Serpent Mound - a 411 m-long prehistoric effigy mound located on a plateau in Ohio (USA) - was excavated by Frederic Ward Putnam in the late 1800s. Putnam didn't find any artefacts in the serpent, but two nearby mounds yielded finds belonging to the Adena culture, dating to between 800 BCE and 100 CE. Putnam also found traces of an Adena village near the mound, which is why it was widely believed that Serpent Mound was an Adena effigy mound. However, Putnam also found traces of a large village of the Fort Ancient culture overlying the earlier village, and another nearby mound contained Fort Ancient artefacts.
     In the early 1990s, Brad Lepper was part of a team which reopened one of Putnam's original trenches and recovered charcoal that seemed to indicate the Great Serpent was built by the Fort Ancient culture around 1120 CE. This date roughly corresponds to the age of the vast majority of other effigy mounds in eastern North America, including Ohio's other effigy mound, the Alligator. Unfortunately, the charcoal did not come from a discrete feature such as a fire pit, but consisted of small flecks mixed into the body of the mound.
     Lepper says that Serpent Mound makes more sense as a Fort Ancient culture effigy mound. The art of this period throughout eastern North America is rich in serpent symbolism.
     William Romain has argued for years that the Serpent was built by the Hopewell culture. Last year, Romain and a team of scientists recovered numerous flecks of charcoal in soil cores from across the mound, including several which yielded dates between 400 BCE and 80 BCE. These appear to confirm that the Adena culture built the mound as originally thought, but have the same provenance issues as those recovered two decades earlier.
     Lepper counters that serpent imagery is virtually absent in Adena art. There are serpents in Hopewell art, but they appear to have been regarded by the Hopewell as just one among many potential spirit guardians. In contrast, serpents are a hugely important component of Mississippian symbolism. Fort Ancient is not strictly a Mississippian culture, but the people did live in a Mississippian world and according to Lepper would have shared much of its cosmology. For him the mound fits within a broad, regional tradition of effigy mound building that includes Alligator Mound and the Kern stone serpent effigies of the Little Miami Valley.

Edited from The Columbus Dispatch, Ohio Historical Society Archaeology Blog (17 November 2013)