Glasford
Key facts
- Country
- United States
- Diameter
- 4 km
- Age
- less than about 430 million years
- Location
- 40.60° N, 89.78° W
Nothing: the structure is buried beneath farmland and glacial deposits near the village of Glasford in Peoria County, Illinois.
Glasford is a buried impact structure about 4 km across in Peoria County, central Illinois, in the United States.
Discovery. The structure was found during the drilling of wells in the area, including wells drilled to test rock layers for underground gas storage. The wells showed that rock layers were disturbed in a round area, with a raised centre and a surrounding ring. Study of the drill samples found breccia and quartz grains with features produced by shock, showing that it was an impact structure.
What it hit. The target was a sequence of Cambrian and Ordovician sedimentary rocks, mainly sandstones and dolomites, on the floor of a shallow sea. After the impact, younger sediments filled and buried the crater, and later glacial deposits covered the whole area.
Age. The oldest rocks lying undisturbed over the crater are Silurian, and the youngest disturbed rocks are Ordovician, so the impact happened in the Late Ordovician or early Silurian, less than about 430 million years ago.
Why it matters. Glasford is one of a group of buried impact structures of similar age in the American Midwest, which together suggest that the Earth was struck unusually often in the Ordovician. The impactor is unknown.
Did you know?
Glasford was found through drilling, not from anything at the surface.
It formed on the floor of a shallow sea.
Glacial deposits completely hide the structure.