ARCHAEOLOGICAL GEOLOGY OF THE BIG BEND REGION, SOUTHWESTERN TEXAS

The arid to semiarid Big Bend region of southwestern Texas has been occupied by humans for the past ca. 12,000 years. However, most of the recorded archaeological sites in the region are less than 2,500 years old. An intensive geoarchaeological investigation is underway to determine whether a large proportion of the archaeological evidence predating 2,500 B.P. has been removed by erosion and/or deeply buried by alluvial and eolian deposits.

Big Bend is in the northernmost part of the Chihuahuan Desert. Its rugged landscape has been shaped by the erosion of upfaulted blocks and deposition in the downfaulted basins. There is about 600 m of relief, and mean annual precipitation ranges from 17 cm on the basin floors to 43 cm at high elevations. The Rio Grande is the only stream in the region that carries water throughout the year.

Alluvial deposits dating between ca.10,000 and 3,000 B.P. underlie the floodplain of the Rio Grande. Despite extensive outcrops of these deposits and associated buried soils in deeply entrenched arroyos, they have yielded few archaeological materials. In contrast, most of the alluvium stored in the valleys of tributaries to the Rio Grande is less than 3,000 years old and often contains deeply buried Late Archaic and younger cultural deposits. Dunes on basin floors have yielded late Paleoindian and Early Archaic materials, and a few rare early archaeological deposits are on and within alluvial fans.

The data suggest that the paucity of cultural deposits predating 2,500 B.P. is more related to human adaptations than to geologic processes that may have affected the archaeological record. Specifically, a major population increase appears to have occurred in Big Bend during the Late Archaic and transitional Archaic/Late Prehistoric periods. The catalyst for this phenomenon probably is post-Altithermal climatic change (greater effective moisture) and concomitant faunal and floral responses.