Enigmatic boulder trains, supraglacial rock avalanches, and the origin of “Darwin's boulders,” Tierra del Fuego

摘要

Charles Darwin considered himself to be a geologist and published extensively on many geologic phenomena. He was intrigued with the distribution of erratic boulders and speculated upon their origins. In his accounts of the voyage of the HMS Beagle,Darwin described crystalline boulders of notable size and abundance near Bahía San Sebastian, south of the Strait of Magellan, Tierra del Fuego. Influenced by Charles Lyell?s reflections upon slow, vertical movements of crust, submergence, and ice rafting to explain drift, Darwin proposed that the boulders of Bahía San Sebastian were ice-rafted. Benefiting from 170 years of subsequent study of the glacial history of Tierra del Fuego, petrography, and terrestrial cosmogenic nuclide measurements, we revisit the origin of “Darwin?s Boulders” at Bahía San Sebastian. We suggest that they, as well as another train of boulders to the west, at Bahía Inútil, represent rock falls of Beagle-type granite from the Cordillera Darwin onto glacial ice flowing into the Bahía Inútil–Bahía San Sebastian lobe. These supraglacial rock avalanche deposits were subsequently elongated into boulder trains by glacial strain during transport and then deposited upon moraines. The cosmogenic nuclide exposure dates support the correlation of Andean glaciations with the marine oxygen isotope record and the glacial chronologies recently proposed for Tierra del Fuego.