A new multistage construction chronology for the Great Serpent Mound, USA

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Effigy mounds occur across the midcontinent of North America but their cultural purposes and construction chronologies are rarely known and often controversial. Determining the age and construction history of monuments is important to relate religious symbolism, scientific knowledge, and cultural continuity to groups within a region. Based mainly on circumstantial evidence, researchers have long held that Serpent Mound in Ohio, USA, was constructed 2000–3000 years ago during the Early Woodland (Adena) or Middle Woodland (Hopewell) periods. Excavations in 1991 recovered charcoal buried at shallow depths (35–45 cm) in fill units of the mound and the 14C ages from two of these units indicated that Serpent Mound was built ∼900 years ago, during the Late Prehistoric (Fort Ancient) period, much later than originally thought. Our recent multidisciplinary work provides a more complex, robust construction history of Serpent Mound. We used geophysics to map the mound, and solid-earth cores to provide accurate stratigraphy and organic samples for 14C age estimates from the base of the mound. Bayesian statistical analyses of the seven 14C ages from Serpent Mound suggest that it was first constructed ∼2300 years ago during the Early Woodland (Adena) period but was renovated 1400 years later during the Late Prehistoric (Fort Ancient) period, probably to repair eroded portions of the mound. Modification of the mound is also indicated by a possible abandoned coil that is located near the head of the Serpent and visible only in the magnetometer survey.