The first anomalous thing people living in Manson noticed was the water. Iowa’s groundwater typically has a fairly high dissolved mineral content, mostly calcium and magnesium, absorbed as the water passes over and through the limestone formations underlying the state. It’s considered “hard water.” But water coming from wells in the small western Iowa town, settled in the 1870s, is soft, with low mineral content. That’s because the limestone that would have been beneath Manson was obliterated by a massive meteor strike during the Cretaceous Period.
Of course, no humans were around 74 million years ago to witness the arrival of a stony meteorite measuring more than a mile across. As for any dinosaurs and small mammals at that spot on the coast of the Western Interior Seaway, none would have survived the impact of the 10 billion-ton space rock.
The blast created by the impact was “the equivalent of about 10 trillion tons of TNT,” according to UI geologist Raymond R. Anderson. “An electromagnetic pulse moved away from the point of impact at nearly the speed of light, instantly igniting anything that would burn within approximately 130 miles (most of Iowa). The shockwave toppled trees up to 300 miles away (Chicago, Minneapolis, St. Louis) and probably killed most animals within about 650 miles (Detroit, Denver).”
In his A Short History of Nearly Everything, Bill Bryson offered a pithy summary of what happened 74 million years ago: “The Manson impact was the biggest thing that has ever occurred on the mainland of the United States — of any type, ever.”
It’s possible that Iowa pride led Bryson, who grew up in Des Moines, to overestimate the Manson impact. But if he’s wrong, he’s not wrong by much.
The impact left a crater approximately three miles deep and 24 miles in diameter. But there’s no sign of that crater in the flat landscape around Manson. The glaciers that later covered most of Iowa filled and buried the crater with glacial till as they moved across the Midwest.
The first sign that something unusual was deep beneath Manson came early in the 20th century, when people drilling water wells turned up unusual rocks. More rocks showing signs of some massive shock surfaced during some wildcat oil drilling attempts in the 1930s. The weird rocks and soft water got the local geology classified as the “Manson anomaly.”
The first investigation of the anomaly by geologists happened in 1955. At first, the general consensus was there might be a crypto-volcano under Manson. In 1959, Robert Diaz concluded the anomaly was a massive meteorite impact crater. Evidence for the impact crater continued to be found, and by the mid-’60s, it was clear that was the answer.
As the scale of Manson impact structure became better understood, some floated the notion that the big blast was partially responsible for the extinction of the dinosaurs at the end of the Cretaceous Period, 65 million years ago. But improvements in dating technology eventually exonerated Manson. The dinosaurs still had another 9 million years ahead of them after “the biggest thing that has ever occurred” in Calhoun County happened.
This article is from Little Village’s December 2025 Peak Iowa issue, a collection of stories drawn from Hawkeye State history, culture and legend. Browse dozens of Peak Iowa tales here.
