The Maltese Lithologic Sequence
The Island’s rock sequence is relatively simple being divided into five main layers, namely the Lower Coralline Limestone layer, the Globigerina Limestone layer, the Blue Clay layer, the Greensand Formation and the Upper Coralline Limestone layer as shown in figure1. Each layer has a distinct composition, as well as distinct properties such as resistance to erosion dependent on rock hardness. These layers essentially lie horizontally but they are displaced at intervals by various faults across the island. These faults in turn control the weathering and erosion of the exposed rock layers.
The sedimentary rocks forming the Maltese geological sequence are lime-rich, being predominantly limestones. This is due to the fact that the deposits consisted mainly of skeletal remains such as shells and shell fragments, dead fish and plant detritus all of which are lime-rich, essentially consisting of calcium carbonate. This is evidenced by the abundant and clearly visible fossils within the strata.
The properties of the layers depend on the grain size of the sediment as well as its layering, the depth of deposition, the fossils residing within it, any disturbances caused by organisms during the time of formation as well as any changes following deposition. For instance if a layer is made up of fine particles with no large ones, this indicated that deposition took place at a large depth with little or no agitation by waves. The fossil remains also shed light upon the organisms that may have inhabited the layer. From these revelations we can then infer whether the sea bed at the time was within light penetration, and hence at a shallow depth or at large depths with little penetration of light.
1.The lower Coralline Limestone
The oldest, visible, exposed unit of the Maltese islands is the Lower Coralline Limestone layer (figure 2). It is hard and pale grey, outcropping to a height of about 140m in the cliffs near Xlendi, Gozo. It is the most compact and crystalline of the Maltese rocks. In fact due to the latter, it is more resistant to erosion, and it therefore forms sheer cliffs. The layer itself is heterogeneous with permeability, colour and porosity varying throughout the layer resulting in a transition from a hard and compact layer to a soft, “rubbly” chalk (Bonello, 1988). The stratum’s heterogeneity hints at a formation which took place in shallow waters, with a very agitated sea environment. Another outcome resulting from its shallow setting is the numerous fossil corals and marine calcareous algae found within the layer due to the fact that light could easily reach the sea bed resulting in an abundant coral and algae formation (Pedley, 2002). Significant portions of rock are composed of small particles of shells such as brachiopods and bryozoa, corals and coralline, skeletal remains of calcareous algae, molluscs and echinoderms (Bonello, 1988). The Lower Coralline Limestone formations have been subdivided into five main facies, which are namely the reef limestones, the shallow lime muds, the cross-bedded lime sands, the foraminiferal limestones and the ‘Scutella’ bed. The latter is a layer consisting of one to three beds with a thickness between a few centimetres and a metre thick. This layer consists of a vast amount of flat sub-circular shells of burrowing sea-urchins (Cachia, 1985). This layer identifies the transition between the Lower Coralline Limestone and the overlying Globigerina Limestone. The latter horizon is sometimes absent replaced by a fine- grained yellowish limestone. The LCL members identified are the Maghlaq Member, the Attard Member, the Xlendi Member and the Il- Mara Member. All Lower Coralline members are identified by the colour pink in the geologic map (Oil Exploration Directorate, 1993).
2. Globigerina Limestone
Overlying the Lower Coralline Limestone is the Globigerina Limestone. This is a softer fine grained rock layer than that below it and therefore, when weathered, it forms gentle slopes (figure 3) unlike the sheer cliffs formed by the former. The thickness of the Globigerina layer varies between 20 metres near Fort Chambray, Gozo to about 200 metres at Marsaxlokk, Malta. This layer’s texture is homogeneous, unlike the Lower Coralline. The composition of the Globigerina Limestone is entirely Globigerina and related deep-sea micro-organisms, which indicates that at the time of its formation 34 million years ago, a sinking of the land mass took place since the components suggest sea-depths of around 600 feet. The variation in thickness of the layer also illustrates that a warping of the sea bed took place. The homogeneity of the layer further illustrates that deposition took place in deep waters with little agitation and wave action upon it. The layer itself has three sub-layers, namely the Lower Globigerina Limestone, the Middle Globigerina Limestone and the Upper Globigerina Limestone (Pedley, 2002). Lower Globigerina limestone is present at different locations with a varying thickness, this horizon thins out completely towards the North West of Malta. The layer itself often exhibits honeycomb weathering when exposed (Bonello, 1988). Thickness variations within the Middle Globigerina Layer are similar to those within the Lower Globigerina. This layer is absent in Eastern Gozo. The Upper Globigerina Limestone layer is subdivided into a further three layers, namely an upper and lower horizon of yellow biomicrites, with an intermediate division of grey marls. The Upper Globigerina Layer is absent in East Central Malta as a result of erosion which took place within the post- Miocene period.
The above three layers are in turn separated by two harder and coarse conglomerate and hard-ground layers. The latter two layers, easily distinguished by their brownish colour, indicate that the sea-level must have fallen at the time of the latter’s formation since their composition hints at an agitated water action. These layers are evident on the coast at Dahlet Qorrot, Gozo. Common fossils easily visible in this layer are scallop shells and burrowing sea urchins.
The Globigerina Limestone layer is identified by the yellow in the geological map of Malta (Oil Exploration Directorate, 1993).
3. Blue Clay
Overlying the Globigerina Limestone is the Blue Clay layer which is the softest layer of the main rock strata. It is compact when dry and malleable when wet. Its colour varies from grey to yellow or brown, with lighter colour bands present in upper layers. It is easily weathered forming rounded slopes covered with landslip debris, shown in figure 4, with the stratum thickness being variable over the archipelago. The layer consists of fine lime grains, clay mineral content as well as skeletal material from planktonic organisms. The blue clay layers consist of variable proportions of calcium carbonate, with the amount increasing in regions where the clays progress to Globigerina. It is compact when dry, with its colour varying according to composition between grey, brown or yellow (Bonello, 1988). The paler clay bands such as those found abundantly in Xemxija, consist of a high proportion of carbonate of lime due to the sizeable content of fossil foraminiferal shells (Testa, 1989). The relatively homogeneous composition of the Blue Clay also hints at a deep sea deposition. Its thickness varies across the islands being absent in certain locations.
This stratum is very important since it forms the basis of the Maltese water table due to its impermeability to water, also, water percolating through the overlying Upper Coralline Limestone and fissures in the rock forms major spring lines and caves. The mixture of clay particles interspersed with calcium carbonate inhibit the binding together of the lime particles. It is for this reason that this layer is the softest and most easily weathered layer. The clay content’s source can only be a land source and therefore Pedley et al. (2002) speculates that the main source must have been the erosion of the uplifted northern Sicily mountain ranges at the time as well as volcanic ash resulting from the movements which first formed the Pantelleria Rift system. The BC layer is identified by the blue colour in the geological map of Malta (Oil Exploration Directorate, 1993).
4. The Greensand Formation
This is the thinnest layer when compared to the Upper Coralline, Blue Clay, Globigerina and Lower Coralline layers. Its thickness is usually about a metre thick reaching a maximum thickness of 11metres at Il-Gelmus, central Gozo. It contains fossil debris, grains of Glauconite and brown phosphatic grains. The most common fossils found within this layer are sea urchins and fossil foraminifera. Unweathered sections are green but exposed glauconite grains undergo oxidation resulting in an orange-brown colour as a result of iron oxide released in the process. Contrary to Blue Clay, Greensand is porous. It was most probably formed in shallow waters in fact the transition between the greensand formation and the blue clay formation is a distinct one whilst the transition between the Greensand formation and the Upper Coralline formation is more vague. The stratum may be of a compact nature at certain locations whilst soft and loose at others. This layer erodes easily and is susceptible to weathering (Bonello, 1988).
5. Upper Coralline Limestone
The Upper Coralline layer, pictured in figure 5, is similar to the Lower Coralline Limestone on both chemical and palaeontological grounds. It is a hard layer of thickness reaching 160m in the Bingemma area, Malta.
Its composition shows that deposition took place in shallow water with a very agitated sea environment. The shallow deposition took place due to the uplift of the northern border of the Pantelleria Rift, which resulted in a shallower sea level. This layer contains an abundance of the fossil algae species Coralline as well as other organic remains including cetacean bones, sharks teeth, and different mollusca species. Outcropping UCL constitutes a third of Malta’s total surface (Bonello, 1988).
The layer is further subdivided in different strata, namely the Reef Limestone, the Tidal Flat Limestone, The oolitic cross-bedded sands, muds with large foraminifera and planktonic muds (Pedley, 2002). The UCL layer is identified by the green colour in the geological map of Malta (Oil Exploration Directorate, 1993). The Reef Limestone stratum consists of large compact beds and is characterised by a pebbly pattern (Bonello, 1988). This feature result from the weathering of the embedded ball-shaped calcareous red algae, termed “Rhodoliths”. The Tidal Flat Limestone is fine grained with a mottled graininess due to interspersed faecal pellets (from snails and crustaceans feeding on mud within the layer) and bacterial precipitates. The Oolitic cross-bedded sands layer usually exhibit the direction of flow and settling of submarine sand dunes at the time of deposition. These indicate that strong marine currents acted on the layer at the time. The Muds with Large Foraminifera contain the remains of unicellular animals of the Protozoan order, Foraminifera as well as their shells, which dominate the sediment in certain beds. Finally, the Planktonic muds, of which only few outcrops can be found, contain planktonic fossils and are chalky white and fine grained in consistency.
The Maltese Geological Map
As illustrated by the Maltese geological map, central and south-eastern Malta comprise of Globigerina Limestone outcrops while the northern and north-western regions of the island consist of upper coralline limestone outcrops. Gozo has a more varied geology than that of Malta, including frequent outcrops of Blue Clay.
Any Pliocene deposits that may have been present were exposed to erosion for an extensive period of time following the uplift of Malta during the Late Miocene Period. This uplift was enough to avoid re-submersion of the islands when the Mediterranean was re-flooded. Following the emergence of the Maltese Islands above sea level, any Quaternary epoch deposits, found intermittently overlaying the main strata mentioned previously, include terrestrial, aeolian and alluvial. The latter were most likely deposited by streams and lakes and are most frequently found near shorelines and in caves. Soil deposits result from climatic and geological actions on rock. Examples of which are erosion due to water and wind, plant root growth within cracks in the rock, due to fungal action and geological movements such as faulting. Sand deposits result from wind action, sea action and crushing action on rock.
Miocene strata are characterised by their regular and relatively uniform formation as marine sediments whereas post-Miocene strata are characterised by localisation and irregularity.
The Geomorphology of the Maltese Islands
Faulting, erosion and land movement are all causes of Malta’s variable relief and landform despite its small size.
A factor that has considerable effect on the strata is the amount of vertical displacement at a fault. Shattered segments of rock, as a result of faulting, are more susceptible to erosion, resulting in the enlargement of the fault line and hence the creation of narrow river valleys.
Due to Malta’s tilt down to north-east, drainage of Malta’s surface water has been guided in that direction resulting in many valleys aligned along or parallel to the SW-NE trending faults.
Following the uplift and faulting processes, erosion caused further changes in the islands’ topography. It has contributed to reduce the uneven topography caused by the former and hence cause a flattening of the land surface. The strata of the archipelago’s geological sequence consist of alternating layers of hard and soft bands, with variations present in individual strata as well. Erosion therefore also causes a terracing effect with softer intermediate layers recessive between harder upper and lower strata.
In the case of uplift, especially in the case of harder overlying rock with softer underlying rock, erosion affects the exposed overlying rock. Erosive factors undercut the capping, harder layer in the case of the uplifted segment. Erosion cuts down deeper into the fault, enlarging it and collapse of the hard capping occurs as a result of undercutting erosion. Once the higher uplifted plane is eroded, the downthrown plane is now more exposed and the whole process repeats itself. This process is illustrated in figure 6.
Erosive processes also give rise to table-topped hills capped by Upper Coralline Limestone cliffed ledges. These are very commonly found in Gozo as shown in figure 7.
4. Coastal Erosion
Coastal Erosion sculpts, shapes and moulds the Maltese coastlines. Following erosion, other coastal processes such as transportation, deposition, creation of wave cut platforms, caves, arches, headlands and bays occur. Many such features can be found on the northern coasts of Malta, an example of which is shown in figure 8.
Coastal erosion is both a hazard and a risk, especially with about 60% of the global population living within coastal zones (Farrugia, 2008). Coastal erosion takes place at various locations around the Maltese archipelago, an example of an ongoing erosive process is shown in figure 9. Two such examples are at Mistra Bay and Ghajn Tuffieha Bay. The greatest risk posed by this ongoing process is at locations where human settlements are present, and where there is a possibility of loss of land and property. This may also lead to landslide hazards, especially when slopes of Blue Clay, such as those at Ghajn Tuffieha, are present.
Sand results from the erosion of rock surfaces. For instance the sand at Golden Bay is of a pale yellow-brown hue, similar to that of the Globigerina limestone present in the cliffs. At Ghajn Tuffieha Bay, the sand has a red-brown colour probably originating from Greensand erosion. Coastal erosion on the other hand leads to a decline of sand in bays such as has occurred in Mistra Bay. As well as erosion in the bays themselves, erosion is also evident at both the toe and top of cliffs. Such a case is the cliff of Rdum Rxawn, with erosion at the top being predominant in this case. The latter result due to the fact that at the foot of the cliffs, accumulation of large boulders as a result of down-slope movement act as a shield against erosion, hence diminishing the process effectiveness at that point.
A similar process takes place at Ghajn Tuffieha Bay though coastal erosion at the latter is more predominant when compared to the former. This is mainly due to the fact that Ghajn Tuffieha Bay is strongly affected by the North Westerly Wind making the location more exposed and hence vulnerable to a greater erosive power. In fact since 1939 the emergent Qarraba headland at Ghajn Tuffieha lost about 2677.5m2 of debris from the foot of the cliff and about 1530m2 of land from the top (Farrugia, 2008).
5. Formation of Caves
The rock surface is constantly assaulted by the elements, namely rain, sea water and wind. Acid rain aids erosion even further by dissolving the calcium carbonate composition as well as enlarging any fissures and cracks present in the rock sequence. This repeated action and dissolution by rain water, facilitates the formation of a network of waterways and eventually land caves. Erosion due to the sea leads to the formation of sea caves, illustrated in figure 10a . The formation of the latter is assisted by the chemically erosive action of the salt crystals in sea water, as well as by various marine flora and fauna which burrow and bore within the rock. Sea caves form at the prevailing sea level. The latter has changed over time, rising and falling multiple times in recent geologic times. Therefore current caves, located in high positions were at sea level some time when the sea level was higher than it is today, or at a time before tectonic uplift of the land took place. An example of the latter is shown in figure 10b.
Figure 10 (a) (above) (b) (below)
These dents resulted from cave collapses in the past. These depressions are then filled by younger sediments as evidenced by the tal-Maqluba or Baħrija collapses.
Channels and caves are often formed at the junction between Upper Coralline Limestone (UCL) and Blue Clay (BC). The reason for the latter is the lateral flow of water between the permeable UCL above and the impermeable BC below resulting in the formation of major spring lines. There are two main collapse features, namely, younger caves and collapse features formed during Pliocene or early Pleistocene and older cave collapse features which took place in early Miocene times.
The caverns formed are covered by sediment until the cavern roof collapses under the sediment weight or due to instability. In the case of older cave sequences, these depressions are then filled with an abnormally thick sediment which later on leads to either a depression mimicking the original collapse feature or else to mesas, depending on whether the new sediment is softer or harder than the surrounding rock respectively. This process is illustrated in figure 11 and two examples of this phenomenon are shown in figure 12, where on the left a sink hole at tal-Maqluba, Qrendi is illustrated and on the right a sink hole near the civil protection, Xemxija.
Bonello, S., 1988. Engineering properties of rocks and soils of the Maltese islands. B.E.&A. dissertation, University of Malta.
Cachia, J., 1985. The mechanical and physical properties of the globigerina limestone as used in local masonry construction. A.&C.E. dissertation, University of Malta.
Farrugia, M.T., 2008. Coastal erosion along northern Malta: geomorphological processes and risks, Geogr. Fis. Dinam. Quat., 31, pp.149-16.
Oil Exploration Directorate, 1993; Geological Map of the Maltese Island. Sheet 1 – Malta - Scale 1:25,000. Office of the Prime Minister, Malta.
Pedley,H.M. Clark,M.H. and Galea,P., 2002. Limestone Isles in a Crystal Sea: The Geology of the Maltese Islands. San Gwann: Publishers Enterprises Group Ltd.
Testa, S.J., 1989. A down-column geochemical study of lower globigerina limestone with special reference to the ’soll’ layers. B.Ed. (Hons.) dissertation, University of Malta