COMBINING ARCHAEOLOGY, SEDIMENTOLOGY, AND GPR STRATIGRAPHY TO ELUCIDATE FLOODPLAIN DEVELOPMENT, CHARLOTTE CREEK, NY

This paper documents the stratigraphy of a floodplain at the Pine Lake Environmental Campus of Hartwick College, by Charlotte Creek near Davenport, NY. The floodplain lies between the active channel and the hummocky topography of a moraine. Ice from the last glacial period retreated from this area about 14-12 kyr, which places an upper age limit on the floodplain. Humans utilized the floodplain intermittently. A few artifacts exhumed from the area date from 9700-8000 BP and 5000 BP, but most correspond either to 4000-1000 BP, or post-European settlement. The floodplain is a site of ongoing archeological excavations for a field school run jointly by Hartwick College and SUNY Oneonta. We conducted A GPR survey (500 MHz antenna) to facilitate location of potential excavations. The survey comprised a series of 1 m spaced profiles, where shot density along each profile was 0.025 m. The radar stratigraphy reveals laterally continuous “humps and hollows”, as well as localized reflectors, which we interpret as longitudinal bars, channels and clusters of cobbles respectively. The summer 2009 field school penetrated several of these features. We present here a comparison between radar stratigraphy, photographic appearance, and sedimentologic characteristics of several excavations in the floodplain. We find a high degree of fidelity between isolated radar reflectors and cobbles. In addition, radar stratigraphy maps well to contacts between layers of silt and sand or gravel.

The emerging picture of floodplain development is complicated. The presence of bars and channels in the shallow subsurface (< 1.3 m) raises the possibility of intermittent stream occupation of the floodplain, which leads to strong diachroneity in the deposits. Equating age with depth or distance from the modern channel is not possible. We view the modern floodplain as a shattered record in an active zone of erosion, transport and deposition that extends at least to 1.3 m depth. The relatively flat appearance of its present-day surface belies the stochastic character of floodplain deposition and preservation. Vertical and lateral accretion occurs over time in the presence of intermittent scour from abrupt shifts in channel location as well as lateral migration of the channel.