Geoscientist Online

Darwin’s first long journey in Argentina began in early August 1833, when he left the Beagle at Patagones, at the mouth of the Río Negro, and travelled via Buenos Aires to Santa Fé - a total journey of almost 1200km lasting three months. The first part, from Patagones to Buenos Aires, took him across the pampas, where he found mammalian and molluscan remains in an extensive series of deposits that he named the Pampean Formation. Darwin regarded the faunas as of Recent age. Marcelo Zárate and Alicia Folguera review more recent understanding of this formation, showing that it consists of varied continental sediments deposited in four cycles spanning the last 12 million years, from the Late Miocene.

During this traverse Darwin revisited the area around Bahía Blanca, which he had previously explored from 7 September to 20 October 1832 in his first Argentine expedition. It was there that he made some spectacular giant mammal finds, referring to his first locality, Punta Alta, as “a perfect catacomb for monsters of extinct races”. One was Megatherium, a spectacular trackway of which is illustrated by Quattrocchio et al. in their review of the geology and palaeontology of the Bahía Blanca area (Darwin never saw this but collected tons of bones).

Darwin’s fascination with the giant Pleistocene mammals that he had seen, coupled with his visit to the lush Brazilian forest, led him to speculate on whether those animals actually needed luxuriant vegetation to survive. He concluded that they did not. Vizcaíno et al. review “recent contributions on the palaeobiology of Pleistocene fossil mammals of South America to provide answers for Darwin’s questions’.

From Buenos Aires to Santa Fé, Darwin traversed an extensive area of predominantly aeolian Pleistocene sediments and recorded a marine Miocene section. His detailed observations are reviewed in the light of modern work by Martin Iriondo and Daniela Kröhling who, like so many contributors, emphasise Darwin’s “acute observation powers and precise descriptions”.

 
Much of the surface of Patagonia is covered with a layer of gravel (the Rodados Patagónicos in modern terminology), which Darwin first saw at Bahía Blanca and which he named the “Patagonian Shingle Formation”. Oscar Martínez et al. stress Darwin’s surprise at the vast extent of these deposits and note his conclusion that they initially accumulated as alluvial sediments at the foot of the Andean Cordillera, to be subsequently redeposited during a marine transgression. But the marine shells that Darwin recorded are now believed to represent man-made accumulations. The Rodados Patagónicos actually includes gravels of various ages and provenance, including piedmont deposits (in northern Patagonia) and glacifluvial deposits further south.

Darwin also recorded some large erratic blocks and associated gravels during a journey along the valley of the Río Santa Cruz (S. Patagonia). Jorge Strelin and Eduardo Malagnino stress the continuing validity of Darwin’s detailed observations on their stratigraphy and geomorphology. However, while Darwin interpreted these sediments as glaciomarine, they are now regarded as terrestrial, some marking the maximum glacial expansion in Argentina, others “linked to moraines and glacifluvial terraces of the Penultimate Glaciation”.

At the mouth of the Río Santa Cruz, Darwin recorded some marine sediments (now included in the Early Miocene Monte León Formation). Ana Parras and Miguel Griffin show that these were deposited in an inner shelf to subtidal or even intertidal environment. From here and from another locality further north at Puerto Deseado (Port Desire, described by Silvio Casadío and Miguel Griffin) Darwin collected fossils that were later described by G B Sowerby.

HMS Beagle visited Tierra del Fuego on two occasions, and Darwin recorded a clay-slate “formation” in the Fuegian Andes from which he obtained a few Cretaceous fossils. Eduardo Olivero et al. review the trace and body fossils to show that the constituent formations extend from Aptian to Maastrichian.

Darwin made a hand-coloured geological map of parts of Patagonia, either while he was on the Beagle or shortly afterwards. This is preserved in Cambridge University library, and its significance is reviewed by Eduardo Zappetini and José Mendía. It was never published in Darwin’s time, but the authors conclude that if it had, “it would have constituted a singular initial contribution to the geological cartography of the South American continent”.

Darwin’s last visit was made while HMS Beagle was surveying the coast of Chile, in 1835. Then he crossed the Andes from Santiago to Mendoza via the Piuquenes Pass and made many geological observations, including on the tectonic development of the Andes. Laura Giambiagi et al. note that he was one of the first scientists ever to realise that “in an orogenic system the sequence of uplift and deformation proceeds from hinterland towards foreland”.

Once over the Andes, Darwin made a particularly interesting discovery in Mendoza, at Agua de la Zorra, east of Uspallata – a small, coniferous fossil forest, in which he observed 52 stumps. These are silicified and still in growth position. Darwin thought the forest was Tertiary, but it is now known to be Triassic. Two papers are devoted to this locality, now known as “Darwin Forest”. Stella Poma et al. consider the depositional environment and comment on how detailed and accurate Darwin’s basic observations were. However, while he thought the forest had been buried by marine sediments, we now know it was swamped by pyroclastic material. Mariana Brea et al. look at the taxonomic composition of the forest, and conclude from the structure of the vegetation, growth ring analysis and sedimentation that the trees lived in a dry, subtropical region subject to strong seasonality.

On his return journey to Santiago Darwin visited the Punta del Inca, a natural bridge that Victor Ramos notes “was formed as an ice bridge associated with snow and debris avalanches later cemented by the minerals precipitated by the adjacent hot-water springs.” Ramos comments that Darwin did not publish most of his notes on this feature (mainly because he was not very impressed by it!). But he was impressed by the complex structural geology of the Uspallata area, and Ramos shows how Darwin recognised both the episodic nature of Andean uplift and that uplift is still continuing. These fundamental tectonic observations provide a fitting climax to Darwin’s Argentine studies.

Author affiliations

* (1) Emeritus Professor, Department of Earth Sciences, UCL; Honorary Professor, CEMS, University of Hull (Scarborough); Corresponding Member, Associación Geológica Argentina. (2) Professor of Palaeontology, University of Buenos Aires and Principal Researcher, CONICET