Edward Evenson, Earth and Environmental Sciences, Lehigh University
When: Friday, February 20, 2009, 10:30 a.m. to 11:30 a.m.
Where: Seminar Room 1.603, 10100 Burnet Road, Bldg 196-ROC, Austin, Texas 78758
Host: Sean Gulick, UTIG
Most people consider Darwin a “Biologist” and associate him almost entirely with the theory of evolution. In reality Charles was first, if not foremost, a “Geologist”. He published extensively on geologic subjects and was especially interested in the origin of “erratic boulders”. In the course of our geologic mapping in Tierra del Fuego we discovered two boulder trains of erratics which we refer to as the “Bahia Inutil” and “Bahia San Sebastian” Boulder Trains. The Bahia Inutil Boulder Train is located on the southeast corner of Bahia Inutil (53030S, 690 14W) and consists of approximately 1000 huge (up to 17 x 5m and averaging 3 X 4m), angular, granodiorite boulders. The “Bahia San Sebastian Boulder Train” (“Darwin’s Boulders”) is located on the Atlantic coast just south of Bahia San Sebastian (53024S, 68005W). It consists of approximately 500 large, angular, granodiorite boulders. Except for the boulders in these two boulder trains the area is devoid of boulders. Darwin (1841, p.419), in his classic paper, “On the distribution of the Erratic Boulders and on the Contemporaneous Unstratified Deposits of South America” describes the boulders in the San Sebastian area as “…many gigantic boulders…one of these, composed of syenite and shaped somewhat like a barn, was forty-seven feet in circumference and projected five feet above the sand beach. There were many others half this size, and they all must have traveled at least ninety miles from their parent rock.” Darwin also noted (as we do for both boulder trains) that the boulders appear to be only “on the surface” of the underlying unit. In 1841, Darwin was aware of the ability of glaciers to transport debris but was obviously still enamored with the currently popular concept of ice rafting and, for numerous reasons, argued that the boulders and tills of “eastern Tierra del Fuego…can (not) have been produced like ordinary moraines; and, therefore, that the imbedded boulders cannot have been propelled by the glaciers themselves.” He concludes, “that the boulders were transported by floating ice.” Because of their huge size, uniform lithology, angular shape, clustered but elongate distribution, surficial position on the moraines and their relationship to end moraines we consider the boulders, in both boulder trains, to have originated as large rock falls, or landslides, onto the surface of glaciers high (above the equilibrium line) in the Darwin Cordillera of Chile and subsequently transported, supraglacially, to their present positions by glacial flow. The strain associated with over 300 km of ice flow accounts for the narrow, elongate boulder distributions. Darwin was right, and wrong. The boulders are on the surface of the till and they came from far away - but not by ice rafting.