They found a mineral called shocked quartz.
Shocked quartz has microscopic fractures that form only under high pressure—for instance, from the shock wave of a meteorite impact.
"Most people working in the field would accept shocked quartz as a diagnostic test for meteorite impact," Amor said.
The researchers also found higher amounts of iridium and chromium 53 in the samples than in the surrounding rocks. The levels of both these elements are consistent with those found in meteorites.
The scientists concluded that the layer is part of an "ejecta blanket," or a layer of debris that settles after a meteorite impact.
Based on the length of the outcrop, the team estimated that the object striking Earth must have been at least 1,650 feet (500 meters) wide and formed a crater at least 3.75 miles (6 kilometers) across.
(Related news: "Crater From 1908 Russian Space Impact Found, Team Says" [November 7, 2007].)
Amor and his colleagues published their research in the April issue of the journal Geology.
The impact occurred about 1.2 billion years ago, when Scotland was still attached to North America and the Atlantic Ocean had not yet formed.
Amor suspects that the actual crater is probably beneath The Minch, a strait that separates the Isle of Lewis from the Scottish mainland.
Alexander Deutsch is a meteor expert at the University of Münster in Germany.
"The data seems to be at the edge of being convincing," he said.
But he agreed that given the thickness of the layer, the crater may have had a size between that of the Ries Crater, 15 miles (24 kilometers) and Sweden's Siljan crater, 34 miles (55 kilometers) across.
Smith of the Natural History Museum is impressed by the analysis of the rogue layer.
"[The new analysis] is convincing evidence indeed that [the crater] is an ejecta layer from a large impact," she said.
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