Posted by: Rob Viens | September 18, 2013

The Posta Express 1: Fens and Killer Hailstorms

By the 16th of September Darwin was back on the trail, and making good time – working his way ever closer to Buenos Aires.  Over the next couple of days I thought I’d share his impressions and experiences as he traveled via the Posta Express”.  Let’s start with Posta #6:

Posta #6:

“To the 6th Posta; soil black & very soft, generally covered with long coarse herbage; — laborious travelling. Rancho here very neat; the posts & rafters were made by a dozen dry stalks bound together with thongs of hide. — by the aid of these Ionic looking columns the sides & roof were thatched with reeds.” (Sept 16)

Posta #7:

“To the 7th Posta, country improving, like Cottenham fen in Cambridgeshire. — a great abundance of beautiful wild fowl. — This posta is close to the Southern base of Sierra Tapalken; which Sierra is a low broken ridge of Quartz rock 2 or 300 feet high. — extending to the East to Cape Corrientes, but no great distance within the Interior.” (Sept 16)

Along with swamps, marshes, sloughs, mires and bogs – a fen is a type of wetland. Specifically a fen is a wetlands that is not acidic and has a lot of dissolved minerals. The overall lack of plant nutrients in fens, often results in their being dominated by grasses and sedges (rather than the peats that one might expect in a more acidic bog).

Cottenham Fen, which Darwin compares to parts of northern Argentina (from the Cottenham Newsletter):

cottenham fen

Darwin goes on to describe a killer hailstone that occurred shortly before his arrival at Posta #7:

“I was here told a fact, which, if I had not partly ocular proof, I could not credit. That in the previous night there had been a hailstorm (I saw lightning to the North) & that the pieces of ice were as large as small apples & very hard. — They fell with such force as to kill almost all the small animals. — These men had already found twenty deers & I saw their fresh hides; one of the party a few minutes after my arrival, brought in seven of them; now I well know that one man without dogs would hardly kill 7 in a week; They thought they had seen about 15 dead ostriches; part of one I eat, likewise saw a large partridge with great black mark on its back, where it had been struck. — Many ducks & hawks were killed & ostriches were then running about, evidently blind in one eye. — My informer received a severe cut upon the head. — This extraordinary storm extended but for a short distance.” (Sept 16)

Ah, of all the many things that fall from the sky, hailstones can certainly be the most painful (short of a meteorite). Hailstones are layered balls of ice that form in thunderclouds.  Their formation requires updrafts that blow the icy droplets back up into the cloud.  Through a surprisingly complex process that can take place for up to a half hour, the droplet accumulates layer upon layer of new ice as it continuously moves up and down in the cloud.  Finally, it accumulates enough ice such that the wind can no longer support its weight, and the hailstone falls to the ground.

Although large, Darwin’s hailstones where still nowhere near the largest recorded (8 inches (20 cm)) or even the heaviest (2.25 lbs (1 kg)). Yikes!

Watch a storm of golf/tennis ball sized hail in Texas that occurred in 2006 to get a feel for what caused the deaths Darwin described (posted to YouTube by cld9trs)

It is still rather surprising that so many animals seem to have been killed. This would suggest that the storm came on rather quickly, or else the region was just too open to provide any significant cover. (RJV)

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