Charles Darwin in Southern South America

While HMS Beagle charted South America's coasts, Charles Darwin made pioneering geological observations. An unpublished map of southern Patagonia and Tierra del Fuego showed seven units, whose distributions approximate modern patterns. Along the northern Beagle Channel in Cordillera Darwin, he noted Mica Slate and boulders of granite. The former represents a mid-crustal complex not seen elsewhere in the Andes. This pre-Late Jurassic basement was intruded by syn- and post-tectonic granites as deformation propagated toward the foreland. Darwin's most widespread unit, Clay-Slate, comprises slate and volcanic-rich greywackes deposited upon ophiolites in a Late Jurassic-Early Cretaceous back-arc basin. Darwin mapped ophiolite as Trappean rock. He mapped the Pacific side of the orogen as Granite and Mica Slate with some Porphyry, Tufa (tuff), & Metamorphics. A fold-thrust belt along the Andean margin involving a Late Cretaceous-Early Tertiary foreland basin was not recognized. Foreland plains were mapped as Tertiary, Recent and Basalt with no distinction of glacial deposits. Glacial theory was still in the future and Darwin followed Lyell by ascribing widely scattered erratic boulders to ice rafting during a marine submergence. Inference of large-scale subsidence and uplift demonstrated by marine fossils within thick sedimentary sequences high in the Andes and by elevated coastal marine terraces was a major contribution, providing a prelude for Darwin's coral atoll theory. Surprisingly, he did not recognize the importance of compression in mountain building, although in his writings he described folds and slaty cleavage cutting across stratification. Like Lyell, Darwin considered only vertical tectonics to be of paramount importance. After the voyage, Darwin became very active in the Geological Society of London, contributed many talks and articles about South America, and considered himself first and foremost a geologist.