The first geological map of Patagonia
Eduardo O. Zappettini and José Mendía
Servicio Geológico Minero Argentino (SEGEMAR) - Av. Julio A. Roca 651 (1322) Buenos Aires Emails: ezappe@mecon.gov.ar, jmendi@mecon.gov.ar
ABSTRACT
This contribution analyses the first geological map of Patagonia drawn by Darwin around 1840, and colour-painted by Darwin
himself. It had remained unpublished and only a small version in black and white had been printed before. The different units
mapped by Darwin are analysed from a modern perspective, and his ability to show a synthesis of the complex geological
structure of Patagonia is stressed.
Keywords: Geological map; Patagonia; Patagonian Shingle; Darwin geologist.
RESUMEN: El primer mapa geológico de la Patagonia. La presente contribución analiza el primer mapa geológico de la Patagonia realizado por Darwin cerca de 1840, pintado en colores por el mismo Darwin, que ha permanecido inédito y del que sólo se conocía una versión de tamaño reducido en blanco y negro. Se analizan las diferentes unidades mapeadas por Darwin desde una perspectiva actual, destacándose su habilidad para mostrar en esa síntesis la compleja estructura de la Patagonia.
Palabras clave: Mapa geológico; Patagonia; Rodados Patagónicos; Darwin geólogo.
DARWIN AND THE VOYAGE OF HMS BEAGLE
At the time Charles Darwin set sail on board HMS Beagle on a journey that was to last two years and ended up lasting five, he was not more than an amateur naturalist that had quitted his medical courses and after that abandoned his intention of applying for a position in the Church of England, just to embrace the study of natural history. In those years Charles Lyell (1830-1833) published his theory of gradualism (the present is the key of the past) as a counterpart to catastrophism then prevailing in geological thought. During the five years (1831-1835) that his journey on board HMS Beagle lasted - of which three years and one month were spent ashore - Darwin made countless observations on geology and biology and even on the communities that he visited (Fig. 1). These altogether paved the way towards his famous theory on natural selection and the origin of species and the influence that the environment - including its geological constitution and evolution - has on the success and adaptability of species.
Figure 1: Itinerary of HMS
Beagle and landings and
journeys of Charles Darwin
in the interior of South
America.
DARWIN'S CONTRIBUTIONS TO GEOLOGICAL KNOWLEDGE
Darwin's contributions to the geological
knowledge of South America were recorded
in four main papers, i.e., On the
connection of certain volcanic phenomena in
South America (1838), On the distribution of
erratic boulders and on the contemporaneous
stratified deposits of South America (1841),
An account of the fine dust which often falls on
vessels in the Atlantic Ocean (1846a,[Read 4
June 1845), On the Geology of the Falkland
Islands (1846b), and finally the book Geological
Observations on South America (1846c)
reprinted in 1876 as part of his Geological
observations on the volcanic islands and parts of
the South America visited during the voyage of
H.M.S. Beagle. On the other hand, a first
unpublished essay of his geological observations
was written during his voyage
on board H.M.S. Beagle ca. 1834 which is
titled Reflection on reading my geological notes.
In this work Darwin put forward a theory
on the geological formation of South
America and included his interpretations
on the history of life in the continent
which, albeit not evolutionist is nevertheless
sequential (cf. S. Herbert 1995).
In these works there are numerous and
detailed descriptions of geological units.
However, figures representing his observations
are few and of limited interest.
Despite of this, Darwin at some stage of
his life as a scientist tried to synthesize in
a map the main units that were subject of
his work, although such a map was never
published.
The Library at Cambridge University
keeps an original unpublished map painted
with water-colours by Darwin himself.
This is the earliest known Regional
Geological Map of the Patagonian region
of Chile and Argentina (Fig. 2). A
black and white version was published by
Rhodes (1991, Fig. 3) who stated "the
magnificent structure of this map" in contrast
with the other Darwin's unpublished
map of Patagonia which is rather simple
(Herbert 1991 and 2005, plate 6). The
geological observations were represented
on the topography produced by (1826-1830) and published in London by John
Murray. The watercolour is from the time
of Darwin's voyage on the Beagle or
maybe just a few years later, circa 1840.
On the back of the map there is an annotation
reading "p 94", which suggests
that it was drawn for inclusion as an illustration
to one of the volumes of the
Geology of the voyage of H.M.S. Beagle (1846c), but it was never printed.
The only previous regional maps are those
published by d'Orbigny (1842) together
with the geological sketch of the
provinces of Misiones, Corrientes, Entre
Ríos, Santa Fe, and Buenos Aires, including
northern Patagonia down to Valdés
Peninsula.
Figure 2: Geologic map of Patagonia
(Darwin, circa 1840, unpublished).
GEOLOGICAL MAP OF PATAGONIA
The observations by Darwin recorded on the map are mainly referred to coastal areas surveyed by HMS. Beagle (Fig. 2) and involve seven geological units:
1) Granite, Mica slate,
2) Trappean rock and porphyries,
3) Purple porphyries and infra metamorphics,
4) Clay slate,
5) Tertiary [newer - crossed out -] (Pliocene?),
6) Recent,
7) Basaltic lava.
About units 2 and 3 he states "These two are perhaps very closely allied".
UNIT 1 (Granite, Mica slate). It mainly involves the Cretaceous so-called Coast
Batolith that Darwin stated as exposed in
Chilean Patagonia south of the Strait of
Magellan. It includes outcrops at the
notheastern corner of Hoste Island and
at the northwestern sector of Navarino
Island. In this unit are also included the
Mica slates of the southernmost end of
the Chonos Archipelago in the Taitao
and Tres Montes peninsulas.
In his description of this unit Darwin
recorded different lithologies, among
which he distinguished - west of the
Beagle Channel bifurcation - a complex
of gneiss and granitic slates that comprise
the Cordillera presently bearing his
name (Eastern Metamorphic Complex of
Aysen and Magallanes in Chile and Lapataia
Formation in Argentina). Towards
the northeast he pointed out the predominance
of gneiss and granites (Coast
Batolith in Chile). Between Cabo Tres
Montes and the north of the Chonos
Islands the predominant rock is a Mica
slate with organic matter (presently Canal
King Formation).
UNIT 2 (Trappean rock and porphyries). It
includes intrusive and extrusive rocks
exposed in Hoste and Londonderry
Islands (Chile) and also Mesozoic ophiolitic
rocks exposed in Wollaston Island.
It comprises Paleogene monzodiorites,
diorites and gabros and Mesozoic basaltic
lavas of the Hardy Formation, and the
Sarmiento and Tortugas ophiolitic complexes
in Chile.
In Wollaston Island Darwin described "greenstones" with pyrite and epidote with
evidence of metamorphism, and basaltic
submarine volcanic rocks. In Hardy Peninsula
he recorded the pyrite-rich "trappean
and basaltic rocks", trachytes with
columnar jointing, and diorites rocks.
Darwin's precise drawing of the contact
between this unit and Unit 1 is especially
remarkable when compared to modern
maps.
UNIT 3 (Purple porphyries and infra metamorphics).
This unit was recognized by
Darwin on the coast of Santa Cruz. It
comprises the volcanic rocks of the
Chon Aike Formation, which in this sector
represents the easternmost exposures
of the Deseado Massif. Darwin described
these volcanics as trachytic porphyries.
The unit includes the tectonic silicified
breccias assigned to the Bahía Laura
Group, and exposed at Cabo Blanco.
These were erroneously interpreted by
Darwin as siliceous sedimentary rocks
and compared them to those of the Malvinas
Islands.
Off-limits he describes purple to pink,
and sometimes laminated, porphyric rocks, which are slightly vesicular with
feldspar and quartz crystals. He stated
these as the dominant rocks in between
Port Santa Elena, Camarones and Malaspina
up to the "Paps of Pineda" (sic).
At Puerto Deseado he records the presence
of porphyries up to 40 km west of
the coast, covered by Tertiary deposits.
He indicates that the composition is
similar to that of those described further
north and points out the presence of
local chert beds.
UNIT 4 (Clay slate). According to Darwin's
description, it comprises siltstones,
feldspar volcanics, graywackes, scarce
black limestones with Lower Cretaceous
fossil remains, and metamorphic schists.
These are the core of the mountain ranges
of southern Tierra del Fuego and
Navarino Island. He describes slates exposed
along the Beagle Channel between
its bifurcation and Lemaire Strait, constituting
Navarino Island to the south and
the eastern sector of Hoste Island and
Hardy Peninsula. North of the Beagle
Channel this unit runs in a northwestern
direction towards Brunswick Peninsula
on both coasts of Admiralty Strait, continuing
north along the eastern flank of the Cordillera. Darwin collected fossils
from the limestone beds he recorded.
These indicate a Cretaceous age.
Darwin stressed the change in the direction
of the Cordillera, that north of the
Strait of Magellan runs with a N-S orientation
changing south of it to a E-W
orientation. This unit includes the present
Lemaire and Yaghan Formations in
Argentina and Cerro Toro, Yaghan and
Punta Barrosa Formations in Chile,
spanning the Late Jurassic-Cretaceous
(see Olivero et al. 2009).
Again, we must stress the precision with
which Darwin drew the contact between
units 2 and 4 in Tierra del Fuego.
UNIT 5 (Tertiary, Pliocene?). Darwin includes
here the Tertiary units exposed in
extra-Andean Patagonia right across to
the coast of Santa Cruz. He observes
that these overlie porphyries of Unit 3.
In the map, he represents this unit in the
hills of northern Tierra del Fuego and in
the northern areas of Brunswick Peninsula.
Darwin's observations on this unit
include the identification of several faunal
assemblages with abundant Turritella,
Pecten and Ostrea. He called this complex
the "Patagonian Tertiary Formation" (see
Parra and Griffin 2009, Casadío and
Griffin 2009).
From a lithologic point of view he recognized
the presence of beds with pumiceous
siltstone with abundant gypsum,
sandstones, and claystones with calcareous
concretions. At the headwaters of
the Santa Cruz River he observed that
the sedimentary rocks are covered by
basaltic lavas of unit 7.
UNIT 6 (Recent). This unit represents Quaternary deposits that Darwin recognized at the southern end of Santa Cruz between the Río Gallegos and Cabo Vírgenes, in the northeastern coast of Tierra el Fuego, along both coasts of the Strait of Magellan in its northeastern sector, and in the sector between Useless Bay and Bahía San Sebastián. He includes here the "Rodados Patagónicos", which he called "Gravel Formation of Patagonia". In his description he indicates that not all the gravel beds are from recent times, particularly those of higher tablelands (see discussion in Martínez et al. 2009). He recorded interesting observations on the origin and transport mode of the gravels, noting the difficulty in explaining the distribution covering the entire area from the Andes to the coast. He did point out, however, that the present distribution of the gravels was due to marine action.
UNIT 7 (Basaltic lava). It represents the basaltic exposures along the middle and upper course of the Río Santa Cruz and the Meseta La Siberia. Darwin placed these outcrops as contemporaneous of the higher sections of the Tertiary formation. In fact, the basalts are of a Pliocene-Quaternary age (see Streling and Malagnino 2009) and include the units presently called Meseta de las Vizcachas Basalt and La Siberia Formation. In his observations he states that the outcrops predominate on the Northern side of the Río Santa Cruz valley and that they reach the foothills of the Andes. He recorded that the basalts were olivinic and that columnar disjunction was common. He identifies at least three overlying beds. Acknowledging the great extension covered by the exposures he concludes that "This great deluge of lava is worthy, in its dimensions, of the great continent to which it belongs. The aggregate streams have flowed from the Cordillera to a distance (unparalleled, I believe, in any case yet known) of about 100 geographical miles" (Darwin 1846, p. 116)
CONCLUSIONS
The brief description presented herein shows one of Darwin's less known facets, i.e., his ability to represent his geological observations in a map. This contrasts Stoddart (1995) who emphasized, when analyzing the geological results of Darwin's voyage on the Beagle, "the contrast between Darwin's verbal facility in describing landscapes and evoking mood, and his general inability to translate his images into visual representations". Lastly, it should be stressed that if it had been published when originally planned, it would have constituted a singular initial contribution to the geological cartography of the South American continent.
WORKS CITED IN THE TEXT
1. Casadío, S. and Griffing, M. 2009. Sedimentology and Paleontology of a Miocene marine succession frst noticed by Darwin at Puerto Deseado (Port Desire). Revista de la Asociación Geológica Argentina 64 (1): 83-89.
2. Darwin, C. 1838. On the connexion of certain volcanic phaenomena, and on the formation of mountain-chains and volcanos, as the effects of continental elevations. Proceedings of the Geological Society 2: 654-660, London.
3. Darwin, C. circa 1840. Geological Map of Patagonia image from the Cambridge University Library (Manuscript MS.DAR.44:13). Available at the Keeping time exhibition web-site: http://www.lib.cam.ac.uk/exhibitions/Keeping_Time/images/large/darwin.jpeg
4. Darwin, C. 1841. On the distribution of erratic boulders and on the contemporaneous unstratified deposits of South America. Proceedings of the Geological Society 3(2): 425-430, London.
5. Darwin, C. 1845. Journal of researches into the natural history and geology of the countries visited during the voyage of H.M.S. Beagle round the world, under the Command of Capt. Fitz Roy, R.N. John Murray, 2d. edition, 519 p., London.
6. Darwin, C. 1846a. An account of the fine dust which often falls on vessels in the Atlantic ocean. Quarterly Journal of the Geological Society 2: 26-30, London.
7. Darwin, C. 1846b. On the geology of the Falkland Islands. Quarterly Journal of the Geological Society 2: 267-279, London.
8. Darwin, C. 1846c. Geological observations on South America. Being the third part of the geology of the voyage of the Beagle, under the command of Capt. Fitzroy, R.N. during the years 1832 to 1836. Smith Elder and Co. 280 p., London.
9. Darwin, C. 1876. Geological observations on the volcanic islands and parts of South America visited during the voyage of H.M.S. 'Beagle'. 2d ed. Smith Elder & Co. 648 p., London.
10. D'Orbigny, A. 1842. Voyage dans l'Amérique Méridionale. Bertrand ed., 3(Géologie), 290 p., Paris.
11. Herbert, S. 1991. Darwin as a prospective geological author. British Journal for the History of Science 24: 159-192.
12. Herbert, S. 1995. From Charles Darwin's Portfolio: An early essay on South American Geology and species. Earth Sciences History 14(1): 23-36.
13. Herbert, S. 2005. Charles Darwin, Geologist. Cornell University Press. 485 p., Ithaca.
14. Lyell, C. 1830-1833. Principles of Geology. John Murray 1(1830) 511 p.; 2(1832) 330 p.; 3(1833) 109 p., London.
15. Martínez, O.A., Rabassa, J. and Coronato, A. 2009. Charles Darwin and the first scientific observations on the Patagonian Shingle Formation ("Rodados Patagónicos"). Revista de la Asociación Geológica Argentina 64(1): 90-100.
16. Parras, A. and Griffin, M. 2009. Darwin's Great Tertiary Patagonian Formation at the mouth of the Río Santa Cruz: a reappraisal. Revista de la Asociación Geológica Argentina 64(1): 70-82.
17. Olivero, E.O., Medina, F.A., and López C., M.I. 2009. The stratigraphy of Cretaceous mudstones in the Eastern Fuegian Andes: New data from body and trace fossils. Revista de la Asociación Geológica Argentina 64(1): 60-69.
18. Rhodes, F.H.T. 1991. Darwin´s search for a theroy of the earth: symmetry, simplicity and speculation. British Journal for the History of Science 24: 193-229.
19. Stoddart, D. 1995. Darwin and the Seeing Eye: Iconography and Meaning in the Beagle Years. Earth Sciences History 14(1): 3-22.
20. Strelin, J. and Malagnino, E. 2009. Charles Darwin and the oldest glacial events/episodes in Patagonia: the erratic blocks of the Río Santa Cruz Valley. Revista de la Asociación Geológica Argentina 64(1): 101-108.
Recibido: 3 de octubre de 2008
Aceptado: 4 de noviembre de 2008