Archaeologists have discovered the remains of a 5,000-year-old shell mound, which is believed the oldest existing mound complex in New World. Constructed of cement-like floors, the mound, researchers say, is the oldest known platform intentionally built in Mesoamerica, the cultural region comprising Mexico, Belize, Honduras, Guatemala and El Salvador, and it could completely change our understanding of the prehistoric people who once inhabited this area.
     The mound, built almost entirely from marsh clamshells, is 73m long, 27m wide and 6.5m tall. John Hodgson, a University of Wisconsin-Madison doctoral degree candidate in anthropology discovered it last October on a remote island in a swampy area along the Pacific coast of Chiapas in Southwestern Mexico. Hodgson has named the site "Alvarez del Toro".
     When Hodgson reached the mound after spending two days traveling by car and foot and then cutting through miles of marsh grass nearly twice his height, he immediately saw the floor layers made from shells. "A trench created by a dynamite explosion about thirty years ago exposed a large number of floor layers formed of clam shells capped with a cement-like material made from burned shell and sand," he explains.
     To determine when the mound was constructed and occupied, Hodgson collected six samples of charred wood taken at different floor levels about two feet apart and then used Carbon-14 dating to ascertain the age of the wood material. The results show that the mound was used for about 500 years. "The time differences of the Carbon-14 dates," explains Hodgson, "suggest that the floors of the mound were either resurfaced or the mound was enlarged about every 20 to 30 years." Based on the analysis, the very top floor layer dates to 2575 BCE. Each level below this layer increases in age with the lowest tested layer dating back to 3024 BCE. Hodgson notes that this bottom layer, approximately 3.6m from the top of the mound, is not the base floor layer: "The dynamite crater only exposed the top half of the mound - there are about 3m of undisturbed floors underneath where we were able to recover samples for dating." As a result, he says the first construction of the mound could even be several hundred years older.
     The mound's age and its floor layers make this site unlike any others previously known for this time period in Mesoamerica, says Hodgson, adding that all other archaeological sites in Chiapas are shell middens - piles of shells and garbage that gradually accumulated from human activities. "The new site did not form accidentally from trash deposits," he explains. "The people who built it planned on creating a very large raised platform." Given the durability of the construction, Hodgson suspects that each floor layer of the mound once served as the floor for a wooden building. He plans to return to the site this September to search for holes in the floor layers where posts supporting walls or roofs may have been placed.
     The idea that people were living at Alvarez del Toro for long periods of time, however, counters decades of anthropological thinking, says Hodgson. He explains, "The common interpretation for this time period is that people were fairly mobile and often moved to different locations to collect seasonally available food resources." Additional research, he adds, could provide solid evidence that these people may have been much more stationary than previously thought.

Source: University of Wisconsin (12 April 2004), Science Daily (14 April 2004)