THE comet or asteroid that wiped out the dinosaurs also left some giant chunks of sandstone perched on top of hills in Arkansas, a geologist claims. He says that the boulders must have been swept up onto the hills from coastal regions by a giant tidal wave, or tsunami, that followed the impact.
The rocks, roughly 120 kilometres northeast of Little Rock, don't appear to fit in with the surrounding landscape. "You've got boulders that are 25 feet (7.6 metres) in diameter, that are up on a 250-foot (76-metres) tall hill," says Gary Patterson of the University of Memphis in Tennessee. "It looks like the Planet of the Apes up there."
At a meeting of the Geological Society of America in Toronto last week, Patterson suggested that the famous impact of a comet or asteroid on the Yucatan Peninsula in Mexico 65 million years ago may have been responsible. This impact probably spelt doom for the dinosaurs and myriad other species.
Scientists believe the impact would have created a tsunami that swept across the Gulf of Mexico and the Caribbean. The rush of water could have carried rocks from the Louisiana coast inland over huge distances in the direction of Arkansas.
This theory fits in with the structure and composition of the Arkansas boulders, according to Patterson. The sandstone boulders contain a mineral called glauconite, which is abundant only in marine environments and is not found in high regions of Arkansas. This suggests the rocks came from a coastal area, he says.
Patterson has ruled out other explanations. One possibility is that the boulders were pushed into position by glaciers, for instance. But glaciers never came that far south, he says. A river could have moved the rocks, but its power would have been such that it would have left other visible signs.
"There aren't any other known agents that could transport these boulders, besides something like a tsunami wave," Patterson concludes. He plans to work out the precise age of the Arkansas boulders in an attempt to confirm his idea.
Author: Dan Falk, Toronto, New Scientist, issue 7, November 1998.
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