Much more than bare rock

Much more than bare rock

Dwejra is in the news but for the wrong reasons. The authorities recently justified their decision to issue a permit for a rocky area between the Inland Sea and Dwejra Bay to be covered with quarry “sand” (or, rather, a mixture of sand and soil, which has now caked over the rock surface) because, according to the competent authority, the site is just “bare rock”. It seems we fail to realise the scientific value of the geological record (which is also a climatic record) and the importance of leaving it in a pristine state for posterity.

The rocky surface now soiled by quarry “sand” not only hosts a bed of the Oligocene sea urchin Scutella but also several metres of rock densely strewn with trace fossils that consist of a reticulate network of burrows made by soft-bodied animals when the rock was still an ooze on the seabed some 20 million years ago. The trace fossils show evidence of tiering and succession to more oxygenated environments that reflect a temporary drop in sea level. The area covered by the “sand” is one of the few places in the Maltese islands where such trace fossils are exceptionally well preserved and conveniently exposed along a wide wave-cut platform.

The geological features at Dwejra and much of the western coast of Gozo make this a site of scientific importance that merits the status of a protected geological park. The designation of Dwejra and the western coast of Gozo as a Natura 2000 site may not afford sufficient protection when the main consideration is the geological importance of the site. There are other sites of exceptional geological interest: Fomm ir-Riħ, Għar Lapsi to Ras il-Ħamrija, the coast of Xgħajra, Daħlet Qorrot, to name a few, although Malta has not designated a single area as a geological parks albeit the geology of the Maltese islands is as intriguing and exceptional as is its land and marine ecology which is afforded some level of protection. The exceptional geology of Malta has attracted tourists and international scientific interest. Few in this country realise that the International Commission on Stratigraphy has designated Fomm ir-Riħ as a Global Standard Section and Point after a meticulous study on nothing more than “bare rock”!

What makes the Dwejra region so special? The spectacular landscape of Dwejra is dotted with 400 metre-wide circular depressions that are the result of catastrophic geological processes that go back millions of years and reflects the dynamic nature of the land and seabed. The story of Dwejra needs to be known although there is nothing in the area that explains to the visitor why this area is so important or the catastrophic geological process that have created it.

The Inland Sea is well-known and appreciated for its picturesque setting, yet, there are at least five other similarly-sized subsidence structures just metres away from the Inland Sea. Some of these circular features have been flooded by the sea, as in the case of Dwejra Bay, which has its entrance guarded by the legendary Fungus Rock. Other circular features immediately north of the Inland Sea have been eroded by the sea so that only a small part of the wall of these structures remains.

Subsidence features are typically associated with karst processes in limestone regions. Rainwater is naturally slightly acidic and slowly dissolves limestone until a cavern is produced. When the lateral extent of the cavern reaches a threshold size and its roof thins by rockfall, the entire roof of the cavern collapses, leaving a circular-shaped depression. The most well-known of such karst features in Malta is at Il-Maqluba on the outskirts of Qrendi. In contrast to Dwejra, this circular feature appears to be solitary and measures less than a fifth of the size of the Dwejra circular subsidence feature.

Dwejra does not fit in this model of limestone dissolution. It seems that rather than having formed by collapse of a thinning roof of a cavern, the Dwejra features were formed by the collapse of a cavernous void beneath western Gozo, hundreds of metres below the present surface. The collapse created a depression over an area of four square kilometres with the lowest point now defaced by the officially sanctioned quarry “sand”. The cavern collapse also triggered the several closely-spaced large circular sinkholes. We know about the existence of deep underground caverns because they were intercepted by deep exploratory oil wells drilled in Naxxar in the 1950s and about a decade ago in Kerċem, just two kilometres east of Dwejra. The caverns are closer to the surface towards western Gozo and formed when a deep layer of gypsum was rapidly dissolved by the infiltration of freshwater.

Ground collapse triggered by gypsum dissolution can be found in a number of places in Europe. In northern England, sinkholes formed by this process pose a threat to homes and constructions and are being monitored by the British Geological Survey. But the Gozo sinkholes remain truly exceptional at a European level because of their large size. The process of sinkhole collapse may have even continued to historical times. The mediaeval story of the calamitous sinking to below the sea of the San Dimitri chapel (when such events were explained by superstition that became legend) actually fits in the model of sinkhole formation along western Gozo.

This makes Dwejra and the coast of western Gozo not just an area of bare rock but a site with a story that is the stuff of legends and catastrophic geological events. It is no wonder the timeless landscape of Dwejra attracts the film industry and was the backdrop to a film released in 1981 featuring Lawrence Olivier, based on an adaptation of the ancient Greek myth of Perseus. No damage was done to the site even though Malta did not have a planning authority then. Hopefully, we have learned something from the recent “planning” blunder that has defaced part of a sensitive geological site so that we do not repeat this blunder. But have the competent authorities learned the value of such geological sites and will they now endeavour to protect them? Only time will tell.

The author is a geologist and researcher on Maltese geology at the University of Durham, UK.

Comments not loading? We recommend using Google Chrome or Mozilla Firefox with javascript turned on.
Comments powered by Disqus