Grind of the Navir. Scars and sockets from recent block erosion, with the block beach behind
Boulder ridge to the rear of the cliff top platform. Grind of the Navir.
Villians of Hamnavoe. Blocks left by wave wash beyond the limits reached in the 1990s. Lichen-cover indicates a residence time of at least several decades.
Origins of cliff-top storm deposits
On vertical cliffs with upper platforms or ramps, the source of boulders now found in CTSDs is indicated by scars and sockets. Inspection of the lower cliff shows a general absence of scars but scars and sockets occur frequently at the apex of the cliff top and on platform or ramp itself. These scars and sockets are often free of a weathering patina or lichen, indicating recent boulder removal. Rock steps are a particular common location for boulder quarry. The main source of the boulders is therefore at the top apex of the cliff (Robinson, 1990).
Scars and sockets vary greatly in size. At Grind of the Navir, it is possible to match the dimensions of boulders of 0.5-1.5 m a axis length on the landward edge of the upper platform to scars within the staircase geo on seaward edge. Typical carry distances are 10-50 m. At two sites, there are indications that blocks up to 20 m a axis length can be quarried by waves and moved en masse, prior to cracking on reaching the base of the storm ridge. At Villians of Hamnavoe, a block of welded tuff 19.6x9.6x2.1 m forms part of a step at the landward part of the cliff top ramp. It was seen in 1993 to be underlain by unweathered and lichen-free chock stones, implying that the block had been lifted during one of the storms of 1/1/1992 and 17/1/1992. This evidence at this locality suggests that the size of blocks gives only a minimum estimate of the size of the bedrock block that is initially lifted because jointed or large tabular blocks tend to fracture and break up during wave transport.
Boulder generation requires detachment from bedrock surfaces, lift and carry. Detachment is achieved along joint surfaces in well-jointed rocks, such as the Devonian ignimbrite at Grind of the Navir. The injection into joints of water and air under high pressures is indicated as the mechanism of lift. The total amount of lift is unclear but it must exceed the maximum thickness of chock stones.
The depth of water crossing the upper platforms during recent storms cannot be greater than the 2-6 m height of the backing ridge. The emplacement of boulders on the crest of the ridge implies surging waves in which the water is moving at sufficient velocity to carry upslope boulders of >1 m3. On cliff top ramps, surging waves are capable of travelling upslope over considerable distances to emplace boulders on the rear of the ramp. At Virda Field, Papa Stour, the ramp is 60 m wide and slopes at 10º to seaward.
Much material in the beach ridges is recycled from earlier deposits. The boulder ridges are being actively eroded on their seaward faces. Over-turned boulders, with lichen and weathered surfaces on their undersides, occur occasionally on boulder ridges and also within boulder groups behind the ridges. Erosion acts to strip out the gravel matrix of the beach ridge and shift the larger boulders landward. Small boulders may also be quarried directly from the bedrock at the base of the ridge. On Orkney and Shetland, advance of the boulder ridges is across older, weathered breccias formed of a combination of debris from wave wash and air throw. This landward shift is directly analogous to the evolution of more conventional storm beach systems.
On Shetland and Orkney, CTSD’s extend for up to 100 m behind the beach ridges and include isolated boulders, groups of imbricate boulders and breccias. These materials lie within and beneath vegetated surfaces and the exposed clast surfaces may be weathered and covered with lichen. Preliminary OSL dating of intercalated sands indicates that these materials have accumulated over timescales of 102-103 yr (Sommerville et al., 2003). Recent accumulation is a product of wave wash and air throw. On Shetland, significant accumulation of air throw debris accompanied the storms of 1/1/1992 and 17/1/1993 but wave wash generally did not extend more than a few metres on to the leading edge of the vegetated zone. The contrast between the upper openwork boulder breccia and the lower gravel and boulder breccia may reflect a range of processes. Initial wash over of the boulder ridge during major storms leaves a coarse, openwork gravel. This is infilled by air throw, floating and smaller wave wash debris during subsequent storms. The movement and vibration of debris during major storms may lead to downward migration of smaller clasts.