Posted by: Rob Viens | February 16, 2014

Ammonites on Top of Mount Tarn

February 6th found Darwin back on the climbing trail as he ascended nearby Mt. Tarn ) located a  few miles south of Port Famine.  His diary entry today leaves me with a few stories to tell. So let’s get started…

Mt Tarn (from Panaramio by Patagonia Chilena)

Mt. Tarn

Let me began with Darwin’s description of the start of his day – a slog through the forested lowlands at the base of the peak:

“I left the ship at four oclock in the morning to ascend Mount Tarn; this is the highest land in this neighbourhead being 2600 feet above the sea. For the two first hours I never expected to reach the summit. — It is necessary always to have recourse to the compass: it is barely possible to see the sky & every other landmark which might serve as a guide is totally shut out. — In the deep ravines the death-like scene of desolation exceeds all description. It was blowing a gale of wind, but not a breath stirred the leaves of the highest trees; everything was dripping with water; even the very Fungi could not flourish. — In the bottom of the valleys it is impossible to travel, they are barricaded & crossed in every direction by great mouldering trunks: when using one of these as a bridge, your course will often be arrested by sinking fairly up to the knee in the rotten wood; in the same manner it is startling to rest against a thick tree & find a mass of decayed matter is ready to fall with the slightest blow.” (Feb 6)

It is interesting that the hardest part of the ascent appears to have involved no climbing at all.  It was the slow slog through the decaying underbrush at the foot of the mountain.  Having tried to traverse similar forested terrain in Alaska, I wholeheartedly agree with the assessment that it is “impossible to travel”. In fact, I can recall the same sorts of experiences sinking into rotten logs. Presumably this dense forest was just found along the coast, because if it had gone on for very long, Darwin would have never made the ascent.

As it turns out, Darwin has very little to say about the rest of the ascent, and quickly jumps to the tremendous view from the top (which seems to overcome the discomfort brought about by the weather):

“I at last found myself amongst the stunted trees & soon reached the bare ridge which conducted me to the summit. — Here was a true Tierra del Fuego view; irregular chains of hills, mottled with patches of snow; deep yellowish-green valleys; & arms of the sea running in all directions; the atmosphere was not however clear, & indeed the strong wind was so piercingly cold, that it would prevent much enjoyment under any circumstances.” (Feb 6)

Check out the 360 View from the top of Mt. Tarn posted to YouTube by bomberocroata.  I suspect it does not look much different from the view Darwin saw 180 years ago.

Here is a link to another short video that follows a hiker through his ascent of the peak (in the beginning you can see the dense forest growth that Darwin discussed).

A modern climbing team at the top of Mt Tarn (from Panaramio by patriciocaceres). Note the layers of rock – these are Cretaceous sedimentary rocks (sandstone and siltstone).

Mt. Tarn

In concluding his diary entry, Darwin offhandedly notes some shells he found at the top of the peak:

“I had the good luck to find some shells in the rocks near the summit. — Our return was much easier as the weight of the body will force a passage through the underwood; & all the slips & falls are in the right direction.” (Feb 6)

These shells turned out to be the first ammonites ever described from the continent of South America. Ammonites are a type of cephalopod (octopus, squid, etc.) that lived during the Mesozoic Era.  They are so abundant (and come in so many forms) that they have become one of the more iconic fossils from that period of time.  Dinosaurs may have “ruled” the Earth, but it was ammonites that were found throughout the oceans of the world, and left behind there shells for the masses.

Ammonite from the genus Maorites (from the Paleontological Research Institution)

Maorites ammonite

Darwin’s fossil did not look like a traditional ammonite – in fact it appears to just be a piece of one.   In a 1846 publication it was identified as Ancyloceras simplex – a name that has not stood the test of time.  Geologists today suggest that these rocks where too young for this species, and some have suggested it is a deformed piece of another genus of ammonite known as Maorites (similar to the one shown above).

The ammonite sample collected by Darwin (on display in the London Natural History Museum):

Mt. Tarn ammonite

(For more on ammonites in this region see the 2009 journal article The stratigraphy of cretaceous mudstones in the eastern Fuegian Andes: new data from body and trace fossils. This article is also the source of the picture above.)

A fossil of the genus Ancyloceras (from Centre d’Etudes Méditerranéennes):

ancyloceras ammonite

Now the interesting implication, of course, is that the sandstones, siltstones and ammonites found at the top of Mt. Tarn (at 2707 feet) where at the bottom of the sea back in the Cretaceous Period when they formed.  That means they were lifted up at least 3000 feet since they were deposited about 75 millions years ago!

One last story to tell – the origin of the name of Mt. Tarn. My first thought was that the mountain was named after a glacial feature called a tarn.  These are small lakes that fill the bowl-shaped depression carved out at the head of a glacier. (When the glacier melts away these “bowls” are left behind just below the glacial-carved peaks – often forming a number of small alpine lakes common in glaciated terrain.)  But I was incorrect.  Mount Tarn was actually named after the surgeon on the first voyage of Beagle.  John Tarn was the surgeon on the HMS Adventure, and is credited with the first ascent of the mountain (the first recorded European ascent anyway).  Tarn was about 34 years old when he first climbed “his mountain” in February 1827. He spent most of his life serving as a ship’s surgeon, and was later was awarded the Sir Gilbert Blane’s Gold Medal for his service. He died in 1877.

Sir Gilbert Blane’s Gold Medal
Sir Gilbert Blane Gold Medal

So in this case, Darwin was literally following in the footsteps of the previous expedition… (RJV)

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