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Stop 5 (450 m) Wooden bridge at the top of Castlebeach – SM 818 050
Next to the wooden bridge over the small stream are the ruins of a limekiln. As with most of Pembrokeshire, the soils of the Dale peninsula are acidic, and benefit from treatment with lime to improve the growth of crops.
Limestone and coal were brought in by sea and placed in layers in the kiln. Using the coal as fuel, the limestone (calcium carbonate) was burnt in the kiln for several days and allowed to cool. The whole process took about a week. The end product was quicklime (calcium oxide), used to neutralise the acid soil.
The last time this kiln was used was at the beginning of the 20th century. The coming of the railways and industrial scale production of lime made local lime-burning uneconomic.
Walk down to the beach. Castlebeach always seems to have more than its fair share of accumulated rubbish from ships – plastic containers, polystyrene, driftwood, nets, bottles and the like. It is a peculiarity of the tides and ocean currents that cause so much pollutant debris to end up stranded on this beach.
The rock making up the cliffs either side of the beach is known as Old Red Sandstone (ORS). ORS is a sedimentary rock. The sediments that formed ORS were produced in hot, dry desert conditions and deposited in coastal plains and river deltas during the early Devonian period, about 400 million years ago.
The red colour of ORS is due to iron oxide coating the quartz crystals of the rock. This oxide is similar in composition to rust, and dissolves in seawater. If you compare the cliffs with the sand on the beach, you can see what happens when the rock is eroded and weathered by the sea – the iron oxide dissolves and the quartz sand grains left behind are the more familiar sandy colour. This tells us that unlike most sedimentary rocks, ORS can’t have been laid down in a marine environment.
Geologists call the layers of sedimentary rock ‘strata’. When they were first deposited they would have been horizontal, but during the Earth’s history they have been folded by movement of tectonic plates in the Earth’s crust. So a section through the rocks shows the strata lying at an angle. This is called the angle of dip. The ORS strata of the cliffs dip to the south at about 70°. This is easier to see if you take a look back at the cliffs of Dale Point.
The rocks of the cliff walls are covered by living organisms. These form distinct horizontal bands. At the top of the cliff is a green band made up of land plants such as grasses and sea pinks. These can only survive above the splash zone, where they are not often subjected to splashing by salt water. Below this is a band containing pale grey and orange lichens. These organisms can survive splashing with the salt water, but not prolonged immersion in the water. Next is a black band, made up of a lichen species that is better at surviving longer periods under water.
Below the black lichen we start to see the truly marine organisms. On Castlebeach there is a thin layer of yellow-brown seaweeds, mainly on the sheltered sides of the rocks, and below this a grey layer of barnacles and limpets.
Biologists call the development of these bands of organisms ‘zonation’. Further up the cliff, the organisms will have to spend less time immersed in seawater as the tides go in and out. Each is adapted to a different period of immersion, and so they grow in bands called zones. Again, if you look across the bay towards Dale Point you can see the zonation on the rocks there.
Walk back to the bridge and continue along the coast path. The walk takes you uphill and through a couple of gates across the path.