Esha Ness, with a cliff-top ramp with wave-tossed boulders at >30 m OD
Wave-quarry site, Grind of the Navir 1995
Wave-scoured ramp, Villians of Hamnavoe
Cliff-top boulder ridges, Out Skerries
Blocks on turf, Grind of the Navir, with demolished wall in the background
Splash and air-throw debris, Grind of the Navir
Geomorphology of CTSDs
Grind of the Navir, showing wave-scoured platforms above 15m high cliffs and beach ridges. Image courtesy of Shetland coastguard
Wave-generated cliff-top boulders may comprise individual clasts, spreads and ridges. The boulders occur at various distances back from the cliff edge, behind rock platforms and ramps are largely swept clear of debris by wave activity. Storm-generated boulders may extend inland of the limits of modern storm wave wash on to and beneath vegetated cliff-top surfaces.
The most extensive wave-scoured rock ramps occur at Villians of Hamnavoe (Mactaggart, 1999). Here the ramp is 40-150 m wide, 3 km long and developed in Devonian tuffs. The ramp is generally bare of sediment, except for a few scattered storm boulders. At South Head, the stepped cliff shows rock ledges at 18 m OD that periodically carry boulder accumulations in which individual boulders are up to 3 m a-axis. The headland of Grind of the Navir is scoured by waves in major storms that overtop cliffs 15-18 m high. The central part of the cliff incorporates a staircase geo that acts to focus wave activity. The cliff-top platform shows multiple sockets, with fresh joint-bounded surfaces and angular edges, from which large boulders of ignimbrite have been removed by wave activity. The cliffs at Esha Ness show some of the highest wave-scoured surfaces on Shetland, at up to 40 m OD. Here CTSD’s occur at the rear of ramps inclined to seaward.
Beach ridges at Old Cro, Out Skerries
Locally, the cliff-top boulder accumulations form a series of ridges parallel to the coast. On the unusually straight section of coast of South Ward, Out Skerries, a sequence of three parallel storm ridges is sited on a 10 m OD platform some 60 m landward of a 20 m high cliff. A particularly fine sequence of 3 boulder ridges, up to 3.5 m high and with ignimbrite boulders up to 2.5 m a-axis, occurs at Grind of the Navir. The ridges rest on a platform some 50 m landward of the cliff edge. More usually, the cliff-top platform or ramp is backed by a single, coast-parallel, asymmetric ridge, 1-4 m high, with a steep, eroding seaward face and a gentler landward slope, locally turf covered. Evidence of rock quarrying by waves may be seen locally at the base of the ridge. The ridge orientation generally follows closely the detailed configuration of the cliff line around bays and headlands. The beach ridge is composed of CTSD’s, including boulders, whose characteristics are described below.
Beach ridges at ~15 m asl at Grind of the Navir (August 2003)
As with storm beaches close to current sea level, the CTSD’s may impound boggy depressions or ephemeral lochans. Where the ramp slopes to landward at the upper limit of storm-wave wash, spillway channels are locally developed parallel to the coastline and represent evacuation routes for wave water that overtops the cliff edge.
Beyond the limit of wave wash, isolated boulders, imbricate boulder groups and boulder spreads lie on relatively gently-sloping and turf-covered platforms or ramps. At Villians of Hamnavoe, large boulders occur well inland of the limit of modern wave wash. At Grind of the Navir, dumps of debris on to turf by waves in the major storms of 1/1/1992 and 17/1/1993 have been mapped up to 50 m inland of the main CTSD ridge. Excavations within shallow peat deposits at Grind of the Navir and elsewhere have revealed partly and wholly buried isolated boulders. These boulders appear to have been deposited by wave wash on the bog surface and then buried by peat accumulation.
Turf-covered surfaces wholly or partly cover gravel breccias and carry variable densities of angular pebble-sized clasts. Transects normal to the coast behind the boulder ridges were undertaken at Villians of Hamnavoe 5 months after the major storm of 1/1/1992, in which wind speeds reached x kmh in gusts on northern Shetland. They revealed large numbers of isolated and clustered clasts resting on turf that was still green, in contrast to adjacent pebbles beneath which the grass was dead. These pebbles had been moved during the recent storm and yet lay above the trash-line that marked the upper limit of wave wash. This debris can be traced up to 100 m inland of the limits of wave wash. The debris was transported in a number of different ways by wind, including sliding and rolling along the ground and being thrown into the air with wave-generated spray and carried landward.