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Using Geophysical Methods to Locate Archaeological Features and Artifacts from La Placita de Los Trujillos, Buried by the Santa Ana River during the Great Flood of 1862
In the mid-1800s, La Placita de Los Trujillos (La Placita) was situated alongside the Santa Ana River (SAR) in what is now Colton, CA. The settlement had grown to be the largest non-native community between New Mexico and Los Angeles, until the river washed away or buried all adobe structures and settlers’ belongings during the Great Flood of 1862. Historical archives reveal that a significant portion of San Salvador is buried approximately 3 meters beneath sandy flood deposits of the vacant, 200-acre lot of Pellissier Ranch. The motivation for this study was to provide the Spanish Town Heritage Foundation with data they could use to advocate the cultural importance of Pellissier Ranch in their attempt to stop the city from developing the site for warehouses. From April 2018 to January 2020, we conducted eleven days of geophysical field surveys in search of buried adobe structural remains, household items composed of wood and iron, and large farming equipment that would have existed in La Placita. Over 7,000 meters of ground-penetrating radar (GPR) profiles were acquired with both a GSSI-SIR 3000 and 4000, using the 400 MHz and 350 MHz HyperStacking antennas respectively, and 25 km of magnetic gradiometry data was acquired using a GEM GSM-19T Proton Precession Magnetic Gradiometer. GPR profiles imaged several 'adobe melt' signatures at a depth range of 1.4 - 3.8 meters, and two buried canals with surfaces reaching 1.3 meters below the surface, and bottom depths of 2.5 meters. Additional structural GPR anomalies vary in size from 9 - 70 meters at a depths of 1 - 3.8 meters. We hypothesize that some of the shallower buried (sub-2-meters) anomalies exist for a few reasons: the portion of the anomalous feature closer to the river absorbed the majority of the river's force, which shielded the more intact part, or the location did not experience the full strength of the river's force, causing less destruction and burial beneath the sandy flood deposits. Contemporary debris from trespassers resulted in many false-positives in gradiometer results that needed to be excluded from mapped vertical gradients. We therefore focused more heavily on GPR results, using the gradiometer as a complementary technique to attempt to verify anomalies we imaged with GPR. In two locations, we observed a positive correlation between the two data-sets, attributed to a sub-2-meter burial of anomalies. This confirms claims made by other researchers that features / artifacts with low magnetic content cannot be seen if buried deeper than 2 meters. From our results, we conclude with confidence that several anomalies we imaged in GPR profiles can be related to structures / features of La Placita, and the depth range of the settlement at Pellissier Ranch is primarily 2 - 3.8 meters. Based on our surveys, we suggest to focus any future investigation in the middle of Pellissier Ranch. There is one east-west dirt road that cuts through the middle of the 200-acre site. At about the halfway point of that east-west road, we found a collection of promising anomalies.