Page 342 - Fundamentals of Geomorphology

COASTAL LANDSCAPES 325

cliff base, and the amount of abrasive material churned
up at the cliff–beach junction.
Ramps occur at cliff bases and slope more steeply than
the rest of the shore platform.They occur on sloping and
horizontal shore platforms. Horizontal shore platforms
may carry ridges or ramparts, perhaps a metre or so high,
at their seaward margins.
Marine potholes are roughly cylindrical or bowl-
shaped depressions in shore platforms that are ground
out by the swirling action of sand, gravel, pebbles, and
boulders associated with wave action.

Caves, arches, stacks, and related
landforms
Small bays, narrow inlets, sea caves, arches, stacks, and
allied features usually result from enhanced erosion along
lines of structural weakness in rocks. Bedding planes,
joints, and fault planes are all vulnerable to attack by
erosive agents. Although the lines of weakness are eroded
out, the rock body still has sufficient strength to stand as
high, almost perpendicular slopes, and as cave, tunnel,
and arch roofs.
A gorge is a narrow, steep-sided, and often spectac-
ular cleft, usually developed by erosion along vertical Plate 13.5 Geo at Huntsman’s Leap fault cleft,
fault planes or joints in rock with a low dip. They Castlemartin, South Dyfed, Wales.
may also form by the erosion of dykes, the collapse of (Photograph by Tony Waltham Geophotos)
lava tunnels in igneous rock, and the collapse of mining
tunnels. In Scotland, and sometimes elsewhere, gorges
are known as geos or yawns (Plate 13.5), and on the
granitic peninsula of Land’s End in Cornwall, south-west Where waves attack a promontory from both sides, a
England, as zawns. hollow may form at the promontory base, often at a point
A sea cave is a hollow excavated by waves in a zone of geological weakness, to form a sea arch (Plate 13.6).
of weakness on a cliff. The cave depth is greater than If an arch is significantly longer than the width of its
the entrance width. Sea caves tend to form at points entrance, the term ‘sea tunnel’ is more appropriate.
of geological weakness, such as bedding planes, joints, Merlin’s Cave, at Tintagel, Cornwall, England, is a
and faults. Fingal’s Cave, Isle of Staffa, Scotland, which 100-m-long sea tunnel that has been excavated along a
is formed in columnar basalt, is a prime example. It is fault line.The toppling of a sea arch produces a sea stack
20 m high and 70 m long. A blowhole may form in (Plates 13.7 and 13.8). Old Harry Rocks are a group of
the roof of a sea cave by the hydraulic and pneumatic stacks that were once part of the Foreland, which lies
action of waves, with fountains of spray emerging from on the chalk promontory of Ballard Down in Dorset,
the top. If blowholes become enlarged, they may col- England. On the west coast of the Orkney Islands,
lapse. An example of this is the Lion’s Den on the Lizard Scotland, the Old Man of Hoy is a 140-m stack separated
Peninsula of Cornwall, England. from towering cliffs formed in Old Red Sandstone.