In their report for Environment and Heritage Service on slope failures at the Giant’s Causeway, McDonnell and Smith (2000) drew attention to two areas within the site that have particular geomorphological significance: the ‘Amphitheatre’ and Port Noffer. Moreover, the present report has focused its attention on these two sites, plus Port Ganny and Portnaboe.
The ‘Amphitheatre’, together with the cliffs above Lacada Point, has consistently been identified as the most active and hazardous cliff section within the World Heritage Site. The headland is especially prone to the toppling of basalt columns and deep-seated rotational failures through the Inter-Basaltic Bed that runs along the cliff at mid-height. Within the Amphitheatre is a superb example of an active scree. This provides an accessible opportunity for the general public and educational visitors to view and understand the dynamic nature of the site, the processes responsible for shaping it and the direct link between slope failures and underlying geological controls.
The Amphitheatre from the top of cliff, looking West. Alternation of well developed basaltic columns and interlayered bols and palaeosols (a bol layer can be followed near the upper part of the cliff, and palaeosol is on the right side of the photo) have triggered major failures that turned into complex debris falls, one of which can be seen in the centre of the panorama. Such complex debris falls keep the lower scree active.
Port Noffer is the location for numerous shallow translational slides and flows above and within vegetated scree around most of the embayment. In addition, there are regular block falls from the cliffs and well-marked rotational failures and slides associated with the Inter-Basaltic Bed below Roveran Valley Head. The shallow flows, in particular, are valuable for maintaining biodiversity and the variety of active and recent slope failures provide an excellent opportunity to explain the ongoing development of the site to the ordinary visitor.
Many of the slope failures across the site can be linked to springs or, more commonly, seepage lines (so-called ‘percolines’). These percolines also produce marshy areas, for example, in Portnaboe and Port Noffer, which contribute significantly to the overall biodiversity of the site.
Panoramic view to the East of Port Noffer from Aird Snout. Shallow debris flows can be seen, due to moisture accumulation within the debris mantled slopes of the embayment. Rockfalls, as well as complex debris falls, have lead to the formation of screes near the eastern headland. Some marshes, fed by the ‘percolines’ have developed over the old beach near the sea shore.
This site is characterised, besides of being the place where Giant’s Causeway is placed, for having a wide cover of dormant, partially vegetated screes. These screes where fed by major topples and block releases, and they are actives no more, except those located on the eastern part of the site. Between these screes and the sea shore lays an old beach, with some marshy areas, which have a high environmental value. However storm surges together with high tides can nowadays overflow the old platforms, and are actually threating the Causeway Road and dormant screes by undercutting them.
Panoramic view to the West of Port Ganny from Aird Snout. Dormant screes, old platform and beach, and Causeway Road can be seen. On the right of the photo it can be distinguished a traslational landslide at Great Stookan, that happened in summer 2008.
Although being one of the least spectacular bays within the Giant’s Causeway site, Portnaboe is an extremely interesting place for Geomorphology, landscape and landslide assessment. Due to the till mantle that overlays all the western and central part, Pornaboe is a prone area for mudflows, translational landslides and slope creeping. Moreover the marine undercutting that is taking place on the western extreme of the bay, at the small abandoned jetty, has created a very unstable slope. On the eastern part of Portnaboe the undercutting created by the Causeway Road on the slope profile has also lead to the happening of a wide range of slope failures. The wall built to stop them is only a temporal solution.
Eastern part of Portnaboe. The Causeway Road cuts the slope profile, triggering a continuous instability along the cliff.
The dynamic nature of the site, and the importance of natural erosion processes in maintaining its character have been identified as a key element in the WHS Management Plan.
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