Samuel's Port

Also within

Down County Mourne Scenic Area

Climate exposure

This is about exposure to a nationwide pressure this habitat type carries — not a verdict on this site's own management.

Habitats

Coastal vegetated shingle

Geology & landscape

Geological Feature: Igneous Intrusion · Rock Type: Basalt, Gabbro, Granite

Landscape processes: Coastal, Volcanic / Igneous

Description

Samuel's Port Igneous - Tertiary The area is of special scientific interest because of its geology and intertidal habitats. Samuel’s Port is of significant geological importance. A dyke records multiple intrusions of magma with each intrusion of a different composition. The wide inner portion of the dyke is of acidic com position and it shows basification against the earlier marginal basalt intrusion. There is also a later tholeiitic member of the dyke that carries xenoliths of gabbro and granite. This dyke is a fine example of the intrusion of both acid and basic magma and the variable com position of the dyke provides inform ation on the formation of the Mourne Mountains. The main dyke, classed as dyke No.124 in an earlier survey of the Mourne Coast dyke swarm, is well exposed across the beach and is approximately 14m wide. From northeast to southwest the dyke is composed of approximately 30cm basalt, 3m acid-feldspar porphyry, 30cm basalt, 1.5m tholeiite, and 1.5m olivine dolerite. The order of emplacement has been determined as oldest to youngest olivine dolerite, basalt, an intrusion of the feldspar-porphyry into the ba salt with slight marginal assimilation, and finally a tholeiite intrusion between the basalt and olivin e-dolerite dykes with a well marked chilled margin against both. The basalt and olivine dolerite are both dark grey, however the dolerite contains phenocrysts of olivine. The porphyry is reddish in colour and characterised by phenocrysts of anorthoclase and other alkali feldspars; occasional small phenocrysts of hornblende occur but quartz is conspicuous by its absence. The tholeitte is dark grey, fine grained and contains sparse xenoliths of granite and gabbro. The gabbro xenoliths are unique with coarse-grained fragments up to 13 cm in size. Just north of this composite multiple dyke occurs a felsite dyke, classed as dyke No.120, with an anomalous northerly trend and well developed flow banded margins. The composite dyke indicates the intrusion of two contrasting magma compositions; acid and basic, and their mixing to produce rock of intermediate composition. In this respect it mirrors the magmatic activity which took place during the emplacement of the cone-sheet found elsewhere on the Mournes Coast. The tholeiite member within the composite dyke is the only recorded dyke in the whole Mourne dyke swarm to bring up from depth fragments of gabbro. The presence of these inclusions supports the proposition that a major mafic igneous body is present under the granites of the northern part of Samuel’s Port ASSI, around Wreck Port and Russell’s Point, has important intertidal rock communities. It is a good representative example of both a moderately exposed shore and a boulder and bedrock shore on the South Down Coast. In the middle shore, the boulder, cobble and stone shore holds rockpools with a high diversity of species more often found in the lower shore and sublittoral. At Wreck Port there is a wide area of coarse sand, pebbles, gravel and patches of seaweed at the top of the shore, above a boulder shore with wide areas of flat bedrock outcrops and ridges, interspersed with frequent rockpools. Large numbers of sea hares Aplysia punctata were noted, mostly in areas of seawater runoff between boulders. The beach is backed by artificial boulder sea defences with a slip running to the shore. A small amount of annual vegetation associated with driftlines is found here. The upper shore has a main area of bedrock with a boulder area to the south-west. There is a narrow sandy/shingle bay in the middle of the rock ridges. The fringe of creviced bedrock on the upper shore contains many rockpools. On the ridges, common limpet Patella vulgata, flat top shell Gibbulaum bilicalis and acorn barnacles Semibalanus balanoides are abundant on areas of bare rock, with abundant Osmundea, a red seaweed, in crevices. The thick top shell Phorcus lineatus is abundant, common periwinkle Littorina littorea is abundant in patches and juvenile mussels Mytilus edulis are found in crevices. The periwinkle Littorina fabalis is also found here. In the upper shore rock pools, there is abundant coral weed Corallina officinalis and green seaweeds such as Cladophora rupestris and species of Ulva are common. Spiral wrack Fucus spiralis was recorded as frequent in patches. The beadlet anemone Actinia equina, carrageen Chondrus crispus and sea lettuce Ulva lactuca were recorded as occasional. Aplysia punctata a sea hare was recorded in this area as well as on the lower shore. Common starfish Asterias rubens, juvenile mussels Mytilus edulis and annelid worms are abundant. The flat top shell Gibbulaum bilicalisis common. The extensive boulder middle shore has stones, cobbles and big boulders on sand. The boulders are very thickly covered with acorn barnacles Semibalanus balanoides, with abundant common limpet Patella vulgata and dog whelk Nucella lapillus. There are small stands of bladder wrack Fucus vesiculosus and occasional toothed wrack Fucus serratus between boulders. The snakelocks anemone Anemonia viridis, beadlet anemone Actinia equina, coral weed Corallina officinalis and some amphipods were recorded here as frequent, while the small brittle star Amphipholis squamata, carrageen Chondrus crispus, bladder wrack Fucus vesiculosus, grey top shell Steromphala cineraria and the common periwinkle Littorina littorea are occasional. The lugworm Arenicola marina is found in the sand between boulders. There are som e rockpools in the ridges deep enough to sustain oarweed Laminaria digitata, dabberlocks Alaria esculenta and toothed wrack Fucus serratus. On the lower shore, the substrate consists of large boulders in sand. This area is dominated by toothed wrack Fucus serratus. Juvenile mussels Mytilus edulis, acorn barnacles Semibalanus balanoides are a bundant and the common limpet Patella vulgata is common. Lithothamnion a type of maerl and false Irish moss Mastocarpus stellatus are frequent. Coral weed Corallina officinalis, the red seaweed Osmundea, dulse Palmaria palmata, sugar kelp Saccharina latissima, sea lettuce Ulva lactuca, other Ulva species, a hydroid Dynamena pumila, the grey top shell Steromphala cineraria and a sponge Halichondria panacea were all recorded as occasional. Dabberlocks Alaria esculenta, oarweed Laminaria digitata, a red seaweed Lomentaria articulata, the sea hare Aplysia punctata, lugworm Arenicola marina, common starfish Asterias rubens, a periwinkle Littorina fabalis, common periwinkle Littorina littorea and the dog whelk Nucella lapillus were also found on the low shore. This area is very similar to that of the sublittoral fringe, but there are more barnacles on the rocks and there is no Laminaria and little Mastocarpus. The sublittoral fringe consists of large boulders embedded in the sand, with occasional bare patches of sand in between. Sand scour was noted at the base of large boulders. On the boulders, common limpet Patella vulgata and acorn barnacles Semibalanus balanoides are recorded as frequent. Around the boulders, false Irish moss Mastocarpus stellatus and the maerl Lithothamnion are abundant and toothed wrack Fucus serratus and oarweed Laminaria digitata are common. Sugar kelp Saccharina latissima, common starfish Asterias rubens, a hydroid Dynamena pumila and a sea mat Electra pilosa are frequent. Carrageen Chondrus crispus, coral weed Corallina officinalis, Laminaria hyperborean cuvie or tangle, dulse Palmaria palmata, two red seaweeds Rhodothamniella floridula and Phycodrys rubens, a sponge Halichondria panacea, a tubeworm Spirobranchus triqueter and Alcyonidium a bryozoan are recorded as occasional. Red rags Dilsea carnosa and Hymeniacidon perleve, a sponge, were also found in this zone. The pallid chink shell Lacuna pallidula, banded chink shell Lacuna vincta, blue rayed limpet Helcion pellucidum and the micromolluscs Rissoa interrupta and Rissoa parva were also found on the shore. At Russell’s Point the intertidal zone is also a bedrock shore dominated by Patellobarnacle rock ridges. There are many rock pools dominated by ephemeral algae on top of the rock ridges. Good diversity of red algae was noted here, as well as a range of bryozoans, crustaceans, echinoderms, insects, jellyfish, anemones, molluscs, ribbon worms, sponges, true worms, brown algae, green algae and lichens. https://www.daera-ni.gov.uk/publications/samuels-port-assi

Species mentioned

Plant communities