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The Lake District Sinks Beneath the Waves After the Caledonian mountains had been worn down by erosion, in the Carboniferous period about 350 million years ago, northern England was flooded by a warm tropical sea. This shallow sea was rich in life, including corals, sea lilies, the early ancestors of squid, and shellfish superficially similar to cockles and mussels, together with early fish and even sharks. With no high land nearby to provide other sediment, limestone accumulated in shallow water from material secreted by algae and from shell fragments, in much the same way as it does on the Bahamas Banks today. Many millions of years later in the Carboniferous period, the seas began to retreat and rivers brought in sediment to build up deltas on top of which lush forests grew in swamplands. The thick vegetation was buried and compacted to form coal, which was mined for many years in the Whitehaven area. Although the Carboniferous limestones do not occur in the central Lake District, they are widespread around the flanks of the region and were originally deposited across the entire region. Uplift of the heart of the Lake District meant that they have now been removed by erosion from above the rocks that we see out of the window today. Limestone is extensively quarried around Cumbria for use in cement and aggregates, and we have found that some limestones ring well and can be used in instruments. If you look at the polished surfaces of some of the keys you will be able to see fossils of the corals that lived in that long past tropical sea. Near Barrow-inFurness, some limestone beds became replaced by iron oxides after they had been buried, and it was mining of this ore for iron that led to the development of steel and related industries around Barrow. Limestones almost always develop where there is warm, clear water with little sand or mud being brought in, as this would swamp the supply of calcium carbonate from organisms. Shallow seas are also important because of the greater biological productivity near the surface. Image courtesy of Ove Hoegh-Guldberg, Centre for Marine Studies, University of Queensland Just as today, the tropical seas in which the Carboniferous limestones were laid down were home to a wide variety of species, including many that produced shells. Image courtesy of Ove Hoegh-Guldberg, Centre for Marine Studies, University of Queensland Modern day mangrove swamps give an excellent impression of how northern England would have appeared later in the Carboniferous when coal beds began to form. In detail the plants are quite different, but the thick, tangled vegetation and matted root systems sometimes invaded by the sea are probably very similar. Image courtesy of Rob Mackay-Wood, Centre for Marine Studies, University of Queensland Funded by: Ruskin Rocks Team and School of Earth and Environment, University of Leeds A collaborative project led by the University of Leeds Further information at www.ruskinrocks.org.uk