Saturday, August 24

The sheer mystical majesty of sea cliffs

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Study the photos and work out the landwards, seawards, and vertical strata dips of the imagined sedimentary rocks.

AS a youth, living on the rugged granite peninsula of West Cornwall, in Southwest England, I would regularly cycle several kilometres to simply sit on a cliff edge and admire the distant horizon and dream of travelling across the seas. As I developed an appetite for geomorphology and geology, with a particular leaning towards how rock compositions, structures and processes affected our landforms, so I was able to understand how these sea cliffs formed and are still forming. The past certainly provided the key to the present.

I was fortunate to have a grammar school education and to be taught by an outstandingly geography/geology teacher, who gave me the best academic advice and tuition that I have ever received. Now 91 years old, he still is very active. He is also a celebrated seascape artist of the world famous Newlyn School of Art. I recently purchased one of Bob Quixley’s coastal paintings of ‘The Crowns at Botallack’, depicting 18th and 19th century Cornish tin mining engine houses perched precariously on the granite cliff edges as the miners extracted tin and copper ores out under the Atlantic Ocean.

For many a year, on our family holidays in West Cornwall, we took our children onto these cliffs pondering the beauty of the sea beyond and recording the past history of tin mining. Several Malaysian family friends have joined me on the castellated granite cliffs at Botallack and at Land’s End and heard me recount the tale of how Malaysian alluvial tin, which was dredged from river beds, saw the collapse of underground Cornish tin mining in the late 19th century.

Around us in Sarawak, we can view equally beautiful and outstanding cliff structures, particularly from the plateau topping Bako National Park.

Sculpturing of cliff profiles

The forces of nature can be summarised as the action of waves as erosive agents, together with the rate and type of subaerial weathering and mass movement of unconsolidated material above the plane of wave attack.

  • Waves act as erosive agents in four main ways:
  • Hydraulic action is caused by the force of waves as they pound the cliff face. In stormy weather the pressure exerted on a cliff face amounts to several thousands of kilograms per square metre. Air becomes compressed within cracks in the rock (vertical joints and bedding planes) and when the wave retreats the compressed air expands and acts like a wedge being hammered into the cliff face. Ultimately, the natural cracks in the rock expand and water will break through on a large wave to gush out onto the ground surface above to form a blowhole or gloup. The latter is named after the resonant sound as the seawater gushes out onto the cliff top surface.
  • Corrosive action occurs usually at the base of a cliff as shingle and beach boulders are hurled against the cliff face. This lends to undercutting and the creation of wave cut notches between high and low tide marks. Such cliff overhangs present challenges to rock climbers. These corrosive notches can be seen at the base of the imposing sea stack as one walks up the beach towards Bako’s National Park headquarters.
  • As the cliffs eventually collapse due to undercutting, attrition action is witnessed as the beach materials are ground against each other by the swash and backwash of waves. Pebbles gradually become more rounded and some even have scratches or ‘chatter marks’ etched on them. These are caused by pebbles of more resistant material grinding against less resistant pebbles. Waves moving shingle up and down beaches make their own music hence; the term ‘chattering’.
  • Solution action. Carbon dioxide is more soluble at lower than at higher temperatures in seawater. Thus, in temperate latitudes solution action on limestone rocks is greater than in tropical regions. The effects of the waves’ spray solution can be seen in the ‘pock markings’ and ‘flutings’ on limestone cliffs worldwide.
  • It must be stressed that these four processes must not be treated in isolation when viewing a cliff face, for their relative effects will depend much upon the geological nature of the cliff itself.
  • Subaerial weathering of rocks takes many forms and can best be judged when looking at the profiles of cliffs from a distance. Many areas in Eurasia that experienced the continental ice sheets of the Ice Ages and, on their fringes, permafrost conditions created near vertical cliffs with a convex slope running up to the cliff top. In the granite cliffs of Cornwall and in Brittany, France, the French geomorphologist, Prof Andre Guilcher, recognised two elements of sea cliffs. The ‘true cliff’ exposed to direct wave action and running up to one third of the cliff’s total height and an upper ‘false cliff’, convex in shape and constituting two thirds of the total height. This, often grassed, slope was and still is subaerial in origin and mostly composed of periglacial melt sludge covering the true sea cliff below, as well demonstrated in the painting.

The gently convex and forested slopes at Damai and Lundu illustrate well how tropical weathering has contributed to the cliff profiles there.

Rates of erosion, shapes of coasts

Many interrelated factors will determine how fast cliffs and coastlines recede:

  • The exposure of a coast, by its very location, to wave action and to the power of the incoming waves in winds of various directions. Such factors to consider are the depth of the sea in the immediate vicinity of a cliff, for the deeper the water, the less frictional loss of energy to the seabed, with greater erosive force to batter the cliff. The distance of open water over which waves travel without loss of energy, commonly known as ‘the fetch’ of waves, will depend upon the prevalent wind directions. On West Cornwall’s headlands in westerly or south westerly storms huge Atlantic Ocean waves travel nearly 6,000km over open water from the Americas before pounding the granite cliffs.
  • The hardness, resistance and character of coastal rocks affect rates of erosion. The very presence of joints and bedding planes helps to accelerate the processes.
  • The structure and the angle of dip of the rock strata will ultimately determine the cliff profile. If the angle of dip of the bedding planes in sedimentary rocks is seawards, then there is a tendency for the rock to slip downwards, creating overhanging cliffs. Should the rock strata possess a landward dip, the cliff profile is less steep. A horizontal dip of folded rock strata leads to easy wave access with undercutting of the cliff face, thus creating wave cut notches between tidal heights. Vertical dipping of sedimentary rocks leads to near vertical cliff faces, which are pretty resistant to wave attack. Such rock structures are best seen in diagrammatic form. Interestingly, whilst sorting out a recent delivery of logs for my heating system, I came across one small log which exemplifies the angle of dip of rock strata on cliff shapes.
  • Study the three photos of these perceived cliff structures and work out the landwards, seawards, and vertical strata dips of the imagined sedimentary rocks. Alas, there are no prizes for the correct answers. Each sea cliff has a geological history resulting from the uplift of land by tectonic processes over millenniums.

Global warming

Throughout the world sea levels are rising and seasonal distributions of rainfall are out of sync. Cliff faces are crumbling, landslides in coastal areas are more frequent and settlements inundated with rising tides. The cost to human lives is vast. We may cry for the fortification of coastlines with sea defence structures but in some cases this is not a cost-effective option. Nature will take its course along many sections of coastlines but where human lives are in danger then large investments are needed in coastal protection measures.

The northern cliffs and former tin mining engine houses of West Cornwall – painted by Bob Quixley 1981 (Newlyn School of Art).

May all readers continue to enjoy family visits to the coasts of Sarawak and Sabah and do try to sit on a high cliff top away from the cliff-edge and marvel at the beauty of the marine scenery below. If walking along cliff backed beaches, look at the shapes of the shingle, wave cut platforms and their rock pools, caves and wave cut notches, but beware of potential rock-falls and landslides. Cliffs are forever crumbling as nature takes its course and are likely to crumble faster in the near future.