Rock mass strength: the persistence of a cliff face requires that the rock mass has sufficient strength to support the rock wall. Rock mass strength is measured using factors that include fracture density, fracture orientation, rock hardness, degree of rock weathering and water content.
Hillswick - steeply-inclined joints in granite generate near-vertical cliffs
Grunna Stack, below Stonga Banks. The caves are developed in vertically-jointed granite. Arches are weakly developed here and at The Drongs because vertical joints are dominant.
Cliff profile and structure
The single most important factor determining cliff profiles on the hard rock coasts of Shetland is the structure of the rock - the way that the rocks are lying and the orientation and number of fractures. The effects can be seen in both plan view - on the map and from above - and in cross section - moving from sea level to the cliff top.
The simplest structure is that of horizontally-dipping sedimentary rocks. Here the detail of the cliff face is controlled by differential weathering and erosion of the rock beds and produces a series of ledges and notches. Joints define cliff edges.
Cliffs in thick beds of near horizontal Devonian sandstone, Sandness. Note the large overhanging mass ready to slide into the sea
Where dip or foliation lies to seaward then the cliff face is inherently unstable. Removal or collapse of rock towards the cliff base leads to rock slides. Recent rock slides can often be recognised at the base of these cliffs and the lack of rounding of debris suggest that it is soon removed by the sea.
Steeply dipping phyllites on the east coast of North Roe, with recent rock falls.
Where rocks dip inland, steep, but sub-vertical and relatively stable cliffs form.
Inland dip of the Devonian sandstones of West Foula generates a steeply-inclined and strike-aligned cliff face, with tilted rock terraces at the base of the cliff. Image courtesy of Andy Gear.
In plan view, the outline of the coast often shows headlands and geos. The geos can usually be traced to lines of weakness provided by faults and joints. The headlands, as well as showing relatively low fracture densities, often relate to resistant rocks. The Grind of the Navir, for example, is developed in ignimbrite, whilst its backing cliff is formed in much weaker slaggy lavas. The cliff face also frequently is aligned with the strike of the rock.