Posted by: Rob Viens | April 13, 2013

Crossing the Shallow Sea

On April 6th the Beagle set sail from Port Louis and began the trip back to mainland South America. Darwin chronicles the 1000+ km trip in a series of short entries over the past week:

“After cruizing about the mouth of the Sound to complete the survey, we stood out to sea on our way to the Rio Negro.” (April 6)

“Our usual luck followed us in the shape of a gale of wind; being in the right direction we scudded before it; by this means we run a long distance, but it was miserable work; every place dark wet & the very picture of discomfort.” (April 7)

“The weather to day is beautiful; it is the first time for three months that studding sails have been set. — We attribute all this sun-shine & blue sky to the change in latitude; small although it be. — We are at present 380 miles from the Rio Negro.” (April 9)

“We expect to arrive at our destination tomorrow morning. — the weather latterly has been tolerably good but there was too much sea to allow me to be comfortable.” (April 12)

The studding sails are the sails that extend out from the right and left of the Beagle‘s primary sails, as seem in the image below (HMS Beagle with porpoises (ca. 1900 by Robert Taylor Pritchett)). Studding sails effectively increase the sail surface area and, therefore, capture more wind. Typically they are used in light winds, as they would make the ship too unstable if the winds were strong. For more on the Beagle‘s sails see Mizzenmasts and Moonrakers.

studding sails

During this week-long voyage, Darwin was sailing across the shallow continental shelf that connects the Falkland Islands to South America. Continental shelves, on average, are only about 100 m (~300 ft) deep, and as noted in an earlier post (see Sailing on the Edge of a Continent) are actually a part of the continental crust. The shelf connecting the Falklands to the mainland, however, is not completely uniform and has some various in the depth of the water.

The following image (from evolutionary biology and author Jerry Coyne’s Why Evolution is True blog) shows the relative depths of the continental shelf between South America and the Falkland Islands.

Falkland bathymetry

The numbered regions represent the land that would be exposed by dropping sea level.  During the last glacial period sea level was about 120 meters lower (III on the maps) though some data suggests there were times that it was 140 meters lowers (IV on the map).  In any case, you can see that even under those more extreme conditions, the Falklands are never completely connected to the mainland – the area shown in light blue would still be covered in water.  It is entirely likely, however, that during glacial times sea ice covered the narrow channel (light blue), making it possible for large mammals, such as the Falkland wolf, to cross over from the mainland.

By the weekend the Beagle was approaching the mouth of Argentina’s Rio Negro – the location of Darwin’s next big adventure. Stay tuned! (RJV)

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Responses

  1. Still enjoying your Beagle Project. Thank you.

  2. In the image “HMS Beagle Among Porpoises” by Robert Taylor Pritchett shown in your article, you also see William Snow Harris’ new lightnig conductors, which were a special feature of the HMS Beagle.

    Best regards from Munich
    Goetz
    http://www.ipernity.com/doc/goetzkluge/22062597


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