Posted by: Rob Viens | November 12, 2013

Sandstone Cliffs of Disappointment

On the 6th and 7th of November, Darwin took a short ride down the coast to spend the night in what is now the resort town of Kiyú (what he called Barrancas de St Gregorio). Sadly, he was not impressed with the geology he saw:

“Had a long gallop to the East end of the Barrancas de St Gregorio: was disappointed in the Geology, but had a pleasant gallop along the coast of the Plata. — It was necessary to cross the St Lucia near its mouth; we passed in a boat, the horses were obliged to swim at least 600 yards; I was surprised to see with what ease they performed it. — We did not return till so late, that I slept at a Rancho, & returned home early in the morning.” (Nov 6/7)

Kiyú (which is said to come from an indigenous word for “cricket”) is a resort that is located on a beautiful stretch of Uruguayan coastline – well known for its sunsets over the Rio de la Plata. Of course, I doubt there was much more than a cattle ranch there when Darwin visited, but he was not really a “resort” kinda guy anyway.

Las barrancas de San Gregorio (posted by Tongas via Google Earth)

cliffs at Kiyu

Kiyú is also known for its sandy cliffs, which (as it turns out) are difficult to find out much about (or at least hard to translate from Spanish).  However, I was able to find out that the rocks exposed here are part of three formations – the Camacho Formation, San Jose (AKA Raigon) Formation, and the Libertad Formation and range in age from roughly about 10 to 1 million years old (the late Miocene, through the Pleistocene). Interestingly, research in that area in the last few decades has uncovered some ground sloth fossils – including a new species.  But clearly, Darwin did find any fossils here worth noting in his journals.

In earlier posts I’ve written a lot about igneous and metamorphic rocks –  since it was mainly these rock types that Darwin encountered along the coast of South America.  (Recall that much of what he was trekking over was the South American Craton, which is made up of these “crystalline rocks” (see Geologizing on the South American Craton).) However, any time he was digging up fossils, Darwin was likely looking at sediment (such mud, sand, or gravel), or the “soft” rocks that form when these sediments are compressed and “glued” together (lithified) into sedimentary rocks. (As a general rule, the processes that create igneous and metamorphic rocks tend to destroy fossils. Furthermore, as you will see shortly, fossils are also more likely to be found in the sedimentary rocks that represent “quieter” environments – such as those made of mud (clay and silt) or sand.)

Let me take a step back before I lose some readers. Sedimentary rocks (rocks made from sediments) come in a couple different varieties.  The kind we are talking about here are made of rock fragments and are collectively called detrital sedimentary rocks (because they are made of “detritus”).  For the most part these rocks are classified by their grain size – and are pretty easy to remember.  See the breakdown below and note that the rock name (e.g., sandstone) generally parallels the sediment grain size (e.g., sand-sized grains).

Detrital Sedimentary Rock Identification (Note this chart goes into more detail than discussed, but you can basically break these rocks down into three simple categories – conglomerate, sandstone, and mudstone.)

Sedimentary Detrital Rock ID

Sedimentary Detrital Rock ID

One of the cool things about rocks is that once you learn how recognize and identify them, you can start to “read them” (i.e., interpret what they tell you about the past).  And, luckily for us, sedimentary rocks are one of the easier rocks to interpret.  For example, if I asked you to find me a bucket of sand (in a natural environment) where would you go to collect it? You might visit a beach, a river bed or desert. Likewise, you might collect gravel in a mountain stream, or mud at the bottom of a lake or in a quiet lagoon.  These are what we call “sedimentary environments” and this distribution of different sized sediment is basically a result of the “energy of the environment” (i.e., movement of the water).  For example, in a mountain stream the fast-moving (high energy) water washes everything away and leaves the gravel behind.  The slow, quiet water of a lake (a low energy environment) does not have the energy to carry larger grains, so only the mud makes it out to settle in the deeper water.

To be fair, that is not the whole picture. Geologists also look at other characteristics of the sedimentary grains to “read” even more about the history of the rock.  For example, we may look at how well sorted the grains are (i.e., are they all one size or a mixture different sizes). Water tends to be sort sediment out (as in the examples above).  On the other hand, when a glacier melts or a landslide stops moving, all the grains (no mater their size) are dropped together creating a poorly sorted mixture of rock fragments.  In addition, if you look carefully at the grains, you can describe whether they have smooth (rounded) or angular (jagged) edges.  All other thing being equal, this roundness will tell you relatively how far the grains traveled between their source and their ultimate resting place.  If a rock contains fossils, too – then wow – you can really get a more complete picture of their sedimentary environment – including information about the climate, ecosystem, etc. in which the rock formed (e.g., tropical plants=tropics; mussel shells = intertidal zone, and so forth).

Darwin knew something about reading he rocks, and often tried to interpret the sedimentary environment of his samples.  Unfortunately, he did not say much about Kiyú, but from what I can tell from a limited amount of online research, the rocks in the cliffs here are primarily mudstones, siltstones and sandstones.  The story they tell (which comes from the fossils in them as much as anything), is that during this period of time sea level rose and covered the land creating an offshore environment that accumulated finer grained sediment and numerous marine fossils. This is a great environment for fossil preservation.  Alternatively, some of the later layers of rock may have been a little coarser grained and formed in as river deposits after the sea level dropped again. (The uppermost layers may be wind-blown deposits of fine silt and clay called loess.)

Too bad Darwin did not have a chance to further explore the cliffs of Kiyú – he just might have encountered some interesting geology after all!

Cliffs of Kiyú (from Puerto Cangrejo on Blogspot)

cliffs at Kiyu

Upon his return from Kiyú, Darwin prepared for his next major outing – a trip into the interior of Uruguay. Much like his experience in Brazil (see Bureaucrats Know No Boundaries) he was frustrated by the bureaucracy involved in obtaining the correct papers.  Though he should know from his experiences in Argentina, that the right papers could make all the difference.  In his only real entry in a week, Darwin wrote:

“I prepared for a ride to see the R. Uruguay & its tributary the R. Negro. — These days were lost by true Spanish delay in giving me my passport, letters &c &c.” (Nov 8-13)

Not to worry Charles – you’ll soon be on the trail again! (RJV)

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