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Cliff/Shore Platform Complex between Portstewart and PortrushAntrim, Londonderry
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Summary Full report
Site Type: Cliff, Coastal section
Site Status: PASSI
District: Coleraine Borough Council
Grid Reference: C815389, C85003995
Rock Age: Quaternary (Holocene)
Rock Type: Basalt
Other interest: No data, Coastal processes, boulder beach, cliff, shore platform

Summary of site:

The cliff coastline between Portstewart in the west and Portrush in the east is a complex of active cliffs, fronted and often flanked by erosion platforms and their residual facets. Bays between headlands show typical features with sediment fills backed by degraded cliffs and accumulations of debris at their feet. This loose material is known to produce local landslips in suitable conditions. The particular form taken by this rocky coastline is strongly controlled by the local geology.

With the exception of a few dykes and pockets of explosively dismembered volcanic debris, this entire coastline consists of lava flows belonging to the final gigantic outpouring known as the Upper Basalts. This eruptive activity finally ceased about 58 million years ago in the Palaeocene, the first epoch of the Cainozoic (the era that extends to the present day). It left behind stacked basalt lava flows derived from very local centres. There are many planar structures in these rocks, particularly the tops/bases of lava flows, near horizontal but often strongly undulating and a variety of joints (natural fractures with no obvious displacement), most caused by contraction of the basalt lavas as they cooled and shrank. The cliffs are fronted by shore platforms and both cliffs and platforms are seen to be active. Cliff heights vary between 9m and 23m above sea level and average 13m. Cliffs in bays are normally inactive with vegetated debris banked against them at angles up to 35 from the horizontal. In contrast, the cliffs on the headlands are vertical, or nearly so, and display complex networks of fractures. Shore platforms cut into the rock flanks of the headlands often extend for some distance around their sides. On the larger scale these platforms are planar but at close quarters they are usually rough and rugged. The platforms appear to have no consistent level and their surfaces undulate. The junction of the cliff with its fringing platform almost always coincides with the junction of two lava flows and in places a notch at this level undercuts the cliff. The instability leads to collapse and sometimes the debris accumulates as ramparts that may lie on the platform or ramp against its flanks. More usually these surfaces are swept clean by high energy waves. On their seaward side the platforms normally end abruptly at cliffs pitching into deep water. The platforms are also cut by steep-sided channels and in some places they intersect, isolating some facets. Pot holes formed by rocks eddying in the beds of channels are almost a characteristic of them. Viewed from above, the coastal features are somewhat irregular in form, although the orientation of the platforms has a north-north-west trend, in line with the dominant regional joint sets caused by crustal stresses and superimposed on the more local cooling cracks. As would be expected on an exposed coast, there are many storm beaches (accumulations of cobbles and boulders thrown back and well above the reach of normal wave activity). Many front the degraded cliffs at the backs of bays. A number of factors have conspired to create this coastline, with its prominent platforms. The main ones are horizontal lava flows with extensive developments of inter-flow junctions a storm wave environment a direction of fetch (open water over which wave generating winds blow) that coincides with its maximum length, here 3,000km a narrow tidal range of just 1.5m, concentrating wave energy into a confined zone a network of contraction and tectonic joints that act as planes of weakness in the face of battering waves. The action of storm waves on joints and lava flow junctions is the most important erosive force on this coast. The platforms exploit the near horizontal flow junctions and zones of heavily jointed rocks which coincide with these levels and their undulations are simple reflections of the irregular lava flow surfaces. The tectonic (large scale regional) jointing sets the orientation of the headlands, bays and to some degree the gullying of the platforms and their fringe facets. Storms can wreak enormous damage, detaching joint-defined blocks, pulverizing or rounding them against the platforms, cliffs and each other. This process provides a steady supply of sediment and a powerful system for sweeping it from exposed headlands and carrying it along the coast. Longshore drift of sediment here is east to west. Raised beaches, created by the coastal erosion of rocks later elevated by the bounce-back of land following the melting of thick regional ice sheets, are common along this heavily glaciated coastline. This has led many authors to seek evidence for them along this stretch of coast and features have been described in the following ranges above modern sea level 4-6m; 7-8m; 8-9m; 9-11m; 10m and 13m. The shore platforms in the area tend to fall below these ranges, most lying only 1-1.5m above sea level. There is a debate about whether such a spread of ranges could represent earlier, elevated post-glacial shorelines. It is known that modern severe storms can reach well above mean sea level and many, if not most, of these features coincide with structural planes in the lava flows. The lack of cliff debris is clear evidence of storm violence and a persistent high energy environment. In these conditions excavation of junctions and joints in the flows can be rapidly extended, making many of the claims dubious. There are some indisputable examples of post glacial shore levels, as at Port Gallen where degraded cliffs stand on a vegetated platform whose outer rim is being carved by the active, modern cliff, but they are few. This interesting coastline not only provides fine examples of modern marine processes but also presents many problem platform levels that could possibly be remnants of ancient post glacial erosion features. More research on marine processes in this kind of geological setting could provide some explanations of such phenomena.

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