Glover Bluff
Key facts
- Country
- United States
- Diameter
- 8 km
- Age
- less than about 485 million years
- Location
- 43.97° N, 89.53° W
Very little: low bluffs in Marquette County, Wisconsin, with disturbed and broken dolomite exposed in quarries. Most of the area is farmland and woodland.
Glover Bluff is a deeply eroded impact structure in Marquette County, south-central Wisconsin, in the United States. It is thought to have been about 8 km across, though only parts of it are exposed.
What it hit. The target was a sequence of Cambrian sandstones and Ordovician dolomites. The impact broke and tilted these layers, and the rebound of the crater floor raised the centre. Since then, erosion, including scouring by ice sheets, has removed most of the crater, leaving only its deeper parts.
Proof of impact. Shatter cones, cone-shaped fractures that form only under the shock of an impact, and impact breccia have been found in outcrops and quarries. The structure also lies on a round pattern of slightly unusual gravity and magnetic readings, which helped geologists trace its outline.
Age. The youngest rocks affected are Ordovician, so the impact happened less than about 485 million years ago; its exact age is unknown.
Why it matters. Glover Bluff is one of several impact structures recognised in Wisconsin, a state where ancient flat-lying rocks preserve the scars of old impacts. The impactor is unknown.
Visiting. Quarries are private and dangerous; enter only with permission and proper guidance.
Did you know?
Shatter cones in quarries helped show that Glover Bluff was an impact.
Most of the original crater has been eroded away.
Its outline shows up in gravity and magnetic measurements.