Arenosa Shelter is a limestone overhang in the north wall of the lower Pecos River canyon near the confluence of the Pecos and the Rio Grande. The shelter, now beneath the waters of Amistad Reservoir, was investigated as part of salvage archeological work in anticipation of the impoundment of that lake in 1969. The shelter was first recorded in 1958, and excavations were conducted during several seasons from September 1965 until July 1968 under the auspices of the Texas Archeological Salvage Project of the University of Texas at Austin and directed by David S. Dibble. The limestone roof of the shelter was about twenty-one meters above the level of the Pecos River and extended out more than eight meters from the back wall. At the time of its discovery in 1958, the shelter was completely filled with stratified, alternating alluvial and cultural deposits. Excavations exposed a series of deposits 12.8 meters thick. Of the forty-nine strata documented, twenty-four contained cultural remains and twenty-five were devoid of cultural debris (these were numbered from the top down, 1 through 49). This remarkable sequence reflects repeated periods of prehistoric human occupation between 9,600 and 1,500 years ago, each followed by flooding. The floods were at times caused by the Pecos River, at other times by backwater from the Rio Grande, and at still other times by both rivers. The most recent (1954) and biggest buried such twentieth-century artifacts as tobacco cans and fishing tackle in the shelter under nearly half a meter of silt.
A series of thirty-two radiocarbon dates of charcoal aided in developing a chronology of the site. The Paleo-Indian period is represented by bison bones and a few nondiagnostic stone artifacts (175 flakes and four unifacial tools) in stratum 38 near the base of the sequence. A single, uncorrected, uncalibrated radiocarbon date of 9550±190 years ago derives from charcoal in stratum 38. From stratum 36 upward into stratum 4 were recovered artifacts resulting from a long sequence of Archaic occupations. These are well-dated with thirty radiocarbon determinations ranging from more than 5,500 to around 1,900 years ago. Cross-dating of diagnostic artifacts in this sequence expands it from ca. 9,000 to ca. 1,300 years ago. Strata 2 and 3 derive primarily from Late Prehistoric occupations, datable from ca. 1,300 to ca. 500 years ago; a single radiocarbon age on charcoal from stratum 2 was determined to be 1,380±60 uncorrected, uncalibrated years old.
Numerous archeological features and important assemblages of artifacts from well-dated, brief intervals of occupation were documented at Arenosa. The site provides the best single-site chronological record in the Trans-Pecos and one of the best in North America. The flood deposits at the site have also afforded evidence for the history of flooding of the Pecos River. Larger floods occurred during drier climatic intervals over the last 8,000 years, a finding dramatically evident in the catastrophic flood of 1954 during the droughty mid-1950s. There has been no comprehensive analysis and reporting of the data from Arenosa Shelter. The collections and records are housed at the Texas Archeological Research Laboratory at the University of Texas at Austin and have often been referred to by students and scholars researching the prehistory of the lower Pecos region. In this way, the site has contributed significantly to understanding of the area, though its full potential has not been tapped.