GEOARCHAEOLOGICAL STUDY OF AN OHIO HOPEWELL EARTHWORK

The Hopeton Earthworks, in Chillicothe, Ohio, were built about A.D. 150 and consist of three circular enclosures, a rectangular enclosure and two long parallel walls. In 1848, the two largest enclosures were estimated to each be 20 acres in size, with the walls of the rectangle being 50 feet wide and 12 feet high, but had already reduced the height of some of the earthen walls.

In 2001, the Midwest Archeological Center (NPS) initiated a program of archaeological, geophysical and geoarchaeological research aimed at learning when and how the earthen walls were built. The use of a cesium gradiometer and other geophysical instruments showed that basal remnants of most of the earthen walls can still be mapped. This has produced the first truly accurate map of actual placement of the walls, and served to guide excavation of eight trenches across the remnant earthen walls of the two largest enclosures, and two of the smaller enclosures.

These trenches indicate that prior to construction of the earthen walls, the A horizon and top of the B horizon was removed. Wall construction was initiated on the exposed subsoil surface. The types of soil material, and the order of deposition varied in each trench. With few exceptions, walls were built with two or three different soil types forming the core of the wall, and these were applied carefully so that the soil types did not become mixed. In all the trenches, at least one of the basal margins of the original wall surface was preserved and could be seen as a dark organic layer that gradually sloped upward from the outer margin of the wall toward the center. These dark layers are in situ soils that developed on the flanks of the walls. Radiocarbon dates indicate that wall construction occurred primarily between A.D. 100 and A.D. 250, with evidence for an episode of either wall repair or remodeling about A.D. 1000.

Soil cores, collected with a Giddings hydraulic probe, within and surrounding the enclosures have produced substantial evidence that between 0.5 m and 1.0 m of soil has been quarried from inside the enclosures, apparently to be used in wall construction. We estimate that nearly 30,000 m3 of soil were required to build the embankment walls of the two larger enclosures, and the majority of this was quarried from inside the these enclosures.