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Geology is the Way

L'antico deserto dell'Isola d'Elba

Elba is a paradise for beach lovers and summer sports, with its delicate coastal sceneries and scented footpaths hidden by the Mediterranean scrub. Certainly, except if you have visited the island in December like me, a desert is not the first thing that comes to mind when thinking about the island. Yet, this little island in the middle of the Tyrrhenian Sea has an ancient desert among its geological wonders.

Spiaggia della Madonna, close to Madonna delle Grazie near Capoliveri, is, perhaps, the place with the best ‘mirages’ of this ancient desert. The beach lies in a small cove, hidden between two rocky coasts.

Spiaggia della Madonna is a small sandy cove on the western coast of the Calamita Peninsula on the island of Elba (Italy).

These rocky cliffs consist mostly of sandstones, deposits of cemented sand that formed in a desert environment, where winds were blowing and pushing around sand dunes. Here at Madonna delle Grazie the spectacular outcrops allow us to travel back in time through the history of this extinct desert.

The first deposits of this desert are visible on the Northern part of the cove. Here, a wall of reddish earth lies on top of some very deformed (fragmented, brecciated) marbles of Jurassic age. This red material is an ancient soil (a paleosol), earth produced by the weathering of the underlying bedrock and that once must have sustained a scrub or maybe some prairie. This soil is about 40 – 20 thousand years old, while the underlying rocks are about 200 million years old (Jurassic) and experienced high temperature metamorphism. This is a surface of nonconformity: a contact between crystalline (metamorphic/igneous) and sedimentary rocks that marks the erosion of the bedrock and the deposition of new sediments on top of it. Do you see the overhanging layers at the top? They are the first layers of sandstone of our desert, but here they are a bit ugly here, so let’s move to the next outcrop!

Detail of the paleosol cropping out in the area. The lower part of the section shows a layer of regolith: a loose soil with fragments of rocks deriving from the underlying bedrock – here mostly marble. The upper part is the topsoil, fine-grained earth. Note: the section above is about 4 meters high.

We are now entering our Pleistocene desert! Dunes move and migrate pushed around by the wind, but they do not remains as landforms in the geological record. What we see are the layers of sand that was rolling down the downwind side of the dune while the wind was pushing the dune forward. This is useful because they can tell which way the wind was blowing. The evident inclined sandstone layers dipping to the right (dashed lines below) tell us that the dunes were migrating rightward and hence the dominant winds in the area were blowing from the WSW to the ENE (towards Capoliveri). The inclined ‘dune’ sequence is separated at the top and the bottom by erosional surfaces (continuous lines) marking the migration of subsequent sets of dunes. This depositional architecture produces layers that intersect at an angle, called cross-bedding.

Here’s a close-up of cross-bedding. As you can see here (and from the slider above) the layers are inclined to the right and tend to terminate tangentially (as sigmoids) to the lower erosional surface. This is because, as the dune migrated and sand was pushed forward, the sand that was depositing at the base of the dune had enough velocity to keep rolling a while on the flatter land in front of the dune.

Some other close-ups of these sandstones that I think are just beautiful.

The holes in the sandstones are called ‘tafoni’. They form because the waves spray salty water on the sandstone. When salt deposits from water, the growing salt crystals expand, break the rocks, and produce these little holes.

Now, if we look better at these rocks we can see some additional layers that tell us better what kind of environment this desert must have been. Wind can move sand around, but certainly does not move gravels or cobbles. Within the sand dunes layers, there are lenses of breccias, deposits of angular rock fragments that largely consist of rocks that can be found in the Calamita (SE Elba), like the marbles below the sandstones. What are they? I think that they witness some debris flow event, likely triggered by sporadic heavy rainfalls. Indeed, it is unlikely that this desert covered all of Elba. There likely were mountains around that could feed debris flows in case of intense rains. This is my guess. If you have other (better) explanations, please write them down in the comments!

A close-up of the previous outcrop.

One last thing. How do we know how old these rocks are? We need to find fossils!

Oh look! There an ammonit… wait! What?

These rocks are full of fossils! The sandstones contain a lot of shell fragments that make geologists think that these dunes re-used the sand deposits formed in a marine/coastal environment. The interesting part are the paleosols, however, because they contain fragments of plant roots (now coal). Researchers have dated these fossils using the 14C within them and obtained ages between 20000 and 40000 years ago (Cremaschi & Trombino, 1998; D’Orefice & Graciotti, unpublished). This age corresponds to the last glacial maximum, when ice caps were at their maximum extent and the global sea level was much lower than today. It is very likely, therefore, that this cold desert was supplied by the -then emerged- sandy seafloor surrounding the island.

Hiking on Elba
Elba is where I work as a geologist. Unfortunately, there is not (much) about its geology available online. I will do my best to show you its beautiful geology. In the meantime, if you go there, here are some hiking tips!

Lonely Planet – Elba

4Land – Hiking Map of Elba (this is the map I use).

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