It is thought to have been caused by a meteor about 200 metres (660 ft) wide which struck during the Middle Ordovician Period, circa 470 million years ago.

[2][3][4] There is no surface evidence of the impact, as the Winneshiek Shale is more than 50 feet (15 m) below the bottom of the Upper Iowa River.

The impact event, equivalent to 1,000 megatons of TNT,[3] did not appear to penetrate the Earth's mantle, but it did push down the underlying Ordovician and Cambrian bedrock several hundred feet.

[6] The Middle Ordovician (Darriwilian) Winneshiek Lagerstätte sediments that filled in the crater contained a wide variety of unusual fauna.

Pentecopterus was scorpion-like in appearance, and the largest predator known from that time, measuring nearly 6 feet in length.