Large meteorite scar identified in Virginia.

If geologist C. Wylie Poag is correct, the Chesapeake Bay owes its existence to an ancient splash.

The U.S. Geological Survey (USGS) researcher from Woods Hole, Mass., and his colleagues have uncovered evidence that a large buried crater underlies the southern section of the bay. According to their theory, a meteorite impact in this spot 35 million years ago determined subsequent river flow in the mid-Atlantic region, causing them to drain toward the present position of the bay.

Poag first uncovered signs of the crash several years ago while studying rock samples pulled up from drilling operations in southern Virginia. The drill cores revealed an unusual layer of jumbled sediments and boulders, called breccias, from the late Eocene period.

Because he was studying a meteorite crater of the same age located off the coast of New Jersey (SN: 11/14/92, p.334), Poag reasoned that the Virginia breccias formed during a tsunami triggered by the same offshore impact. In the late Eocene, eastern Virginia would have been underwater, with the coastline sitting more than 100 kilometers to the west.

Poag changed his mind after examining the results of seismic studies that imaged the rocks beneath the Chesapeake Bay. Oil companies had collected such information by sending seismic waves down into Earth and measuring the vibrations that reflect off buried structures.

These seismic reflection studies revealed a pattern of faults that form concentric circles, similar to a well-known impact crater in Germany, report Poag and his colleagues in the August GEOLOGY. "This shows clearly that there is a large impact crater in the southern part of the Chesapeake Bay," says Poag, who titled his paper "Meteoroid mayhem in Ole Virginny."

Judging from the span of the rings, the USGS geologists calculate that the crater measures 85 kilometers across, filling an area larger than Rhode Island. A structure this size would rank as the largest crater in the United States and among the top 10 known on Earth.

It might also explain a puzzling triangle of impact debris stretching from New Jersey to Texas to Barbados. Within this large region, geologists have discovered a layer of taktites -- distinctive rocks thrown into the air by an impact -- dating to the late Eocene period. Poag and his colleagues propose that the crash at the southern end of the Chesapeake Bay created this vast field of tektites.

Poag's report has made a splash among other geologists, who would welcome the opportunity to study a large, relatively young crater. But the limited evidence has yet to bowl over most scientists. "It's not obvious that we're dealing with an impact deposit," comments Richard A.F. Grieve, who studies craters for the Geological Survey of Canada in Ottawa.

Grieve and his coworkers are currently studying rock samples from the Virginia breccias. They are looking for "shocked" mineral grains, which bear distinctive fracture patterns formed by the high-pressure shock waves generated during impacts. A preliminary search has not turned up clear examples of any shocked grains, but his group will continue this work for the next several weeks.

Lacking any shocked minerals, Poag may have a difficult time making his case. Grieve remains skeptical about the seismic reflection data because such images are equivocal. "Seismics are a matter of interpretation. People see what they want to see," he says.