In a few localities around the Maltese islands it is possible to find what look like mounds of reddish-brown soil with stones and pebbles embedded in them. On close inspection one realises that they are actually a solid structure as hard as the surrounding rocks.
These geological features were formed after the Maltese islands emerged from beneath the surface of the sea, where they formed. These structures can be found at Wied Magħlaq and Pembroke, among other places.
The Maltese islands are made up of five layers of sedimentary rock. The oldest layer started to be deposited 30 to 35 million years ago, making the islands relatively young in geological terms.
The topmost layer, and therefore the most recent layer to be formed, is known as upper coralline limestone and is usually more than 150 metres thick. The second layer, which is made up of greensand, is absent from many areas and is nowhere thicker than 13 metres.
During ice ages, evaporated water precipitated as snow and ice and formed thick glaciers leading to lower sea levels
This is followed by the blue clay layer underneath where one finds the globigerina limestone which can be as thin as 20 metres in some places and thicker than 100 metres in others. The oldest layer is the lower coralline limestone which is very similar to the upper coralline limestone and also very thick.
Beneath these five layers are even older layers.
The emergence of the Maltese islands above the surface of the sea did not mean the end of rock formation.
The rocks which formed afterwards are known as quaternary deposits because they were formed during the Quaternary period. This period started just over two-and-a-half million years ago. It is defined by a number of ice ages with warm periods in between. This period is divided into two: the Pleistocene which ended about 12,000 years ago and the Holocene, the period in which we are living. The Holocene is considered the most recent interglacial warm period.
During the Quaternary period, sea levels went up and down, depending on the temperature. During ice ages, evaporated water precipitated as snow and ice and formed thick glaciers leading to lower sea levels. In higher temperatures the glaciers melted and sea levels became higher. The low water level uncovered the sea bottom between Malta, Sicily and the Italian mainland, creating a large continuous landmass.
The weather was also characterised by heavy precipitation and several valleys were formed by large amounts of flowing water which carried with them large quantities of sediment and debris including soil, pebbles, rocks as well as small and large organisms.
The Quaternary deposits were carried down such valleys, although it is possible that many of them were formed during the Holocene period.
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