shore platform
slowly changing
shore platforms
inherited elements
new from NZ
ESPED
Block quarry at the cliff
Abraded platform, inactive in the foreground, with storm beach and stripping of weathered sandstone
Block lift from the crest of small cuesta
Blackened and pitted surface on a relatively stable part of the platform
Shore platform processes
Significance: shore platform in many parts of the world are slow to
change. The high wave energy environment of the exposed outer coast of Orkney
provides high rates of erosion and allows the main processes which shape these platforms
to be identified.
The
shoreline between Stromness and Warebeth shows an actively eroding shore
platform and low cliff. The platform is exposed to westerly storms and
experiences a tidal range at springs of 2-3 m. The Devonian flagstone includes a range
of massively bedded to flaggy sandstones. The sequence dips westward at 10
Process zones
The platform displays a series of process zones from landward to seaward. The
cliff is rarely more than 5 m high and postglacial weathering has often created
a weathering profile of disaggregated and discoloured sandstone 1-3 m thick.
Wave action is locally stripping this weathering profile and quarrying large
blocks from dipping rock faces. Rapid coastal retreat has been partly stabilised
by piecemeal coastal protection works.
At the base of the cliff is a storm beach, comprising cobbles and boulders of
mainly rounded sandstone clasts but with occasionally large angular blocks.
At the foot of the beach is zone of abraded bedrock, with rounded joint edges
and lacking significant lichen growth.
This passes seaward into rock surfaces which are neither frequently nor intensively abraded today. Although some rock surfaces may be smoothed they are blackened with Verrucaria maura and pitted by weathering. Small cuestas show no clear smoothing and a blackened and inactive dip slope and loss of blocks from the scarp face. Scars with little or no lichen cover shows sites of recent block removal. Block transport is from W-E, as show by the carry of blocks of distinctive size or lithology.
Towards the low water mark seaweed cover becomes extensive but evidence of recent block loss from scarp faces is provided by fallen and transported blocks in strike gulleys.
Processes of erosion
The platform is being extended to landward by cliff retreat but wave action is
parallel to the shore, rather than normal, reflecting the orientation of
dominant waves at this site. The platform is lowered by abrasion only at its
inner edge, where mobilisation of coarse beach materials during storms allows
scouring of rock surfaces. Elsewhere block quarrying is the main agent of
lowering - these are wave-cut platforms. Blocks, slabs and flakes are
lost to hydraulic quarrying from the scarp face of mini-cuestas, reflecting
block lift during turbulent flow. Block loss from the scarp face leads to
progressive elimination of the cuesta. Not all of the platform is actively
eroding. Some elevated parts of the platform have blackened and pitted surfaces,
indicating an absence of abrasion and block loss and sufficient time for
significant chemical weathering to occur.
On the generally flat-lying Devonian sandstones and flagstones of Orkney and Caithness there is a strong association between platform width and the height of backing slopes. Platforms are wide and extensive wherever gentle slopes back the shore. This suggests that rock mass strength is the dominant control over platform extent. A corollary inference is that wave energy in even sheltered locations is or was sufficient to mobilise blocks from fissile lithologies, such as flagstone.