Britain's biggest meteorite strike.

An unusual layer of rocks found along Britain's northwestern coast formed from debris thrown out of a crater during a meteorite strike more than 1 billion years ago, geologists say.

The Stac Fada stratum, long thought to be of volcanic origin, stretches for 50 kilometers along the Scottish coast and in some spots is more than 20 meters thick.

"It was a puzzle," says Kenneth Amor, a geologist at the University of Oxford in England. There are no similar strata elsewhere in the region, nor have any likely sources of volcanic material been found that match the stratum's age.

Amor and his colleagues analyzed samples from seven sites in Stac Fada. As much as 50 percent of each sample is made up of chunks of previously melted material that are 2 to 15 millimeters in diameter, the researchers report in the April Geology. Chemical analyses indicate that the layer contains iridium at concentrations approaching 20 times the average in Earth's crust. That anomaly, as well as the distinctive fractures that riddle the samples' quartz grains, is a hallmark of an extraterrestrial impact (SN: 6/15/02, p. 378).

The amount of material exposed in the Stac Fada suggests that the impact created a crater about 6 kilometers in diameter, the largest in Britain, says Amor. That, in turn, suggests that the object that struck Earth was approximately 500 meters wide. Radioactive dating of the sandstones directly atop the ancient ejecta layer indicates that the impact occurred about 1.2 billion years ago, he notes.

Amor and his colleagues haven't pinned down where the ancient impact occurred, but gravitational anomalies about 15 kilometers offshore make that spot a prime candidate.