The Maltese islands are composed of sedimentary rock which was formed under the seawater of Tethys. This began to form about 25 million years ago out of remains of sea creatures, sand and silt. Fifteen million years later tectonic movement lifted the sediment that was on the seabed and thus the Maltese islands emerged above the surface.

Watch these two videos which explain clearly how the sedimentary rocks of the Maltese Islands were formed.

Now that you have seen these videos explain briefly how sedimentary rocks were formed millions of years ago under the Sea of Tethys.

The Five Layers of Rock of the Maltese Islands:

The Maltese islands are made up of five main types of rock, which are the following from lowest to highest: lower coralline limestone, globigerina, blue clay, greensand and upper coralline limestone.

Malta’s rock is formed of five horizontal layers, each one on top of another and the oldest layer is the lowermost, that is, the lower coralline limestone. This does not necessarily mean that all the five layers can be found everywhere. For example at Baħar iċ-Ċagħak there is only one layer and the others are missing.

The geological map below shows the distribution of different types of rock found at the surface. A different colour is used for each type of rock. The greensand layer is not visible in the geological map since this does not exist at the surface, neither in Malta nor in Gozo.

Upper Coralline Limestone:
Upper coralline limestone was the last layer to be formed on the seabed of Tethys. This type of rock is now found in the highest places of the Maltese islands at Mellieħa and Dingli in Malta, as well as in Xagħra and Nadur in Gozo. This type of rock is sometimes hard, compacted and crystallised. At other times it is composed of softer rock which is easily weathered. Even the colour varies. Some layers are greyish white, while others are yellowish or ruddy (red). On the hills and cliffs the upper coralline limestone has an average thickness of 30 metres. However, at a borehole in Binġemma a thickness of 162 metres was registered. This layer is porous and allows water to pass through it. In both the upper and lower coralline limestone layers, many fossils of algae and corals are found. These take up calcium from seawater in order to build their own calcareous shell. Coralline limestone is made up of algae reefs that formed in a clear, shallow (not more than 50 metres deep) and warm waters with temperatures always above 20ºC, hence their name, since they are made up of coralline algae. This means that at the time when these two types of rock were forming, the conditions of Tethys sea were very similar to those of the present day Great Barrier reef of Australia. Upper coralline limestone quarries are obviously found where the rock outcrops, that is, lies at the surface. Spalls and building sand is produced which is much required in the construction industry to produce, concrete, pre-fabricated roofing, bricks and tarmac. Lime is also produced from upper coralline limestone.

Greensand:
Below the upper coralline limestone we find the greensand layer, ‘rina’ or ‘ramli’, in Maltese. Greensand is soft, very grainy and porous rock almost totally made up of the broken shells of marine organisms. This sandy rock is not always found between the blue clay and the upper coralline limestone and often it is only about 1 metre thick. The thickest layer of such rock is situated on the slopes of Gelmus Hill in Gozo. In this layer one can find a large amount of fossils of creatures that inhabited the bottom of a shallow and sandy sea like starfish, urchins and molluscs.

Blue Clay:
Below the greensand layer one finds the layer of soft blue clay. On the practical and technical aspects, this thin layer is of utmost importance for Malta. While globigerina limestone and coralline limestone have provided the Maltese with building stone, blue clay helped the Maltese settle on the islands since clay is the only impermeable rock layer in Malta. Due to this, infiltrated rainwater settles inside the pores of the layers above thus forming the perched aquifer. Moreover at the blue clay outcrops, natural springs formed which provided a much needed ready source of water. Maltese clay has been worked since prehistoric times. Large amounts of locally-manufactured clay ornaments, statuettes, vases, plates, amphorae, basins and other objects were found in many prehistoric sites.

Globigerina Limestone:
The globigerina limestone layer forms the surface rock or outcrop of 70% of Malta’s territory and forms large plains. In some areas there are cliffs made of this rock. The rock is formed out of shells of marine creatures called foraminifera globigerina. This layer is between 23 and 207 metres thick. Globigerina limestone is used for building. This is soft stone that is easily cut and dressed but weathers well and hardens once exposed to the air. Most of the houses in Malta are built with lower globigerina limestone, tal-franka, in Maltese, The excavation of this type of rock in the quarries generates a lot of construction activity. Apart from quarrymen, haulage contractors, stone-masons, plasterers and tile-layers depend on quarrying for their work. Globigerina limestone is made up of silt and creatures which lived in the sea of Tethys between 20 and 8 million years ago. In this porous layer we find fossils of decorated or spiral shells. Remains of crocodiles, seals were found, as well as large teeth of large sharks that are now extinct.

Lower Coralline Limestone:
The lower coralline limestone is the oldest rock in Malta since it was the first to be formed at the bottom of the sea between 35 and 25 million years ago. Lower coralline limestone is found at the surface where the layers above it were eroded in time. Thus we find it on the coast at Baħar iċ-Ċagħak and at Xgħajra, as also at Ta’ Ċenċ and Fomm ir-riħ cliffs where it reaches a thickness of 140 metres. Lower coralline limestone is precisely semi-crystallised stone that is hard, heavy, very strong and almost impermeable. However can still seep through the many joints in the bed-rock. This layer consists of many fossils of algae and corals. In olden times coralline limestone was used to lay pavements, for stairs and for simple monuments. This rock can be polished. In this case it is then called Maltese marble. This is not really marble though, since marble is a metamorphic rock of which Malta possesses none. But since this is a hard and shiny stone, it is similar to marble. When Malta was ruled by the Knights of St John, many coralline slabs were used to lay the pavements in Valletta. In fact many of these slabs can still be seen especially in front of palaces and auberges built under the Knights’ direction. In coralline limestone quarries, the rock is excavated by means of dynamite. Machines will then break it up into smaller pieces which are then dropped into a crusher that grinds them into spalls or building sand. Spalls derived from lower coralline limestone is much sought for the production of pre-fabricated concrete roofing and for street tarmac, concrete and bricks.

As we have seen Malta is composed of five main layers of sedimentary rocks, all formed under the Sea of Tethys. Now make a copy of the diagram seen below according to the colours used.

• write down the name of the rock type next to each layer in the diagram;
• next to each layer write porous, permeable or impermeable;
• next to the impermeable layer write briefly the importance of this layer.

Which is the oldest layer of rock in the Maltese Islands? Why do you think so?

The layers of rock we find in Malta vary considerably from one another. Try to identify each layer of rock with the help of the given clues.
• grainy orange coloured rock
• the oldest layer of rock of the Maltese Islands
• the last layer to be formed under the sea
• porous but impermeable type of rock
• found on the surface of 70% of the Maltese Islands
• an impermeable layer over which water accumulates
• a layer of rock made up of coralline algae
• this forms barren rock surfaces full of hollows and cracks
• very soft rock found above the clay layer.

Look carefully at the image below and then answer the following questions.

• Which rock layer is shown in the image above?

• Using the geological map of Malta name some places in the Maltese Islands where this layer is found on the surface.

• Describe the main characteristics of this rock.

• Explain how this rock was formed.

Look carefully at the image below and then answer the following questions.

• What type of rock is shown in the picture above?

• What is the importance of this rock layer for the Maltese Islands?
• With the help of the geological map name some localities in Malta where this layer is found on the surface.
• The layer of rock seen in the picture above is (porous, permeable, impermeable). Choose the correct answers.

Look carefully at the picture below, then answer the questions.

• Which layer of rock is the large boulder made up of?

Say if the following sentences on the picture above are true or false.
• This rock breaks down easily.
• Large boulders from this layer are undercut from the cliff face and roll downwards.
• Impermeable layer that does not allow water to percolate downwards.
• Very hard limestone that does not erode easily.
• This rock was formed under the Sea of Tethys.
• A layer of rock abundant in fossils of sea creatures.
• This layer is found on the surface of large parts of the Maltese Islands.
Rewrite correctly the sentences that are false in the above exercise.

Look carefully at the image below and then answer the following questions.

• What do we call the layer of rock seen in the picture?
• Write a short paragraph to describe the main characteristis and uses of this rock layer.

Now you can complete the worksheet entitled Rocks of the Maltese Islands available in the resources section.