An unstable landscape
Consideration of the cliffs within the World Heritage Site highlights the fact that this is a complex and constantly changing landscape characterised by a range of microenvironments that support a diverse flora and fauna. The clearest expression of this activity is the almost constant occurrence of slope failures that range from shallow, translational flows to large scale rotational landslides and block falls ranging from individual boulders to major failures measured in hundreds of tons. The final trigger for these failures is often a period of prolonged and/or intense rainfall, but underlying factors include the undermining of cliffs by marine erosion and human excavations and the gradual weathering and weakening of the geology. Whilst posing a potential risk to visitors, these failures are themselves an essential component of the site.
Complex debris fall at Port Noffer (summer 2008). Failures can still threaten the path network.
In general terms, active basal erosion of bedrock produces vertical or near-vertical cliffs. Efficient removal of debris, but limited erosion of bedrock, allows slope form to adjust to underlying geological structure and produces stepped cliffs. Impeded basal removal results in the accumulation of scree that begins to mask the cliffs from which the debris derives. Ultimately, as a scree grows, its source area for new debris is diminished and it becomes less active. This combination of coastal morphology and slope processes has resulted in two broad slope classes – those found on headlands and those found in the bays – that in turn largely dictate geomorphological activity and hazard in terms of landslide and rock fall probability.
Cliffs and Debris Slopes in Bays
Within bays the slope foot zone typically contains an extensive wave-cut platform mantled by large boulders that effectively dissipate wave energy. In places, groundwater in the form of springs and marshes flows on to the platform and this has created areas of considerable ecological interest. The presence of this platform has also allowed debris to accumulate from weathering of the cliffs and extensive scree slopes to form. Many scree slopes are now vegetated and may have formed during earlier, more active periods of weathering and cliff retreat during the early Holocene (post-glacial) Period. The scree is mainly angular basalt debris (>50cm), but when stabilised this is covered by approximately 50cm of angular cobbles and gravel set in an organic rich, sandy clay matrix, strongly bound together by a dense root mat.
Slope failures within the scree are often associated with poor drainage, and undercutting of slopes by the road and footpaths. These shallow failures are particularly common in Port Noffer. Although conditioned by undercutting and the natural failure plane between topsoil and scree, failures are triggered by periods of prolonged and/or intense rainfall. Above the scree are vertical or stepped cliffs which regularly shed debris either as angular blocks or as toppled basalt columns. Often, the fall of an individual block or column triggers the release of surrounding material and a considerable area of cliff may ultimately collapse. There is therefore a regular 'leakage' of debris from cliffs, interspersed with occasional larger falls.
This overall pattern of erosion is widespread along the coast and at any one time it is possible to see a number of active and vegetated scars from falls and slides. These indicate long-term slope instability and suggest that these failures can occur without the necessity for human intervention.
Most failures in the western part of Port Noffer do not take place because of human activity, but because of excessive moisture within the debris mantle, usually along so called ‘percolines’ or ‘seepage zone’. Till overloading and flowing from top of the cliff infills also the basalt joints and can lead to the occurrence of rockfalls, as it happens at the western extreme of Port Noffer, on the right of the photo.
Cliffs and Debris Slopes on Headlands
Basal erosion and removal of cliff foot debris are efficient and effective in these areas, in response to concentrated wave attack. Distinctions can, however, be made between the ends of headlands and their sides. Once initiated, headlands create wave refraction that concentrates wave energy along their flanks. The most active erosion and steepest cliffs are therefore not at the ends of the headlands, but on their sides where they join the scree of the intervening bays. The effect of concentrated basal erosion is to diminish geological control on slope form. In particular, the gentler slopes associated with the Inter-Basaltic Bed are steepened and in places disappear altogether to leave a vertical cliff. All other factors being equal, major collapses are therefore most likely to occur on the sides of headlands. This is particularly the case where the Inter-Basaltic Bed has been excavated in the past to accommodate the lower cliff path. There is, however, evidence that this is a long-term pattern of erosion that is not dependent on human intervention. For example, along the coast it is possible to see natural arches, where basal erosion has cut through headlands, and numerous remnants of previous headlands in the form of sea stacks.
An example of rotational failure on basalts near Hamilton’s Seat. Path undercutting on Inter – Basaltic layer led to failure of the columns of the Middle Basalts, that took with them part of the underlying palaeosol and Lower Basalt.
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