Frederick Gomez

Gibraltar's Caves

The following text was provided by Frederick Gomez and the  Gibraltar Tourist Board.


The caves have been explored over the years, and numerous fossil remains found, including the skull of a Neanderthal Woman. This was discovered in 1848 in Forbes Quarry, at the foot of the North Face of the Rock, before the famous discovery of the Neanderthal Valley. Its discovery was not publicised and the skull lay forgotten in a cupboard in the Garrison Library for many years. Had its importance been recognised at the time, we might well talk today about Gibraltar Woman instead of Neanderthal Woman.

A fine display of prehistoric tools, implements and fossil remains found in various caves on the Rock can be seen in The Gibraltar Museum, situated at Bomb House Lane. Fossil remains include bones of elephants, rhinoceros, deer, wild horses, hyenas, leopards and African lynx.

The following is a report prepared by Frederick Gomez from The Gibraltar Cave Exploration and Protection Group.

Gibraltar is honeycombed by natural caves as well as tunnels, some 150 principal and natural caves have been distinguished to date situated above present day sea-level and more are known to occur below.

Some, if not all, these caves, originated in two ways:

  1. Solution Cavities
    Most of these formed as weak acidic ground-waters slowly reacted to the Gibraltar Limestone. Some caves developed along faults or major joints because these fractures facilitated ground-water movement, thereby rapidly removing the reaction products and introducing fresh acid into the rock's surface. Others developed within beds of dolomitic Limestone that occur within the Gibraltar Limestone succession. The mineral dolomite is more soluble than calcite and dolomitized Limestone tend to have a higher porosity (dolomite being denser than the original calcite from which it formed) thereby exposing a larger surface area of rock to the dissolving acidic ground-water. Most caves developed initially at horizons related to former relatively stable ground-water levels, because of the enhanced solution and frequent replenishment of acid within the zone of fluctuating water levels. These caves are controlled in part by the prevailing position of the sea-level and variations have occurred reflecting the various different sea-level stands that existed previously.

  2. Sea Caves
    Several levels of caves were eroded by marine action during times when sea-levels differed to those of today. Some are former solution cavities breached by the sea, but they indicate periods when sea-levels were stable enough for such erosion to take place. The most prominent and well developed caves of this kind are to the south and east of Windmill Hill Flats.

Commonly, most of the Gibraltar caves situated close to the sea, usually contain one or more characteristic sediments.

Flowstone layers

Strictly termed speleothem deposits, and often loosely termed travertine, For these to form the cave must be wet, so that calcium carbonate can be precipitated from acidified water carrying calcium bicarbonate dripping from the cave's roof, walls or floor as carbon dioxide within the water dissipates in the cave's air. Many of the caves in Gibraltar today are too dry for flowstone formation, yet the contents of this deposit are spectacular, sometimes buried beneath other sediments. These are indicative of periods of more humid climate in the past, such as climate encouraging growth of more vegetation than at present at ground surface, with the release of greater quantities of humic acid, thus enhancing the acidity of the percolating water necessary for the initial dissolution of the limestone.

The nature of the flowstone depends on the rate of flow of the water, the discharge point and the chemistry of the host rock. Magnesium-rich limestone, thus palettes of flowstone can be left behind as the dolomite-rich rock supporting them is dissolved away. If the water drips from the roof, stalactites can form with stalagmites growing on the cave floor beneath, cave columns developing when these two structures grow sufficiently to join together.

However, flowing water tends to produce the sheet-like deposits properly known as flowstone, whereas slow seepage produces more delicate cave forms such as helictites (thin carbonate tubes), cave flowers (sprouting clusters of helictites) and cave shields (semicircular sheets of carbonate attached to fractures in the cave's walls)

If pools of water remain on the cave floor for long periods of time, special flowstone deposits can develop, notably calcite rafts (paper-thin sheets growing on the water surface which can develop into thick ledges at the waters edge) and moon milk (white paste formed by micro-organisms causing disintegration of the limestone wall rock)

Cave-earth

In some caves the surface deposits on the floor consist of a loose or damp semi-compacted red clay, usually soil transported into the cave by water, but sometimes with components of chemical weathering or of organic detritus. Such cave-earths tend to be fine grained, since large sediment particles tend to be filtered out though tight fractures and pore-spaces, and to be well stratified; in contrast to these caves, those with loose, unstratified cave-earths indicate the effects of wind deposition (and are suggestive to a more arid climate, of greater sediment availability through low sea-level, lower vegetation cover, or variation in global dust flux)

Cave breccias

These consist almost exclusively of angular pieces of rock identical with those in which the cave is situated, with no significant wear observed on the fragments. They must therefore, have formed from material detached from the walls and rook of the cave. None are forming at the present day, and the climatic conditions under which the formed must have been different from the other two cave deposits just described. Such breccias are thought to indicate physical weathering in the form of fresh fracturing, and developed best in a climate with frequent frosts.

Solution Cavities

These are the consequence of ground-water solution within the cave, weathering at cracks, and at soil deposits these cavities are nearly always in a dump and arid condition making the process of flowstone and other cave formations possible.

Faults, Joints, Fissures

The Gibraltar Limestone is almost everywhere fractured by faults, joints, fissures and fractures, these are most obvious in highly tectonized areas where brecciation also occurs, some tend to be discontinuous through the rock, whilst others continue into natural chokes, and physical access becomes impossible, very large fissures and fractures, however do occur inside the rock (these are sometimes situated at/or near tunnels, excavated after the 1900's) and many of these have served as passage-ways for ground-water movement, as indicated by solution widening and the coating of flowstone, and other cave formations.

Gibraltar's caves are far and wide, and in many ways are connected to Gibraltar's history, some have served as gun-power magazines, or used to accommodate troops (Beefsteak Cave) others as war time shelters (Glen Rocky) or as peace time storage (Poca Roca) and some even as potential water reservoirs (Ragged Staff), whilst few have been the subject of excavations by archaeologists (Genista's, Devil's Tower, Gorhams) others are known as tourist attractions (St. Michael's).

The most potential of the caves are;

Genista caves (Busk & Falconer, 1865; Warren 1865; Brown 1867)

Earliest scientific cave exploration on Gibraltar began with the Genista caves, named after Captain Frederick Brome, Governor of the Military Prison. (The word Genista derives from the formal Latin name for the plant called Broom in English).

These caves were accessible from Windmill Hill, and yielded rich deposits of fossil bones, including those of bear, rhinoceros, aurochs, deer, ibex, wild horse, leopard, hyena and many other animals now extinct or no longer living on or near the rock, the most accessible of the Genista caves yielded human remains found lying in every imaginable direction and position (Busk 1869) illustrated and described the Genista's and many other Gibraltar caves as then known, and discussed their archaeological contents.

(Gibraltar Schedule of Antiquities, 1947 (?)=a very unreliable source of information) infers that the Genista lies somewhere in and under Genista Magazine (?) at Windmill Hill, a very deep cave discovered by Brome in 1862, yielded many human and animal remains, not completely excavated, the top cavern was blasted out into an ammunition store in 1895, during which operation, the entrance to the rest of the cave was lost.

Present day: Even though the Genista Cave's complex has been lost(?) inside the Lathbury Barracks, and opposite the Parade Ground, there stands a rock plaque (which appears to have been moved to its present position) indicating the existence these caves, however and because of the effects of weathering to the rocks surface, this plaque is hardly legible.

Devil's Tower Cave (Garrod & others, 1929)

This famous rock-shelter overlooking the isthmus, from the base at North Front. It north-east corner has yielded a Neanderthal human skull ( a child aged around 5 years) and flint implements of Mousterian culture type. Deposits yielding these remains rested on a marine beach 8-9 meters above present sea-level. The vertebrate fauna, as reported by Garrod & others, are indicative of a climate somewhat cooler and damper than at present, with ibex being frequent and the great auk and alpine chough also present.

Present day: Although called Devil's Tower Cave, this is not a cave in the full context of the word, butt a natural fissure within the rock. At present it is inaccessible, as it forms part of the north-east arial farm, situated opposite Devil's Tower Road, at the Eastern Beach Road junction.

Gorham's Cave (Waechter, 1951; Zeuner 1954; Zeuner 1958)

Records show that this important cave on the east side of the Rock of Gibraltar was formed during the main and late Monastirian sea-levels, a concept now known to be invalid.
In the present the cave has been the subject of re-investigation under the joint directive of Dr J Cook of the British Museum and Dr C B Stringer of the Natural History Museum in London.
Based on J.Cook's preliminary report, æthis cave is a phreatic passage which formed part of a subterranean system in the Gibraltar Limestone and was subsequently exposed by the erosion and massive collapse which created the cliffs along this section of the coast'.
æSediments within the cave appear to have accumulated without interruption during the late Pleistocene. It remains to be proved whether or not they rest on beach material deposited during the high sea-level of the Last Interglacial about 120,000 years ago, when sea-level is unlikely to differ greatly to that of present day. If so, the over-lying deposits, were laid down between 100,000 and 10,000 years ago during the period of fluctuating climatic conditions known as the Last Ice Age. More certain is the evidence that during the period represented by the cave fill, Gibraltar must have been part of the mainland with a broad coastal plain covered with dunes stretching in front of the site.
The sediments incorporate Middle and Upper Palaeolithic stone artefacts, large and small animal remains, coprolites (fossil excrement), marine and terrestrial mollusc shells, as well as charcoal and plant ash which are occasionally concentrated in a manner which suggests they are the remains of hearths. These indicate that the cave was initially occupied over a long period by people who used locally available pebbles and cobbles to make a range of stone tools which falls within the general category of æMousterian' and elsewhere (Devil's Tower) are associated with Neanderthal skeletal remains (the well preserved adult Neanderthal skull found at Forbes Quarry in the North Face in 1848 and originally referred to, before the discoveries in the Neander Valley in Germany, as Homocalpicus, cannot be dated because no details have been preserved of its context). Some flint is known from the Gorham site, which must have been brought or traded from distant sources, possibly even across the straits from North Africa.
The presence of anatomically modern hunter-gatherers during the later Pleistocene is attested by the presence higher in the cave sequence of both large and smallstone blade industries based partially on imported flint. Burnt animal bones exhibiting butchery marks have been found in association with the stone tools throughout the sequence'.

Present day: Access to this cave is at present restricted. Only few personnel are allowed access, obtainable from the Gibraltar Museum, whilst conducting excavations with other relevant bodies.

St Michael's Caves (Shaw, 1953, 1955 a,b)

The most easily accessible and celebrated cave in Gibraltar is St Michael's Cave, a noted tourist attraction and with a chamber large enough to serve as a concert hall. Shaw (1955 a,b), provides an extensive account of the history of exploration of the cave and the numerous bibliographic references to it since its earliest citation by the Augustan geographer Pomponius Mela in 45 AD. The cave is now properly named Old St Michael's, to distinguish it from New St Michael's Cave discovered in 1942. During 1942, a tunnel was being driven into the largest chamber of the Old St Michael's Cave so that it could be used as a store with reasonable air ventilation. This broke into the system now known as New St Michael's Cave, which was controlled and looked after, until recently by the Royal Engineers. (Shaw's 1953, account provides the best description of the system). New ST Michaels' not only contains stalagmite formations rivalling those in splendour of Old St Michael's, but also has a lake some 30 meters long and up to 11 meters wide and 6 meters deep. One of its remarkable features is the size of the calcite ledges formed at the lake margin by deposition from the surface of calcium bicarbonate-saturated water. Access into this cave system tends to be hazardous and can only be visited with the assistance of a guide.

Martin's Cave, on a path leading off from the Mediterranean Steps on the Upper Rock was rediscovered by chance by an artillery soldier in 1821. Two ancient 13th century swords were found there. The cave, situated some 600 feet (185m) above sea level, contains a thriving bat colony.

Poca Roca Cave, near the old Isolation Hospital on the Upper Rock, was prepared for use in 1789 as an emergency residence for the Governor of Gibraltar. It never needed to be used.

Sources

The documentation has been prepared by Frederick Gomez from the following sources:


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