Erosive Factors

SURFACE WATERS: During the Pliocene and early Pleistocene, surface waters attributed to a marked change in scenery. This period was marked by the Pluvial periods coinciding with the Ice Ages. The increased rainfall, coupled to the connection to the continent, resulted in a number of river tributaries coming from a northern direction. These have left their mark in the large number of eroded valleys, included among which are Wied il-Ghasel (Malta), Wied il-Kbir (Malta), Wied Dalam (Malta) and Wied il-Ghasri (Gozo). Surface water now plays a less evident part in the Islands' present denudation. There are presently no permanent rivers or marshes on the Islands, but during heavy rains the valleys may be filled with torrents which carry off soil and cause widespread destruction to the fields. Such heavy rains are fortunately rare, but the normal rains also play an important part in the Islands' present erosion. After a period of prolonged rainfall, water continues to flow down defined water courses, contributing towards their erosion.

Rainwater falling onto soil quickly percolates through causing the soil to become waterlogged. At this stage, the rest of the water will run away as small streams carrying with it light matter such as clay, soil and sand. The particle size carried is dependent on the water velocity. Clay when dug up from deep layers is quite hard, but on the surface, it absorbs a certain amount of water becoming impervious to more water. When the rains fall on the surface clay, the clay quickly becomes waterlogged and viscous. This allows landslipping of this strata and the overlying strata, resulting in the characteristic Talus Formation of clay slopes. The clay horizon never forms cliffs, but rather forms comparatively gentle slopes on which the overlying strata is piled up in great heaps. In these slopes are also found evidence of erosion by running surface waters causing grooves cutting through the slopes. The undercutting of the clay facilitates the fracturing of the overlying greensand and upper coralline limestone, which slide down to the botton of the clay slopes.

Surface water erosion of the soil cover is minimized by an adequate plant cover, which helps bind soil particles together. Local farmers attempt to preserve the precious soil by terracing their fields, thus reducing the stream water velocity. The surface waters described above run down usually towards the sea. When the stream enters the sea, the velocity of the water is immediately checked causing coarser material to precipitate, the finer mud being carried on to deeper waters. In this way a form of delta can be formed, and probably accounts for the Greensand horizon formed during the Helvetian series. The talus formation of the clay slopes also "flows" downwards, and in localities near the sea, notably in the vicinity of Slug's Bay (Malta), the clay strata may be noted to cover the sea-bed.

Rainwater, helped by sea-water near the coast, falling on the non-resistant limestone causes chemical erosion of the surface of outcropping limestone. This results in grykes consisting of narrow grooves up to several inches deep, a feature of the Karst phenomenon. These grykes are notable on outcrops of coralline limestone, which being more resistant than globigerina maintains the formation. Globigerina is too easily eroded, hence being more easily smoothed out.

UNDERGROUND WATER: Because the Islands are partly composed of porous limestone, percolating rainwater finds its way into rock fissures and reaches the impermeable clay strata. Rainwater is acidic in nature since it has dissolved in it carbon dioxide and other gasses, and hence proceeds to dissolve away part of the rock to form watercourses and caverns. These are very often unseen and unsuspected at the surface, though the water occasionally finds its way to the surface through faults or the Greensand/Blue Clay transition, accounting for natural springs. Cave-ins of caverns have occurred, usually causing no great damage. However on December 22, 1923 a cave at Bahrija collapsed killing a shepherd boy. A spectacular result of such cave-ins can be seen at Il-Maqluba to the south of Qrendi. This cave-in has resulted in a 108 feet deep hole with a circumference greater than 1000 feet.Though to have been formed by similar processes are the inlets on the western side of Comino and Qala Dwejra (Gozo).

Caves due to subterranean water erosion are numerous on the Islands. The more important ones include Ghar Hassan (Malta), Ghar Dalam (Malta), and the Alabaster Caves at Xaghra (Gozo).

Another spectacular cave of this sort is found at St. George's Bay known as Chark El Hamien. Lying at a grid reference of 538762, this cave is found in the fields off the old military road leading to St. Andrew's Barracks from St. George's Bay. The cave opening is a slit-like man-made aperture in what from afar appears to be a smooth quarry face of coralline limestone. The opening, about 3 feet in width, leads to a small area of stones re-enforced by concrete. This elliptical dome-shaped cave has an overall area of just under 600 square meters, and is completely filled with water. The water depth is generally deep except along a ledge to the right of the cave's opening by the wall where one can progress by wading. This ledge leads to a medium-sized area above water-level, the result of a past fall. This is composed of lower coralline and globigerina limestone boulders bound by smaller stones and soil. From this boulder area, leading to about midway to the opposite wall, was a narrow path of boulders and soil in shallow water.

The process of formation of the cave can only be speculated at, but it is plainly obvious that the main causative factor was the action of underground water enlarging faults and fissures, thus forming the initial cave. Later cave-ins, as evidenced by the boulder area, further enlarged the cave dimensions. The entrance has been created by a local fault line breaking off at 5/10^o from the vertical. This geological event brought down with it the original cave's cumbersome overhanging roof. The original entrance was a narrow shaft in the ceiling, but in 1772 Count Giovannantonio Ciantar relates about the cutting of a conduit trench on the south side facilitating entrance to the cave. This trench was cut in an attempt to use this freshwater reservoir for irrigation purposes at Bailiff Spinola's residence some distance away. This plan was eventually abandoned, however recently the cave water is being pumped out for use.

Count Ciantar also relates about the passage of two "streams" of underground water, one of which winds its way via Qormi to Marsa and the other to Chark El Hamiem. This latter stream supposedly originated from the hills to the west and South. This idea is based on a previous suggestion made by Fra Girolomo Manduca who tells of a place not far from St. Paul's Bay where sea-water is taken inland and made potable by its passage through the rock. One wonders on what basis Fra Manduca arrived at these conclusions of underground streams originating from sea-water. This idea is now known to hold true for the presence of the Lower aquifer at Sea water Level. It may also holds true for the origins of Chark El Hamiem's water, since studies done on the cave water level showed the water level to be roughly in unison with the sea-water level in the bay. The water line fluctuates about 0.3 m between the dry and wet seasons, being accounted for by different rates of evaporation.

The other important source of water comes from rainwater which percolates through the fissures in the upper Coralline Limestone, and the permeable Greensand strata to reach the impermeable Blue Clay layer. Here it forms the perched water tables. In some areas such as Pwales, Marfa, and Comino, the Blue Clay horizon lies below sea-level, hence the water table becomes a lower water table.

The underground waters containing dissolved carbonate continues to slowly percolate to the cave, trickling down the cave walls and from the cave roof. The carbonate in solution is gradually redeposited in the crystalline form of calcite, forming stalagmites, stalactites or encrustation's over the walls or in fissures. Spectacular results of this process may be viewed in the Alabaster Caves of Xaghra (Gozo). Stalagmites and stalactites can also be seen at Ghar Dalam (Malta). The growth of these formations is very slow and variable, depending on the amount of rainfall, water tapping and other factors. On average a stalactite grows at a rate of 0.01 cm per year. Water action in caves also result in the formation of conical holes in the cave roof or wall. Examples of these holes can be found in the water-formed caves of Ghar Dalam (Malta), Ghar il-Friefet at Girgenti (Malta) and Ghar Hasan (Malta). The roof at the latter cave shows the conical holes and stalamitic incrustations over the walls and roof deposits. Water flow through the caves can vary through time, thus resulting in the cutting of secondary channels with widening at the lower part of the cave walls. This feature can be seen both at Ghar Dalam and Ghar Hasan.
 

SEA ACTION: Another important factor affecting erosion is the effect of the sea on the coastline. It is now generally agreed that ocean currents play a very small part in the erosive and transporting action of the sea. The work of the sea is primarily through wave action, and to some extent through the hydraulic forces engendered by the movement of great masses of water, but far more through the arming of the waves with quantities of rock, stones, gravel and sand. Consider what happens at the base of cliffs. Angular blocks of rock and stones fall from the cliff face. These are picked up by the waves and hurled against the base of the cliffs which they thus tend to undercut. Blocks from above then split off along along joints and fall by the force of gravity.

Such an occurrence occurred in the Summer of 1980 at Ghajn Tuffieha (Malta) when a large part of the overhanging cliff fell into the sea. Where there is a dip of the rocks seawards great masses may slide down the bedding planes. This latter effect is best seen where rock overlies clay, the surface of which becomes slippery and acts as a greased plane - hence the constant slips. This undercutting and slipping result very often in large faults close to the cliff edge, such as the ones at Rdum Rxawn (Malta).

On the shore, the rock fragments are rolled against one another and quickly reduced to rounded boulders, pebbles and sand. These rolled backwards and forwards by the waves, enable the sea to level off its wave action. In addition, hard stones are whirled about by the action of the surf in rough weather. This wears out as smooth and more or less circular excavation in the softer rock giving rise to underwater potholes. These potholes, by virtue of their mode of formation, are commoner in coastlines areas composed of the soft globigerina limestone.

Thus the effect of the sea round the coasts may be described as the creation of a platform or the continental shelf. This effect is more marked to the southeast of Malta, creating the deep natural harbours of the area. The reason behind the marked erosion of this area in mainly due to the southeastwards tilting of Malta bringing the easily erodable limestone to sea-level, while resulting in the high lower coralline cliffs to the west of the Island. A very striking example of sea erosion is seen at St. Thomas Bay on the limits of Zejtun (Malta). here the predominant strata is the very soft and easily erodable globigerina limestone. At this site are fields which have already been halved and old residents remember rooms which stood about 30 meters out to sea. A large farmhouse known as "Tal-Franciz" is gradually being destroyed by the sea, and part of the building has collapsed with the cliff.

In a number of sites, such as the coast off Gnejna Bay (Malta), the platform formed by the action of the waves has been lifted above sea-level. An these sites, the process is very often repeated, resulting in the undercutting of the previous platform.

The strong action of waves has also resulted in the numerous amount of caves, grottoes and natural arches of which the Blue Grotto caves and the caves of Dwejra are the most well known. These caves are usually found in well-jointed rocks such as Lower Coralline limestone. In these rocks, the water is forced into crevices compressing the air trapped within them. When allowed to expand suddenly, the air widens the crevices, and by a continuous process enlarges them to form caves. When the action of the waves on headlands occurs from two sides, erosion occurs equally from both headland faces. In this way a natural arch may be produced such as the one found at Dwejra Bay in Gozo. With continued erosion, the arch may collapse leaving a stack such as the one found at Il-Mara (Malta).
 

Over very large areas, the continental shelf is covered by a verneer of sediments laid down by the sea itself. These are derived from both the near-by land by wave action and from the smoothing of the shelf itself, as well as from sediments brought by rain water. There is normally a gradation from the coarse shingle and stones of the beach to sand, becoming finer seawards, which in turn passes into silt and sand. This process accounts for the sandy beaches of the Islands. Microscopical studies of sands collected from various sites from the Islands showed these to be subangular to subrounded. The sand particles are prevented from becoming exact spheres by the cushioning effect of the sea. In contrast sands formed by the action of wind are usually rounded, and those formed by a crushing action are angular. In addition different forms of sand are found locally.

The colour and size of the sand particles depend on the bed-rock from which they originate. Thus the sands at Ghajn Tuffieha (Malta) are of a reddish yellow colour and of very fine texture, and are similar to the sands from Ramla Bay (Gozo). These sands contrast very well with the sandstone of the areas which has similar properties. The sand at Mellieha Bay (Malta), on the other hand, is coarser and of a grayish colour being similar to the sandstone of the region.

Broadly speaking, any stretch of coastline is either one of erosion or one of deposition. The rate of deposition in an area is dependent on local factors. Deposition is higher in areas where eddy currents are produced, such as behind a spit of land. An example of such a process produced artificially can be seen at Pretty Bay, Birzebbugia (Malta). In relatively recent years, a breakwater built there to protect fishing boats has resulted in the bay becoming shallower with the creation of a small sandy beach.

CLIMATIC FACTORS: Denudation on the Islands of Malta has been indicasted to be mainly due to water action on the easily eroded limestone. However other less spectacular factors are also responsible. Wind action carrying small particles, sea water spray, or rain has an erosive action on the softer limestone. The efect of wind action  is to smoothen out the globigerina limestone face, a feature best seen along the globigerina cliff at Mtahleb. When rain-water plays an important role in erosion of globigerina, then uneven weathering closely resembling a honeycomb notable in many old globigerina walls and monuments.  This process is undoubtedly helped by the chemical solution of the rock. Air pollution from motor cars and industrial wastes would further increase this form of erosion. A similar effect can be seen in natural outcrops composed of variable strata. Acceleration of weathering at the softer strata relative to others results in the phenomenon of degradation so common on most of the natural outcrops.

Wind action also contributes towards altering the scenery by the transportation of light particles of soil and sand. Such transportation, like transportation by water, is dependent on the wind speed and particle size. this wind action has led to the development of embryo sand dunes on the sandy beaches of the Islands, particularly at Mellieha Bay (Malta). Microscopical studies of sand from the dunes show this to be of a more rounded variety.

The processes which break down the bed-rock to fine powder, be it wind or water action, are essential for the formation of soil. This process is very often helped by farmers by breaking down of large stones to smaller ones, thus helping nature's forces. Other soil forming processes include extremes of temperature. High temperatures markedly dry up the rock causing exfoliation, while freezing of water inside the rock cracks the rock mass. Cracking of the rock is also produced by faults and other geological movements. Various changes then take place on the bed-rock material thus forming the soil profile.

The formation of arches and stacks of headlands serve to show the variety of forms which can be formed by the processes of denudation. These form because the erosive forces are equal on both sides of the headland. Similarly, if the forces of denudation are equal in all directions, a circumdenuded remnant is left. This very often takes the form of an isolated conical hill with a flat top, such as the hills of Gozo.

EFFECTS OF THE ICE AGE: About 800,000 years ago, in the Pleistocene period, temperatures began to fall and ice slowly advanced from the north until it covered considerable areas in northern Europe and North America. In Europe, the ice spread mainly from Scandinavia and the Alps over most of the North European plain. The Ice Age was interrupted on at least three occasions by interglacial periods when mild conditions returned. It has been estimated that the present mild period began about 20,000 years ago. The Mediterranean region did not suffer directly from the glaciation of the Pleistocene, but rather suffered from a series of pluvial periods. It is evident therefore that evidence of past glaciation in the Maltese islands is not to be found. Instead we have with us marked remains resulting from the effects of denudation by water - surface and underground. The most noted of these effects are the large number of valleys found on the Islands. In addition, the large number of caves caused by underground water probably date to this period. This same period, because of the increased rate of denudation, also saw the development of the soil cover. An erosive factor which must have been prevalent during the pluvial periods, was the action of freezing and expanding waters in joints and fissures. An interesting effect of this low temperature denudation has been described by Prof. J. Borg (1936). it has already been noted that in coastline areas composed of the soft globigerina limestone, the action of the surf carrying small stones results in the formation of smooth near-circular underwater potholes. These potholes are common at the Sliema front. A number of these, now unfortunately filled up and covered to widen the road, showed a number of deep fissures radiating from the hole. these fissures extended to distances varying from 1-8 meters, and were filled up with a crystalline deposit of calcite. These fissures were the result of the enormous expanding force of freezing water collected in a receptacle of some depth.

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