Destructive Coastlines
4.3 Cliffs and Shore Platforms
Cliff coasts with their striking façades and adja-cent platforms often designated as geoheritage features as they are amongst the most valued
natu-ral landscapes and they may have great symbolic value for the people. There are many poems and songs that allude to the mystery and romance of waves crashing against rocky cliffs: for example the iconic White Cliffs of Dover of Britain, facing the Strait of Dover and France, composed of soft, white chalk, have inspired movies from James Bond to Robin Hood, poetry to express English exiles’ homesickness as they were the last glimpse
of England while crossing the English channel to Europe’s mainland, and diverse music featuring reggae or rock.
However, geologists and geomorphologists are still puzzling over the details of the evolution and genetic origin of cliffs. The debate mainly circles around the relative roles of mechanical wave abra-sion and subaerial weathering processes such as the above mentioned water layer weathering in
Fig. 4.21 Quarrying by wave action on the weathered rock platform of the “Tessellated Pavement” of SE Tasmania. (Photo credit:
A. Scheffers).
Fig. 4.20 A horizontal intertidal rock platform at the “Tessellated Pavement” in SE Tasmania, Australia, as a result of water layer or salt weathering. (Photo credit: A. Scheffers).
the development of shore platforms. We here use the term shore platforms as it is most widely used by coastal scientists and has no reference to their genetic origin. Other synonymous terms include abrasion or intertidal platform, wave-cut or surf-cut terrace, schorre or simply rock bench. Shore platforms are horizontal or gently sloping surfaces cutting across geologic structures and rock for-mations and are backed at their landward exten-sion by a cliff (Figs. 4.23–4.26). Cliff and shore platforms normally occur together, however some cliffs plunge directly into deeper water. These
develop where the postglacial sea level rise was Fig. 4.22 Basalt columns cut horizontally by wave action in Victoria, Australia. (Photo credit: A. Scheffers).
Fig. 4.23 Three examples of shore platforms developed in hard metamorphic rocks in the St. Lawrence River, Quebec, Canada. These were cut by floating ice and debris loaded waves. (Photo credit: D. Kelletat).
much greater than the rate of sediment accumula-tion at the foot of the cliff (Figs. 4.27–4.32).
The development of cliffs and shore platforms and their morphological characteristics depend on the underlying geology, the type of rock forma-tions (rock strength) occurring along the coast, the overall wave climate and energy and other oceanographic factors such as tidal ranges. As the sea pounds rocky shores, cliff faces and shore platforms face the main force of mechanical wave erosion This includes the compression of air in joints and fissures in the rock. Air compression
results from the sudden inrush of water into a joint or cavity and causes an explosive shock through the sudden release of compressed air. This process is very effective even during fair weather wave conditions since water depth determines the posi-tion of breaking waves relative to the shoreline.
Larger storm waves break in deeper water further away from the shore and shoal across the platform
Fig. 4.24 Inclined rock strata on a shore platform on the west coast of Ireland. (Photo credit: D. Kelletat)
Fig. 4.25a,b Shore platforms exposed during low water along the macro-tidal coastline of western France. (Photo credit:
© Google Earth 2010).
4.25b 4.25a
0 1km
North Ronaldsay Orkneys
Shetlands
rock platforms sandy beaches shingle boulders
Isle North Ronaldsay exhibits numerous cliffs and inclining abrasional rock platforms
© 2011 Hans van der Baan / Ingeborg Scheffers
surface rather than directly impacting against the cliffs. Associated with mechanical wave erosion is the process of abrasion, which is the scouring and wearing away under wave action or tidal cur-rents of the underlying bedrock by the movement of sediments ranging in size from silt to boulders.
If sediments generated by abrasion are not trans-ported away by currents and tides, they may accu-mulate on the shore platform and further erosion will cease. In higher latitudes ice grounding by wind and waves is also an important process in platform development.
Cliffs show a vast catalogue of accompanying features, which often exist only for moments in geologic time (Figs 4.27–4.47): Small bays, nar-row inlets (geos), caves, arches or stacks are usual-ly formed by erosion along structural weaknesses in the rock formation. At the cliff foot within the
reach of surf, notches with smooth contours are abraded and polished by sediment loaded waves.
The deepest part of the notch often is situated a lit-tle below the mean high tide level. The occurrence and morphology of cliff caves, tunnels and arch-es is often controlled by weaknarch-essarch-es of the rock due to joints, fractures, faults or other geologic inheritances such as unconformities or changes in sedimentation patterns. It is thought that arches develop through the coalescence of caves eroded in the opposite sides of headlands. Spectacular coastal landforms are sea stacks, blocks of ero-sion-resistant rock isolated from the land by sea.
Their existence begin as part of a headland or sea cliff, however over time relentless pounding by waves erodes the softer, weaker parts of a rock first, leaving harder, more resistant rocks as sea stacks isolated from the coast.
Historical accounts (spanning centuries in cer-tain regions) or old photographs suggest that the average life span of sea stacks or natural bridges from their initial separation to their eventual col-lapse is somewhere between 100–250yrs. Overall, the erosion rate of sea cliffs is rather episodic and site-specific. Reported rates vary from virtually nothing to up to 100m/year depending on the rock strength in regard to resistance to erosion and the frequency of waves exceeding the mini-mum height capable of erosion (Sunamura, 1992).
The persistence of some cliff coasts is well docu-mented by the existence of ancient coastal forts from Neolithic to Iron Age times or Medieval castles in many regions of the world that have withstood wave action for centuries due to the resistant rocks on which theywere built. Cliff pro-files are strongly influenced by the type of rock (harder versus softerr), structural weaknesses, stratigraphic variations or the orientation of the strata. Cliffs may also occur in soft rock forma-tions or unconsolidated material, along coastal dunes or glacial and fluvial deposits, but they are cut back quickly and loose steepness by rain or groundwater seepage (Figs. 4.48–4.51).
Fig. 4.26 The island of North Ronaldsay of the Orkneys (Scotland) is surrounded by cliffs and an inclined abrasional shore platform
Fig. 4.27 40 m high cliffs in limestone in SW Portugal. (Photo credit: D. Kelletat).
Fig. 4.28 A 5 km long cliff section at the Atlantic coast of Morocco at 28° 13´N and 11° 45´W. (Photo credit: © Google Earth 2010).
4.28 4.27
Fig. 4.29a,b,c,d High cliffs with coarse clast cliff top deposits at (a) Moher, western Island (b,c,d) the Aran Islands, Ireland. The origin of the deposits either from storms or tsunamis is under debate. (Photo credit: A. Scheffers).
Fig. 4.30 A mega-cliff (about 300 m high) at the south coast of Gran Canaria, Spain. (Photo credit: D. Kelletat).
Fig. 4.31 These fresh break-outs of an old coral reef rock (several 100 tonnes each) on the island of Bonaire (Caribbean) are the result of 12 m waves from hurricane Ivan in 2004. (Photo credit: A. Scheffers).
Fig. 4.32 Cliff collapse along vertical joints may also occur without the influence of strong wave action, just by normal weathering, in particular where frost is present during the winter. West coast of Inishmore Island, Aran Islands, Ireland. (Photo credit: A. Scheffers).
Fig. 4.33 Quarrying of well stratified limestone rocks at cliffs in western Ireland. (Photo credit: D. Kelletat).
Fig. 4.34 Quarrying along a network of joints and fissures in coastal rocks of the Galway Bay, western Ireland. (Photo credit:
D. Kelletat).
Fig. 4.35 The Aran Islands partly show undermining of slightly seaward dipping limestone strata with accelerated cliff recession forming cliff bays. Inishmore, western Ireland. (Photo credit: A. Scheffers).
Fig. 4.36 Dominant joint patterns and directions are easily eroded by strong waves like in the dolerite of southern Tasmania, Australia.
(Photo credit: D. Kelletat).
Fig. 4.37 Ongoing cliff erosion and formation of cliff bays with stacks and more sheltered places with initial pocket beaches (western Scotland). (Photo credit: D. Kelletat).
Fig. 4.38 A pocket beach dominated by cliffs where erosion is still active during intervals of high water and large waves, Shetland Islands. (Photo credit: D. Kelletat).
Fig. 4.39 Cliffs, stacks and abrasion directed by parallel joints along the “Great Ocean Road” in Victoria, Australia. Position is about 38° 39´S and 143° 04´E. Scene is 2.4 km wide. (Photo credit: © Google Earth 2010).
0 1km
Fair Isle Orkneys
Shetlands
sloping coast active cliffs natural arches
caves © 2011 Hans van der Baan / Ingeborg Scheffers
Fig. 4.40 Fair Isle between the Orkneys and Shetland Islands of Scotland exhibit numerous sea caves and natural arches. (Image credit: D. Kelletat).
Fig. 4.41 A stack about 30 m high in sandstone of Helgoland, Germany. (Photo credit: D. Kelletat).
Fig. 4.42a,b Stacks away from the active cliffs document a wide area of erosion/abrasion in these shales of western Ireland. (Photo credit: D. Kelletat).
Fair Isle exhibits numerous sea caves and natural arches
Fig. 4.42a
Fig. 4.43 A natural arch formed by abrasional forces on granite near Albany (Western Australia). (Photo credit: A. Scheffers).
Fig. 4.44 The “Azur Window” of Gozo Island, Maltese islands, Mediterranean, adapted to the rock strata. (Photo credit: D. Kelletat).
Fig. 4.45 A natural arch in a small island in the Shetland group, northern Scotland. (Photo credit: D. Kelletat).
Fig. 4.46 The so called “London Bridge” at the Great Ocean Road in Victoria, Australia, collapsed in the year 1990. (Photo credit:
D. Kelletat).
Fig. 4.47 A sea stack and natural arch in chert limestone of the white cliffs of French Normandy coast. (Photo credit: D. Kelletat).
Fig. 4.49 Cliffs in soft rock formation cut by gullies due to heavy rain at the east coast of Brazil at 9° 53´S and 35° 55´W.
Coastal section shown is 1.3 km long. (Photo credit: © Google Earth 2010).
Fig. 4.50 Cliffs in unconsolidated deposits show slides and mud flows (eastern England at 54° 34´N and 0° 50´W, about 3 km long section). (Photo credit: ©Google Earth 2010).
Fig. 4.48 Two examples of unconsolidated younger sediments with cliffs and slumps along the coast of Algarve, southern Portugal.
(Photo credit: D. Kelletat).
4.50 4.49
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Kelletat D (1999) Physische Geographie der Meere und Küsten. Teubner Studienbücher Geographie, 2 ed., Stuttgart
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Fig. 4.51 Cliffs may also exist for a short time as quasi stable features in sand as in these mid-Holocene dunes with soil development and vegetation cover of Sylt Island, German North Sea coast. Many houses have been destroyed by rapid cliff retreat in these dunes during the last 200 years. (Photo credit: D. Kelletat).
A.M. Scheffers et al., The Coastlines of the World with Google Earth: Understanding our Environment, Coastal Research Library 2, DOI 10.1007/978-94-007-0738-2_5, © Springer Science+Business Media B.V. 2012