Geographical Evolution

Pre-Miocene Evolution: The geographical state of a small area at any particular period can be generally deduced from the rock formation making up the area, coupled with paleontological studies. In the absence of specific data the evolution of a region may be extrapolated from palaeogeographical studies of the Mediterranean. The Pre-Miocene evolution of the area occupied by the Maltese Islands is summarised in Table 1. The table defines two states, both marine. Marine epicontinental is a state where the Islands' area forms part of the continental shelf found around most lands up to a sea depth of 6000 feet, whereas marine orthogeosynclinal is a state where the Islands' area forms part of the continental slopes at depths greater than 600 feet.

That Malta has been a region of continued carbonate sedimentation for a considerable period prior to the Miocene is illustrated by the borehole sunk by the British Petroleum Co. Ltd., at Naxxar (4974) Malta in 1959. Commencing at the top of the Lower Coralline Limestone, the hole terminated at a depth of 3000 meters in dolomites, which carried spores of Lower Cretaceous affinity. Higher Cretaceous and Eocene rocks were also dolomitized limestones. The uppermost 650m, of shelly limestones and subordinate shales, was referred to the Oligocene.
 
 

Sandstone (GNS) is a granular, non-crystalline rock with rounded grains cemented together by various chemical substances, such as iron oxide, silica, lime or clay. Clay (BC) consists of very fine-grained particles which have not hardened completely, this being indicative of a land uplift on the Islands. Limestone is formed mainly of calcium carbonate with varying amounts of impurities, resulting either from the deposition of calcium salts or from the action of lime secreting organisms. This formation is mainly of two types: Coraline Limestone (UCL & LCL) deposited at sea depths of 30 to 180 feet, and Globigerina Limestone (GLS) deposited at sea depths of about 600 feet. The rock formation indicates that up to the Tortonian series, the state of the Islands' area was marine epicontinental, the depth varying from 30 to 600 feet. The changes in sea level, resulting in the different strata were determined by the third episode of the Alpine movements. These include the formation, filling and folding of the molasse foredeep.

This intermittent uplift is also indicated by seismic studies of the Mediterranean carried out by the American National Science Foundation. Seismic studies of the Mediterranean indicate a salt layer of two or three kilometers deep, over a large area of the seabed. However, if the present sea had to evaporate, it would produce a salt deposit of only 20 meters thickness. This implies repeated flooding and drying out. When it occurred, the reflooding would have been a gradual process from the Atlantic end. But how can a sea, as large as the Mediterranean dry out completely? This is very possible, because at present the evaporation loss from the Mediterranean is very much greater than the input from rainfall and rivers, the difference being made up by an inflow from the Atlantic through the Gibraltar Straits. Assuming present climatic conditions, the Mediterranean would dry up in about a 1000 years if this inflow is stopped.

The Mediterranean during the Pliocene

At about 5 million years ago tectonic activity resulted in a re-opening of the Gibraltar Straits filling up the Mediterranean and isolating the Maltese Islands area from the mainland.

It is not known whether the Islands were directly connected with Africa. The French paleontologist Vaufrey holds that the Pleistocene fossils found in Malta are typical of those in Europe, while typical African fossils have not been found. On the other hand J. Borg holds that the flora typical to Malta is of African origin and is only found in the southern regions of Europe. On palaegeographical basis it is probable that the Islands were connected to Africa, but separation from the African continent occurred long before that with the European mainland.

About 2 million years ago, the world climate underwent a series of cold-warm periods which gave rise to the Ice Ages. These climatic fluctuations caused the periodic growth of ice sheets on land in high latitudes and mountains during the glacial periods, while the interglacial climate was similar to those prevailing today. The ice cap during the glacial periods advanced at a rate of about 100m/yr and has been estimated to have been about 2500 meters thick across Europe. The ice cap however never reached further than 40o latitude and thus the Maltese Islands were never covered with ice. The uptake of water by the increasing European ice-caps resulted in a total drop in the sea level, estimated at a total drop of 150 m in the Central Mediterranean during the last Ice Age. This sea level drop is sufficient to expose the submarine ridge of the Central Mediterranean thus connecting the Maltese islands to mainland Europe. It is estimated that the Pleistocene may have undergone a total of about 17 cold periods. This cycle of mainland connection of the Maltese Islands followed by a period of isolation allowed for the development of a number of endemic species which were generally characterised by dwarfism of the herbivore and carnivore mammals, and gigantism of the rodents, reptilian and aviuan species. Similar species development has also been reported from other Mediterranean Islands. The Pleistocene and Holocene, till recently, saw the Islands covered by forests which were destroyed by early man.

Pleistocene endemism in the Mediterranean

The excavations of the deposits at Ghar Dalam Cave allowed the elucidation of the climatic conditions in the region. The regions south of the ice caps did not suffer directly from the glaciation of the Pleistocene, but suffered from a series of pleuvial periods which were further subdivided into three sub-phases. The first phase or the Pseudo-Pleuvial Period was characterised by a summer which was cooler than today, and a warmer winter. Rain precipitation was less restricted in spring and autumn. These climatic conditions resulted in the extension of the central european forest into the Mediterranean region. The unsettled weather further aggravated in the second phase or true Pleuvial Period. This phase correlated to the period of greatest extension of the ice-sheets in Northern Europe. The summer became more unsettled with much rain and rapid intense temperature changes. The winters were colder. The Pleuvial phase was followed by a rapid return to present day type Mediterranean climate with a decline in total rain precipitation. The Mediterreanean region during the glacials was relatively arid and poorly vegatated, though the period was puctuated by a series of rain showers. The climate was more humid and the region became well vegetated in the interglacial periods.
 
 

Present earthquakes and faulting show that the African and European plates are slowly swinging together, pushing up mountains that may close the Straits and cause the Mediterranean to begin to dry up again. This gradual swinging together is also the cause of the gradual southeastwards tilting of the Islands. Tectonic activity during the Pleistocene and Holocene periods has also altered the Islands area topography giving rise to separation into the various small islands - Malta, Comino, Gozo, and Filfla. The isolation of the various island populations from each other have resulted into the differentiation of various animal subspecies, best exemplified by the Wall Lizard Podarcis filfolensis which has differentiated into various islands subspecies/varieties including filfolensis on Filfla, maltensis on Malta, Gozo and Comino, kieselbachi on St. paul's Islands, generalensis on General's (Fungus) Rock, and an unnamed form on Cominotto. The Lizards on the Pelagic Islands of Lampione and Linosa belong to the same species as that of the Maltese Islands - P. filfolensis laurentiimuelleri. This may suggest either later introduction of the species or that the Maltese Islands landmass included the Pelagic region.

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