MASTODONS AND GROUND PENETRATING RADAR: A LATE QUATERNARY INTERDISCIPLINARY STUDY IN SOUTHEASTERN NEW YORK

The accidental discovery of two large-diameter mastodon tusks protruding from a stream bank in Orange County, NY, in the Wallkill Valley lowland, led to an interdisciplinary study utilizing ground penetrating radar (GPR) to characterize site stratigraphy, distribution of additional disarticulated faunal remains, and guide excavation/recovery. The Orange County study site lies in a former proglacial lake basin of the Hudson Lobe of the Laurentide Ice Sheet, a region colonized by mastodons and paleoindian populations in the Late Pleistocene. Successful excavation yielded material suitable for radiocarbon dating and paleoecological reconstruction.

Observations during initial visits indicated that the tusks were 1.8 m below the ground surface in close association with a marl unit overlain by peat and underlain by silty clay. Two GPR surveys were completed using a GSSI SIR-3000 data acquisition system, with 400 MHz and 200 MHz monostatic antennas. Data acquisition was completed along an 8 by 17 m grid, with a 0.25 m line spacing and sampling interval of 40 scans per meter. Amplitude-slice maps were produced using RADAN processing software to produce a three-dimensional image of anomalies and stratigraphic relationships.

Survey results showed the marl and peat interface to occur at 1.6 m as a sharp contact . While both GPR surveys produced multiple anomalies and delineated the tusks and site stratigraphy, the 200 MHz antenna clearly produced the best combination of penetration and resolution, especially at this site, which is less than ideal for a GPR survey.

Results of the GPR surveys were used to guide a systematic 3 by 3-m excavation to extract and salvage the tusks. Excavation of the site confirmed the marl contact at 1.6 m and that marl draped over the tusks. The thickness of the marl was 0.3 m. In addition to the tusks, the GPR survey identified additional targets including a 0.3 m long fragment of tree at the level of the marl interface. Abundant plant macrofossils, mostly of aquatic plants, and pieces of wood and charcoal were also recovered from various deposits in the excavation. The recovered botanical and faunal remains provide detailed information and chronologic context for paleoecological and deglacial reconstructions in the region.