmental information from these colluvial processes. In the Perigord of France, Henri LaVille (1980) has shown that glacial and interglacial times can be reconstructed on the basis of a quantification of clast size and frequency. If we assume that the erosion is correlated to periods of higher humidity a similar analysis may be possible at El Gigante. Initial evaluation of the grain size histograms do not reveal any patterns that we feel comfortable interpreting as due to changes in climate. However, some of the sedimentary particle distributions may be indicative of the intensity of human use of the shelter. More work is needed to sort out various confounding variables with column sample variations (volume and time-span).

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