Posted by: Rob Viens | February 6, 2014

Earth, Wind and Tierra del Fuego

Yikes – time sure does fly!

After recovering from his lack of water, the middle of January found Darwin in transit – first back to Port Desire to pick up the Adventure, and them back south to the Straits of Magellan.

With little vegetation in Port San Julian (and Port Desire) the geology lay naked and exposed, and Darwin took advantage of the opportunity to examine the rocks. In true naturalist form, he managed to discuss geology, biology, history and archeology in one short diary entry:

“Went out walking, & found some fine fossil shells. — The country precisely resembles that of Port Desire. — it is a little more uneven, & from the absence even of brackish water, there are fewer animals. The Guanacoe who drinks salt water is of course to be seen. — Two things have been found here for which we cannot account: on a low point there is a large Spanish oven built of bricks, & on the top of a hill a small wooden cross was found. Of what old navigators these are the relics it is hard to say. — Magellan was here & executed some mutineers; as also did Drake & called the Island “true justice”. ” (Jan 14)

He actually discusses the geology in much more detail in Voyage of the Beagle.  Here are his first impressions:

“The geology of Patagonia is interesting. Differently from Europe, where the tertiary formations appear to have accumulated in bays, here along hundreds of miles of coast we have one great deposit, including many tertiary shells, all apparently extinct. The most common shell is a massive gigantic oyster, sometimes even a foot in diameter.” (Voyage of the Beagle)

This is the oyster that Darwin refers to as Ostrea patagonica (now Crassostrea pataginica), which was deposited in the late Miocene (around 5-10 million years ago) – another of the many species first described by d’Orbigney.

Here is an image of a Miocene oyster from Patagonia from the blog Coleccionismo a Full (follow the link for more images of the author’s collection):

Patagonian oyster fossil

Darwin goes on to describe the layers of rock found in the region.  I particularly like how he considers the amount of time it would have taken to pulverize all that rock:

“These beds are covered by others of a peculiar soft white stone, including much gypsum, and resembling chalk, but really of a pumiceous nature. … These white beds are everywhere capped by a mass of gravel, forming probably one of the largest beds of shingle in the world: it certainly extends from near the Rio Colorado to between 600 and 700 nautical miles southward; at Santa Cruz (a river a little south of St. Julian), it reaches to the foot of the Cordillera; half way up the river, its thickness is more than 200 feet; it probably everywhere extends to this great chain, whence the well-rounded pebbles of porphyry have been derived: we may consider its average breadth as 200 miles, and its average thickness as about 50 feet. If this great bed of pebbles, without including the mud necessarily derived from their attrition, was piled into a mound, it would form a great mountain chain! When we consider that all these pebbles, countless as the grains of sand in the desert, have been derived from the slow falling of masses of rock on the old coast-lines and banks of rivers; and that these fragments have been dashed into smaller pieces, and that each of them has since been slowly rolled, rounded, and far transported, the mind is stupified in thinking over the long, absolutely necessary, lapse of years. Yet all this gravel has been transported, and probably rounded, subsequently to the deposition of the white beds, and long subsequently to the underlying beds with the tertiary shells. ” (Voyage of the Beagle)

“Stupified”indeed! 🙂

Given the region’s climate, the cross Darwin observed on January 14th could have been the one left by Magellan 300 years earlier.  For more on that story see Treacherous Rocks and Mutineers.

Speaking of climate, much of the next few days was taken up by bad weather.  Darwin wrote:

“A heavy gale of wind from the SW; several breezes from that quarter have reminded us of the neighbourhead of Tierra del Fuego.” (Jan 15)

“Bad weather preventing the completion of the survey has detained us these days.” (Jan 16-18)

The climate of the San Julian region is pretty temperate with temperatures ranging from lows around 0°C (in June/July) to 20°C (December, January and February).  Rainfall varies a little, ranging from only 1 to 2 cm per month. Since Darwin was visiting in the Argentine summer, it is likely that most days were probably pretty pleasant.

Check out the monthly temperature and precipitation averages for Port San Julian from weather-and-climate.com (follow the link for additional climate data).

average temps in San Julian

average precipitation in San Julian

The following week saw the Beagle in transit, about which Darwin didn’t have much to say.  He described the journey in his diary:

“Made sail very early in the morning, & with a fair breeze ran up to Port Desire; next day anchored off the mouth & with the young flood entered the harbor.” (Jan 19)

“I landed directly the ship came to an anchor, & had some collecting. — On an headland projecting into the sea, I found a heap of stones similar to the ones already described. There was a tooth & head of thigh bone, all crumbling into earth. — in a few years no traces would be left: This explains the apparent absence of bones in the grave, made with so much labor, on the top of the hill. The Adventure is ready for sea & with her new square top-sail will doubtless sail well.” (Jan 20)

“The Adventure & Beagle stood out to sea. — At sunset the Adventure steered for West Falkland Island & we came to an anchor under Watchman Cape.” (Jan 22)

“After Latitude observations at noon we made sail for the Straits of Magellan.” (Jan 23)

“With a fair wind, we passed the white cliffs of Cape Virgins & entered those famous Straits. (Jan 26)

We’ll pick up the story in the straits in the next entry… (RJV)

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