The coastal scenery of the World Heritage Site owes much to the presence of numerous north-northwest trending olivine and tholeiitic dykes that cut vertically through all three geological units and may once have acted as feeders for later volcanic activity. The most prominent example of these runs as a dark slab of rock through Roveran Valley Head, but others include the Camel Dyke in Portnaboe, two dykes that separate and define the three causeways and numerous others that can be seen running across the foreshore in areas such as Port Noffer. The variable resistance to erosion along the coastline imparted by these dykes has contributed significantly to the formation of the bays, headlands and spectacular marine erosion features that both characterise the area and drew praise in the original IUCN technical evaluation of the site.
The Camel. A doleritic dyke at Portnaboe.
Included within this landscape are a number of key features, including the impressive columns of The Organ in Port Noffer and The Harp above Lacada Point, the free-standing columns of Chimney Tops, the almost circular bay of The Amphitheatre, and the viewpoint of Hamilton’s Seat from which the basalts were first accurately described in 1786 by the Rev. W. Hamilton.
Chimney Tops: Relict basaltic columns from a former cliff edge.
As indicated by Carter (1991), much of the coastal scenery dates from the end of the last glaciation (around 25-17, 000 years ago) when at various times ice would have covered and surrounded the cliffs, with the North Channel filled with pack ice. Ice sculptured forms were later modified by periglacial processes, especially frost action along exposed cliffs , to produce angular debris that accumulated as scree within a series of embayed ‘amphitheatres’ cut into the lower and Middle Basalts. To some extent these processes continue today through active marine erosion and ongoing collapse of the cliffs that ranges from the toppling of individual columns to major landslides. The pattern of slope failures is, however, sporadic both in space and time and owes much to three key factors.
Firstly, the morphology of the coast that determines both the effectiveness of bedrock erosion by concentrated wave attack and the rapidity and efficiency of basal debris removal by wave action. Secondly, the position of the Inter-Basaltic Bed that for the most part runs along the coast at mid-cliff height, and provides a natural zone of structural weakness and moisture concentration through the ponding of groundwater that percolates down through the well-jointed Middle Basalt. Lastly, the detailed characteristics of slope failures are determined by the joint and fracture characteristics of the basalt lava flows. East of the Causeway, in the area of Dunseverick there is evidence of a higher sea level between about 5-6,000 years ago (Carter 1982). Within coastal embayments this takes the form of a wave cut platform, behind which are relict caves, stacks and arches. As sea level fell, collapse of the former sea cliffs has in places covered the platform in scree.
Port Noffer: Giant’s Boot, a wave modelled rock nowadays situated in an unactive boulder beach. Former marine platform and an ancient stack can be seen at the background.
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