Posted by: Rob Viens | April 9, 2012

Lagoons, Beaches, and the Eternal Motion of Sand

Today, April 9th,the Macaé expedition traveled from Lagoa de Maricá to a place Darwin called Ingetado. He also mentions stopping for lunch in Mantetiba, another location which does not appear on modern maps of the region. Darwin frequently spelled place names based on how he heard them pronounced, so it may just be his spelling. However, if anyone knows the actual places Darwin is referring to, please let me know.

Macaé excursion map– April 9

Map of Darwin's Expedition – April 9. 1832

Today was a particularly long day for the travelers, and Darwin was impressed with how well the Brazilian horses fared:

“I never ceased to wonder, from the beginning to the end of the journey, at the amount of labor which these horses are capable of enduring: I presume it is from being in a country more congenial to their original nature. —and from the same cause they seem far better than English horses to recover [from] injuries & wounds.” (Apr 9)

And I suspect he knew a few things about horses, as he enjoyed riding back home.

However, what struck me as particularly interesting as the party traveled up the coast was the variety of ecosystems they encountered. Forest, cultivated fields, pasturage, freshwater lakes, lagoons, marshes, beaches – all in the course of a couple of days. A few examples, noted by Darwin, include:

“After passing through some cultivated country we entered a Forest, which in the grandeur of all its parts could not be exceeded.” (Apr 8)

“for the few last miles the road was intricate, it passed through a desert waste of marshes & lagoons.” (Apr 8)

“Leaving Mandetiba, we continued to pass through an intricate wilderness of lakes, in some of which were fresh, in others salt water shells.” (Apr 9)

“We at last entered the forest; the trees were very lofty, & what was always to be remarked in them was the whiteness of the boles, this at a distance adds much to their effect.” (Apr 9)

“The road passed through a narrow sandy plain, lying between the sea & the interior salt lagoons.” (Apr 9)

This last entry, from the morning of the 9th, refers to the sandy beaches, called barrier beaches, that separate the lagoons (such as Lagoa de Maricá) from the ocean.  So how do barrier beaches form?  I’m so glad you asked…

Barrier beach around Lagoa de Maricá in modern times (from http://www.revistaencontros.com)

 Lagoa de Maricá

Beaches are dynamic places – ever changing. In fact, if you return to your favorite beach, maybe one you used to visit in your childhood (or even last year) you’d find that although it may look the same, the grains of sand that make up the beach are all new.  The sand that was there last year has washed away and new sand has taken its place.  (It is sort of like your skin – old skin cells die off and fall away and new ones take their place.  You are never “skinless”, it is just a gradual, yet continuous, process.)

The mechanism for this constant turnover in sand is a process called “longshore drift”.  Longshore drift occurs because waves often approach a coastline at an angle. As the waves wash in, they pick up grains of sand and carry them in at an angle, too.  But when the water washes out (called backwash), it tends to carry the sand more or less straight out – simply flowing downhill back to the ocean.

Longshore drift (from Barcelona Field Studies Centre):

 Longshore Drift

Sand travels in a zig-zag pattern down the beach, and the net effect of this process is the overall “drift” of sand down the shore.  In a sense, longshore drift is like a coast current carrying the sand along.  Or by other accounts, the beach is like a “river of sand” flowing down the coast.  In any case, the sand is always on the move.

Fr those who would like a visual – here is short video from YouTube (source a little unclear) on longshore drift:

Now imagine what happens when there is an inlet along the coast (a bay, for example). The sand migrating down the beach reaches the inlet, but rather moving “around” the bend into the bay, it continues to build out into the mouth of the inlet.  At first it will build a sand spit out into the bay, but eventually the migrating sand can completely enclose the inlet with a large sand bar.  And voila – you have a barrier beach that separates the ocean from and a salt water (or at least brackish water) lagoon or lake.

Just like regular beaches, barrier beaches are often temporary features.  A big storm might wash over them and connect the lagoon with the sea again (only to be re-blocked again by the relentless movement of sand).

Darwin didn’t think too much about the formation of the sand bars. I should say that he did not write about it, as I would not be surprised if he thought about it – pondering the nature of these large lagoons.  In his journal,  he was more interested “the number of beautiful fishing birds such as Egrets, Cranes &c & the succulent plants”, the temperature (hovering in the high 90’s today), and the state of his lunch.

Tomorrow the trip moves inland a bit and travels through the rainforest. The Rio Macaé is still a couple of days away. (RJV)

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Responses

  1. […] In this case, they are isolated from the sea by barrier beaches that form along the coast (see Lagoons, Beaches, and the Eternal Motion of Sand ). Where the sand bars form an effective barrier (and there is a regular inflow of freshwater), the […]

  2. […] The barrier beaches that FitzRoy describes sometimes do have openings to the sea (usually if there is a strong current or if the beach has been breached by a storm).  But they can also be completely separated from the ocean.  (For more on barrier islands see Lagoons, Beaches, and the Eternal Motion of Sand.) […]


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