Darwin's geological mystery solved
Origin of odd South American boulders may have defeated the author.
In June 1833, Charles Darwin asked the captain of the HMS Beagle to delay his departure from Tierra del Fuego so that he could study a strange group of granite boulders he had found on the coast at Bahía San Sebastián.
"[O]ne of these, shaped somewhat like a barn, was forty-seven feet in circumference and projected five feet above the sand beach," he later wrote. "There were many others half this size, and they all must have traveled at least ninety miles from their parent rock."
What made the boulders unusual was that, other than these 500 or so big rocks spread out in a long, banana-shaped region, there were no others in the vicinity. Darwin was puzzled. How did the boulders get there?
"Darwin is known mostly for evolution and natural selection," says Edward Evenson, a glacial geologist from Lehigh University in Bethlehem, Pennsylvania — but his interests were broader. "Darwin considered himself a geologist," Evenson said yesterday at a meeting of the Geological Society of America in Portland, Oregon.
In South America, mysterious rocks weren't all that Darwin found. He also saw beach-like landscapes, hundreds of metres above the present-day waters of the Pacific. He saw rocks embedded in glacial ice and heard at least one report of a rocky iceberg floating far out to sea.
The boulders, he concluded, had been scoured out of the mountains by glaciers that calved into the sea. The icebergs then ran aground, melted and dropped their rocky burdens on the seabed, where subsequent uplift raised them above the waves.
In a pickle
It was a good theory, but unfortunately, says Evenson, who recently revisited the site as part of a mapping project, it doesn't quite work. To begin with, he said, there is another, similar group of rocks at a place called Bahía Inútil. This group consisted of 1,000 large rocks, again spread over an elongated zone, whose outline Evenson this time compared to a pickle or gherkin, 8 kilometres long by 2 kilometres wide. "Darwin never saw these boulders," he said.
If both groups of rocks had been carried by icebergs, the icebergs must have been remarkably similar. Nevertheless, both sets of rocks were angular, indicating that they had been carried atop the ice, rather than bulldozed in front of it.
"Where do big angular boulders get onto glaciers today?" Evenson asked. "Rockslides." The shapes of the boulder fields were another clue: because glaciers flow more rapidly in their centre than at their edges, landslide debris tends to get stretched into ever-lengthening ellipses as it moves down a valley.
Finally, he said, the rock must have fallen onto the glacier somewhere above the zone where snow melts faster than it accumulates. That far up, Evenson said, glaciers tend to be concave, allowing rocks to slide far along their surfaces. Lower down, the glaciers are more likely to be convex, trapping rocks near the edges.
Putting all these clues together, Evenson was able to pinpoint the source of the rockfall to one of three locations, the most likely of which was beside a tributary called the Parry Glacier, 200 kilometres from the boulders' present locations.
When the flow of a roughly circular patch of rocks is modelled from that starting location, he said, "we get a pickle at Bahía Inútil and a banana at San Sebastián."
Other scientists were impressed. "It was quite convincing," says Kevin Padian, curator of the Museum of Paleontology at the University of California, Berkeley.
He notes, though, that Darwin wasn't completely wrong. "He had the general idea of where [the rocks] came from and what direction they were going, but he didn't realize they were carried by an ice field," Padian said. "He thought they were carried by icebergs."