History

The first professional survey was conducted in 1846 and published in 1848 by Ephraim George Squier and Edwin Hamilton Davis in Ancient Monuments of the Mississippi Valley, the founding document of American archaeology, which attributed the earthwork to the Adena culture based on burial mounds with Adena artefacts found nearby. That attribution went largely unchallenged for over a century. In the 1990s, the archaeologist Robert Fletcher led radiocarbon dating of charcoal extracted from the mound interior, producing two distinct date clusters: one centring on approximately 321 BC (consistent with Hopewell culture) and one centring on approximately 1070 AD (consistent with Fort Ancient culture). The two-date result prompted a fundamental reassessment: some scholars argue the mound was built once in the Hopewell period and later dates reflect contamination; others propose that Fort Ancient people built a new and larger mound over an earlier structure; a third view holds the original Adena attribution may still be defensible. No consensus has been reached.

The mound has been under conservation management since 1900, when Harvard’s Peabody Museum purchased the site to prevent its demolition. Frederic Ward Putnam of the Peabody conducted excavations in the 1880s and 1890s, finding the mound was built in sections filled with puddled clay and faced with stones or sand — but finding no burials within the mound itself, a puzzling absence that distinguishes it from nearly all other effigy mounds. The Ohio History Connection now manages the site.

The serpent’s three major curves align with the summer solstice sunset to the northwest, the winter solstice sunrise to the southeast, and the equinox sunrise to the east. These alignments were identified by researchers in the 1980s and have been confirmed by subsequent independent analysis, making the Great Serpent Mound one of the most precisely astronomically oriented prehistoric earthworks in North America.