Geoarchaeology at Leetsdale: Reconstructing Prehistoric

Landscapes of the Upper Ohio Valley

Joseph Schuldenrein, Geoarcheology Research Associates

Frank Vento, Clarion UniversitySuanna Selby-Crowley, Geoarcheology Research

Associates2009 GSA Annual Meeting, October 18-21, 2009 Portland, Oregon

Objectives

• Developing linked archaeological and geological stratigraphies at a complex, multi-component site in a region that is relatively unknown.

• Structuring geoarchaeological observations that interface with paleoenvironmental trends.

• Modeling “geo” and “archeo” trends into a diachronic model of site formation.

• Extending the model valley wide, regionally and extra-regionally.

Project Setting

Site area is 40km south of glacial margin. There is an expectation for outwash sedimentation on major valley floors.

Industrialized Landscape on Right Bank, Ohio River

Area 1

Area 3

Area 2

Primary Excavation Areas within Project Footprint

Individual areas selected on the basis of potential impact and archaeological potential. The latter is previewed by subsurface stratigraphy.

Project investigation parcels and terrace relations

Historic Physiography and Land Use

GIS layering generates area-specific images of former land cover.

Archaeological Excavations

Prehistoric features are housed within soil horizons to variable depths.

At depths of ±1 m, excavation extends into terrace substrate which exposes Bw, Bt, or Btx horizons depending on terrace elevation and paleosol articulation.

Baseline Chronology

Initial testing disclosed carbonized residue to depths of 7m but definitive cultural features extended to 4 m, well into Middle Archaic time ranges.

Leetsdale: Initial Sensitivity Map

Isobar delimits extent of Bt horizon. Superposed perimeters offset zones of older and younger prehistoric distributions. Intersection forms zone of multi-component stratification.

Sedimentation Rates and Soil Formation

Terrace construction was most dynamic around 6-5 kya (Area 2N) and rates of alluviation declined after 3 kya.

Evidence for deep soil formation is pervasive in the Late Archaic horizons.

Site Formation in the Back-Channel & Area 1

Lamellae signify episodic flood events and limited pedogenesis in the back channel.

This trend carries over into Area 1, where Late Archaic horizons were found both in lamellar and Bt horizons.

Micromorphology

Medium sands with silty infillings. This is typical of episodic flooding followed by translocation of clays.

Finer textured parent material with clay coatings characteristic of alluvial paleosol (Bt horizon).

Geoarchaeological Variability in Area 2

Leetsdale Composite Section

Back Channel: Changing Vegetation Through Time

AU-2 to AU-3 marks peak of oak frequency and general displacement of tree cover by cheno-ams and herb communities. Is this related to forest clearance at the latter stages of the Late Archaic?

Composite Block Diagram, Leetsdale Prehistoric Site

Leetsdale: Detailed Radiometric Chronology

Phosphates Register Anthropogenic Signatures

Phosphate fractionation differentiates occupational signatures. Locus A depicts long term habitation while Locus B represents single use events (at Leetsdale).

Diachronic Model I: Leetsdale Terrace Construction

Diachronic Model II: Leetsdale Terrace Stabilization

Geoarchaeological Landscapes Along the Ohio

River Valley

Ohio Valley Sites, Landscapes, and Topographies

Landform-Sediment Assemblages, Ohio Valley Site Complexes

Preservation contexts of prehistoric assemblages. They are vertically stacked upstream (one terrace), laterally and spatially zoned downstream (two

terraces).

Geoarchaeological Landscapes Across

Pennsylvania

Representative Geoarchaeological Columns for Pennsylvania’s Trunk

Streams

Implications of the Leetsdale Study

• Allostratigraphic principles accommodate the latitude necessary for explaining complex site formation process.

• Terrace construction at Leetsdale had effectively been completed by 5000 B.P. Subsequent additions to terrace landscapes were the product of dynamic (alluvial) events. The Bt horizon is a marker of landscape stability.

• By 3000 B.P. there is some evidence of terrain change in the form of human interference (deforestation?).

• Broader geoarchaeological correlations are suggested by comparing the Leetsdale sequence with valley-wide (Ohio River) and inter-valley (Pennsylvania) chrono-stratigraphies.