THE GIANT BOULDER TRAINS OF TIERRA DEL FUEGO AND THE ORIGIN OF “DARWIN'S BOULDERS”

In the course of our mapping on Isla Grande in Tierra del Fuego we (re)discovered two large boulder trains deposited by the Bahia Inutil Lobe of the Darwin Cordillera ice cap. 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 lying on the surface of the LGM moraine. The boulder train is approximately 8km long and 2km wide and is oriented east - west – perpendicular to the LGM terminal moraine. In June of 1833, Darwin delayed the passage of the Beagle in order to more closely observe a collection of boulders in the surf and above the sea cliff along the coast at Bahia San Sebastain. The “Bahia San Sebastian Boulder Train” (“Darwin's Boulders”) is located on the surface of an older end moraine on the Atlantic coast just south of Bahia San Sebastian (53024S, 68005W). It consists of approximately 500 large, angular, granodiorite boulders arranged in a boomerang shaped pattern. Except for the boulders in these two boulder trains, the tills of the Bahia Inutil Lobe are nearly devoid of large 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” discusses the boulders at Bahia San Sebastian and concludes that “the boulders were transported by floating ice.” In 1841, Darwin was aware of the ability of glaciers to transport debris, deposit till and build moraines but was obviously still enamored with the then popular concept of ice rafting and, for numerous reasons, argued forcefully that the erratic boulders of “eastern Tierra del Fuego…can (not) have been produced like ordinary moraines; and, therefore, that the boulders cannot have been propelled by the glaciers themselves.” We disagree. Based on their huge size, uniform lithology, angular shape, clustered but elongate distribution, surficial position upon 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. Strain during approximately 200 km of supraglacial transport accounts for the narrow, elongate boulder distributions.