Mark Wilkinson (UK)
The Spanish coastal town of Calpe is dominated by the towering massif of the Peñon de Ilfach (Fig. 1). The 332m, steep-sided summit is surrounded by the Mediterranean Sea on three sides and connected to the land by a relatively narrow neck, rather like a gigantic sea stack. Unless you are prepared to do some serious rock climbing, the summit is accessible only through a tunnel bored for the purpose, complete with rope hand-rails. The half-hour walk to the summit, through a small nature reserve and visitors centre, gives fantastic views of the surrounding coast and mountains.
To the geologically minded, the Peñon offers another aspect. It gleams white in the Mediterranean sunshine, so it’s not too difficult to guess that it is made of limestone. But bedding is quite tricky to spot, especially from a distance. I’ve often wondered as to how such an isolated feature came to be there – and why is the bedding so hard to see? It was almost a relief to purchase the Geologists Association guide to the area and discover that “even experienced geologists may find the bedding hard to locate”.
Phew, it’s not just me then! And how did the Peñon come to be so isolated from the other limestone hills in the area, the nearest of which is several kilometres away? Was there a massive sheet of limestone, which has simply been eroded away, leaving relics in the form of the highest hills, or were there other forces at work? The key to answering these questions lies in a traverse of the north coast of the neck of land connecting the Peñon to the mainland – a series of cliffs with a wave-cut platform making for easy access. Since this is the Mediterranean, you don’t have to worry about the tide, unlike my more normal coastal fieldwork nearer to home.
To locate the exposure, follow signs for the Playa del Penyal, which is a nudist area. In mid-winter, with a north-facing aspect, I’ve not encountered ‘exposure’ of a non-geological nature, though I guess in the summer the situation might be different. And there’s no sandy beach either, just lots of sharp rocks. Great for geology, not so great for sunbathing. The first rocks encountered are mixed mud and siltstones with interbedded sandstones, which are almost horizontal and undeformed (Fig. 2).
Fallen blocks have excellent flute casts, suggesting that the sandstones are turbidites, or at least some sort of mass-flow (Fig. 3).
A feature of one of the beds, exposed as the wave-cut platform just above present-day sea-level, is irregular echinoid fossils (Fig. 4).
These are described in the GA guide as excellently preserved, along with a plea to leave them for future visitors to enjoy. Almost needless to say, only poorly-preserved ones are left, any others having mysteriously disappeared since the guide was written.
Progressing towards the Peñon itself, the rocks begin to change. There are still sandstones and mudstones, but they become increasingly folded and deformed (Fig. 5).
If you are leading a group of students, this would be a great place for a spot of field sketching. Some of the features are quite tricky to interpret – there is a vertical ‘pillar’ of sandstone at one point (Fig. 6) that might be a rotated bed, or some sort of injection feature? Around ten metres above sea level, there is a more-or-less horizontal surface, above which the beds are undeformed and parallel to the surface, visible at the top of Fig. 6.
Those of us brought up in the UK on a diet of the Northwest Highlands of Scotland, and tales of Peach and Horne solving the ‘Highlands Controversy’, will immediately think of thrust tectonics and begin to think that the Peñon was perhaps thrust into place. Certainly, there are no shortage of mountain-scale fold structures in the area, so plenty of evidence of compressional tectonics. While imagining entire mountains-sized slabs of the Earth’s crust being pushed over one another is hard enough (though proven beyond reasonable doubt), the truth, it turns out, is even stranger. The Peñon is believed to have slid into place, along a low angle detachment surface, slightly downhill and under the force of gravity. This explains the lack of easily-observable bedding: it was destroyed, or at least badly disturbed, by deformation within the sliding limestone block.
What makes a mountain-sized block of rock slide from one place to another? There are two factors. The first is having a strong rock overlying a weak one, in this case limestone over mudstones with some gypsum. The weak rock effectively acts as a lubricant below the stronger one. The second factor is that there has to be a slope to slide down. In this case, the slope was provided by uplift associated with the upwelling of Triassic gypsum, forming the type of dome that might be more normally associated with halite (rock salt). Evidence for recent geologically-rapid uplift of the area can be seen as spectacular canyons developed as rivers cut down through the uplifting landscape, for example, the nearby Mascarat Gorge (Fig. 7).
The gypsum itself isn’t well exposed, but hand specimens can be found dotted around the landscape. Just to complicate matters, it is by no means certain that all the deformation seen along the shore is actually related to the emplacement of the Peñon. Instead, at least some of it may be the result of slumping of the sediments when they were still soft and close to the seabed. Sorting out soft sediment deformation from tectonic folding can be quite tricky, and can make for interesting field discussions.
The surface along which the Peñon slid is exposed (Fig. 8), but is not easily accessible due to a low projecting rocky spur.
In summer, wading around this would be perfectly possible. I’ve only been there in winter, when low temperatures make braving the waves rather unattractive. So, I’ve not visited the actual slide surface, although the overall story is clear enough without this. There are however a number of questions that I don’t have answers to. Is the Peñon still sliding, presumably very slowly? If it isn’t slowly moving, did it move quickly, that is, fast enough to see if you’d been there at the time? The thought of a kilometre-scale block of limestone sliding past you makes the recent cliff collapses in the UK seem really rather insignificant. And how far has the Peñon moved anyway? The nearest large limestone cliffs are in the Sierra de Olta, a limestone plateau of several square kilometres which is at least two kilometres away (Fig. 9).
And (wait for it…) the entire Sierra de Olta is also interpreted to have slid into its present position! One thing is for sure – the Peñon isn’t a volcano, despite some ‘alternative facts’ in one of the published tourist guides to the area.
About the author
Mark teaches geology at the University of Edinburgh, including running two of the fieldtrips for undergraduate students. He only visits Calpe on holiday, but it is virtually impossible to ignore the geology here. And why would you want to?
A Geological Field Guide to the Costa Blanca, Spain by F. Moseley. Geologists’ Association Guide no. 43, see: https://geologistsassociation.org.uk/guidesales/.