Serpent Mound
Key facts
- Country
- United States
- Diameter
- 8 km
- Age
- less than about 320 million years
- Location
- 39.03° N, 83.40° W
A rough circle of disturbed hills and ravines about 8 km across in Adams County, Ohio. Within it, on a ridge above Brush Creek, lies the famous Serpent Mound earthwork.
The Serpent Mound structure is an eroded impact crater about 8 km across in southern Ohio, in the United States. It takes its name from the Great Serpent Mound, a winding earthwork more than 400 m long built by Native American people, which happens to lie inside the structure.
A long puzzle. Geologists noticed in the nineteenth century that rocks in this part of Ohio were strangely disturbed, with layers tilted, faulted and broken in a roughly circular area surrounded by flat-lying rock. For more than a century the cause was argued over. Some thought it was a 'cryptovolcanic' explosion from below; others favoured an impact.
Proof of impact. In the 1990s geologists reported quartz grains with planar deformation features, microscopic sets of parallel planes that form only under shock pressures, from drill cores into the central uplift. Shatter cones have also been described. These features cannot be made by volcanic activity, and the structure is now accepted as an impact crater.
What it hit. The target was a sequence of Palaeozoic sedimentary rocks, mainly limestones, dolomites and shales of Ordovician to Mississippian age. In the centre, the rebound of the crater floor lifted older rocks several hundred metres. The youngest rocks affected are Mississippian, so the impact happened less than about 320 million years ago; the precise age is unknown.
A coincidence. There is no evidence that the builders of the Serpent Mound knew of the impact, which happened hundreds of millions of years before people existed. The location of the earthwork on the disturbed ground is generally seen as a coincidence.
Visiting. The Serpent Mound earthwork is a protected historic site with a museum. Respect the earthwork and stay on paths.
Did you know?
The famous Serpent Mound earthwork lies inside the impact structure.
The disturbed rocks were noticed in the nineteenth century but only proven to be an impact in the 1990s.
Rocks in the centre were lifted several hundred metres by the impact.