Seashells, seashells... in the bedrock

The Maltese islands are made up of different layers of sedimentary rock. In geological terms, the islands are very young. They were created long after dinosaurs became extinct and so it is impossible to find dinosaur fossils in any of the islands’ rocks.

The fossils found locally are mainly of marine organisms. The species of these organisms, however, change according to the layer in which they are found.Fossils are the preserved remains of plants and animals or their traces. They provide palaeontologists with important information about both the geology and biology of the earth. An object is considered a fossil if it is older than a certain minimum age, which for most palaeontologists is 10,000 years. The oldest fossils are about 3.4 billion years old.

The oldest layer, the lower coralline limestone, is made up mainly of reef-forming organisms such as calcareous algae and coral, where one is more likely to find bivalve seashells. At the very top of the lower coralline limestone layer is a shallow but very concentrated layer of large flat sea urchins.

The globigerina limestone layer is made up of the remains of planktonic organisms. Some parts of this layer are rich in fossils of free-swimming organisms such as scallops as well as bottom-living creatures such as sea urchins and crustaceans.

Blue clay was formed from a sediment that originated on land, probably from eroding landmasses to the north that were being uplifted as a result of tectonic action. Fossils in this layer are few, and are mainly molluscs and coral.

The penultimate layer, the greensand, is absent in many parts of Malta but forms a thin layer in many parts of Gozo. Greensand is characterised by its orange-brown colour (green before being exposed to air) and large amount of fossils.

The most obvious fossils belong to large sea urchins but fossilised fish have also been found in some places. The topmost layer, the upper coralline limestone, which was the last layer to form, is similar to the oldest layer, the lower coralline limestone.

This article was published in The Times on 3 April 2013.