for National Geographic News
The study findings solve a long-standing puzzle about a layer of rock that stretches for about 31 miles (50 kilometers) in northwest Scotland.
The layer, 65 feet (20 meters) thick in places, is sandwiched between gigantic layers of red sandstone and siltstone, which form the so-called Torridonian sedimentary rocks.
Some scientists have suggested that the layer hails from a volcano—but there is no evidence of volcanic activity in the region.
Also, the rocks below the layer hint at something other than volcanic deposition, scientists say.
"Some of the underlying strata have quite obviously been ripped up, and curled over, and in some places rolled up almost into a ball," said study co-author Kenneth Amor of the University of Oxford, in England.
The layer also contains green bits of shattered rock fragments that had once been molten. Similar pieces have been found in the impact layer of the Ries Crater in southern Germany.
"The layer has always been a bit of an oddball," said geologist Caroline Smith, curator of the meteorite collection at the Natural History Museum in London, England. Smith was not part of the study.
(Related news: "Mysterious Meteorites Stymie Scientists" [March 12, 2008].)
To co-author Amor, these features smacked of a high-energy meteorite impact. So he and colleagues from the University of Oxford and the University of Aberdeen, in Scotland, analyzed samples from the layer for signs of a crash.
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