geomorphology

origins

cliff-top storm deposits

Green Head, Mousa 1995

Clettnadal, CTSD breccias resting on Lateglacial and Holocene lake deposits

Tabular blocks of Devonian sandstone, Scat Ness

 

Scat Ness. CTSD breccia, with blocks embedded in turf

Key sites

Grind of the Navir

Villians of Hamnavoe

Esha Ness CTSD survey February 2012 by Ian Napier

 

Sedimentology of CTSDs

Definition: cliff-top storm deposits

CTSD’s occur as ridges, boulder spreads and isolated boulders. In the Northern Isles, coarse gravel-rich breccias, containing isolated boulders, occur behind boulder ridges and reflect the accumulation of coarse clastic debris from wave wash and wind action. Clast sizes do not reach the block category (>4 m b-axis) of (Blair and McPherson, 1999); boulders (25-205 cm) are dominant.

Sections in the ridge at the upper limit of storm wave wash show a maximum thickness of 4 m of CTSD’s. The CTSD’s are coarse gravel breccias which comprise dominantly clast-supported angular pebbles, cobbles and boulders, often openwork near the ground surface but with a matrix of sand, granules and pebble to boulder size gravel at depth. At Grind of the Navir and Clettnadal, the matrix very occasionally includes comminuted shell. The breccias generally rest on till or directly on bedrock. In places, however, thin soil profiles underlie the breccias, probably representing older turf layers immediately prior to the onset of deposition. At Grind of the Navir and Clettnadal, Shetland (Whittington et al., 2003), fine-grained lake deposits are found beneath the erosive base of the storm breccias, implying that cliff retreat has caused pond and lake drainage.

The seaward face of the ridge is formed of large imbricate and fitted boulders dipping to seaward (PHOTO). Fine examples are seen developed on Devonian sandstones on Ness of Burgi, Scat Ness, and Green Head, Mousa, Shetland, where they form three low ridges parallel to the cliff edge. Measurements of clast orientation on Shetland (Table 1) and Aran (Williams and Hall, 2004) show that the boulders are aligned normal to ridge orientation. Ridge orientation and boulder imbrication provide important information on the direction of wave approach

Clast shape and size is largely controlled by the structure of the cliff-forming rocks. Tabular boulders tend to dominate on schist (Shetland), sandstone (Shetland, Orkney and Caithness) and limestone (Aran) and orthogonal boulders on granite and gneiss (Shetland and Lewis). The tuffs and andesites of the Esha Ness peninsula, Shetland, are strongly jointed and break readily into large cuboidal boulders (Mykura, 1976) . A axis lengths of boulders frequently exceed 1 m and reach 3 m at Scat Ness and Villians of Hamnavoe. The largest boulders tend to occur on the seaward face and crest of the ridges. The seaward face of the ridges carries recently quarried boulders, with fresh, lichen-free surfaces and sharp, unweathered edges. In contrast, the upper and landward faces of the ridge tend to be dominated by older, partly lichen-covered and more weathered boulders. A range of material is firmly embedded beneath large boulders on the crest and back slope of the ridges. This includes shell debris, wood, floats (cork, rubber and plastic), fragments of seaweed and, abundantly, plastic, polystyrene and foam rubber.

Behind the main CTSD ridges on Shetland, 0.1-2 m diameter boulders rest on or are embedded in the modern turf. The turf grows on a thin surface layer of sandy granule and pebble gravel. This rests on up to 210 cm of crudely flat-bedded angular pebble and cobble gravel, with occasional boulders. These back-ridge breccias thin inland and include 2-20 cm thick layers of sandy gravel of similar characteristics to that immediately below the turf but apparently lacking in organic material. Thin lenses of organic sand also occur at a few sites, representing former soil surfaces.