The swampy, semitropical wilderness in southern Florida called the Everglades lies in a peculiar bowl-shaped depression that was bound to arouse the curiosity of geologists. They concluded years ago that the distinctive cavity was probably formed over many aeons as ground water slowly dissolved a surface layer of limestone. Now Geologist Edward J. Petuch, 36, of Florida International University in Miami, has another idea. In a report to the Geological Society of America's national convention in Orlando, he suggested that the Everglades are the mud-filled remains of an impact crater left by an asteroid that struck the earth 38 million years ago and punched a hole in the ancient seabed, which then lay under 600 ft. of water.
Much of the evidence behind Petuch's hypothesis has been available to scientists for decades. As early as the 1940s, geologists noticed an extensive network of fractures that radiates outward in the layers of limestone beneath the Everglades like cracks around a bullet hole in a pane of shatterproof glass. Maps published by the Florida Bureau of Geology in 1974 show a pit-like dip in the area's underground geological contours. Magnetic readings in the Everglades suggest the presence of a subterranean mass of metallic ore that could conceivably be the remains of an asteroid. Finally, scientific journals have noted that a commonly found rock stratum, called the Ocala formation, is suspiciously absent in southern Florida. Petuch suggests that it was hurled into the sky during impact.
That impact, he says, along with two other major hits by either comets or asteroids around the same time, threw enough soot (from the resulting conflagrations) and debris into the atmosphere to blot out the sun for months. It was this catastrophe 38 million years ago, he says, that may have caused the great Eocene-Oligocene extinction, which was similar to the one that many scientists believe killed the dinosaurs 27 million years earlier.
Citing evidence from bore samples taken in southern Florida, Petuch says that, after impact, coral began to grow on the raised rim of the crater, forming a circular atoll-like formation. Later, when the sea level rose, the atoll gradually elongated into an ellipse as the coral (which seeks warm waters) migrated toward the shallows north of the original crater. Some 1.8 million years ago, the atoll contained an inland sea continually replenished by ocean waters. But as the rising coral walls gradually closed out the ocean, newly deposited sediments' piled up in the forming lagoon. The inland sea shrank, the basin filled with fresh water and, in the warm southern sun, soon became clogged with the rich grasses that formed the Everglades. Central Florida's Lake Okeechobee, says Petuch, is the last remnant of that great, sediment-filled lake.