Wetumpka
Key facts
- Country
- United States
- Diameter
- 7.6 km
- Age
- about 83 million years (Late Cretaceous)
- Location
- 32.52° N, 86.17° W
A horseshoe of wooded hills about 7 km across around the eastern side of the town of Wetumpka, Alabama, open to the south-west. Most of it is private land.
Wetumpka is an impact crater about 7.6 km across in central Alabama, in the United States. It formed in the Late Cretaceous, about 83 million years ago, when a shallow sea covered the southern part of the state.
A marine impact. The impactor fell into a sea about 100 m deep. It punched through soft sediments on the sea floor into the hard metamorphic rocks beneath, ancient schists and gneisses of the Appalachian Piedmont. The crater rim, made of these crystalline rocks, survives as a horseshoe-shaped ridge of hills. The south-western side is missing, perhaps because the impactor came in at an angle or because sea water rushing back into the crater destroyed that part of the rim.
Discovery and proof. The structure was found during geological mapping in 1969 and 1970, and was described in the 1970s as an unusual crater-like feature. Its impact origin remained uncertain until 1998, when geologists drilling near the centre recovered cores containing shocked quartz, with microscopic sets of planes that form only under the pressures of an impact.
Age. The age comes from the fossils of tiny sea creatures in the youngest layers disturbed by the impact and in the sediments that filled the crater afterwards.
Why it matters. Wetumpka is one of the best-studied marine craters in North America and shows how sea water reshapes a crater immediately after an impact.
Visiting. The town celebrates its crater with signs and occasional guided tours. Almost all of the rim is private land, so ask permission before exploring.
Did you know?
Wetumpka formed when an asteroid fell into a shallow sea that covered southern Alabama.
Its rim of crystalline rock forms a horseshoe of hills around the town.
Proof of impact came from shocked quartz in a drill core recovered in 1998.