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I sit outside on the balcony here watching life go by, I look at the houses up and down the street and they're all made of limestone. Gozo and Malta are made of the stuff, thrust up from the seabed in some violent and ancient geotectonic movement. They carve it into blocks, stack them up and then live in them. There's Coralline limestone, which as the name suggests is made of coral, and there's Globigerina limestone, which is the crushed and compacted shells of innumerable minute deep-sea organisms. Sedimentary calcium carbonate, CaCO3, 43 million tonnes of it is produced annually, all by the life and death of these organisms.
Some of it here in Gozo is no good to build with, it still has whole shells in it. I wondered how some of the limestone is smooth and some has whole bits of mollusc shell still in it. Also, I mused, coral only grows in shallow water, whilst Globigerina mainly live on the sea bed, so how can we have limestone made of both. It turns out Malta and Gozo have roughly four layers of visible rock. The top layer I stand on is the Upper Coralline, then a layer of Blue Clay, then Globigerina Limestone, then the Lower Coralline at the bottom.
Some of it here in Gozo is no good to build with, it still has whole shells in it. I wondered how some of the limestone is smooth and some has whole bits of mollusc shell still in it. Also, I mused, coral only grows in shallow water, whilst Globigerina mainly live on the sea bed, so how can we have limestone made of both. It turns out Malta and Gozo have roughly four layers of visible rock. The top layer I stand on is the Upper Coralline, then a layer of Blue Clay, then Globigerina Limestone, then the Lower Coralline at the bottom.
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This Lower Coralline layer is hard, grey and compact, resistant enough to erosion that it forms the bulk of the sheer cliffs rising vertically from the sea. The diversity and jumble of shells, corals and skeletal remains, burrowing sea urchins and calcareous algae, molluscs and brachiopods, indicate this was laid down in shallow waters where the sun could reach the sea floor where these creatures lived.
Above that is the Globigerina layer, which is much more homogenous than the Lower Coralline, so at a depth where waves couldn't have agitated the sediment, and entirely made up of deep-sea organisms. 34 million years ago the sea floor here sunk 600 meters in some catastrophic event, turning a shallow sea into a much deeper one in which these
Globigerina organisms could live.
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Next is the Blue Clay layer, a mixture of the remains of deep-sea organisms as above, plus clay particles, which can only have come from a land source. It's thought that they were deposited after erosion from mountains being uplifted in Sicily at the time, and volcanic ash from the intensely tectonically active rift, trough and fault system between Sicily and Tunisia.
Finally, at the top, the Upper Coralline. Formed in much the same way as the Lower Coralline, in a shallow sea, agitated by waves, and again full of mollusc shells, fossil algae coral, the spheres of sea-urchins as well as shark teeth and whale bones.
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After all this sinking and uplift, volcanic deposition and tectonic movement, the Maltese Islands were uplifted again in the Late Miocene Period, some 5-7 million years ago, and have remained above sea-level ever since. So now I understand why some of the stone is smooth, and some is full of shells.