Caves, arches and stacks:
These are features of marine erosion. When a headland has been formed,
incoming waves around the headland. This is known as wave refraction. This is
because sometimes the water is shallower in front of the headland than in the
bay. Wave refraction increases the erosion around the headland.
Destructive waves crash into the coastline and begin to attack lines of weakness
in the headland such as faults or joints. The sea begins to undercut the headland
to form a cave by means of abrasion. This is where boulders, pebbles and sand
are hurled by the waves against the headland. This material crashes off the
headland and breaks off pieces of rock. Abrasion is most effective during storms
and at high tide, when breaking waves throw suspended material against the
roof and sides of the cave. The cave is enlarged and deepened to such an extent
that the cave may break through the headland to the other side to form an arch.
Compressed air is also a very common process in forming arches and stacks.
Air is trapped by an incoming wave in a cave and compressed. As the wave
retreats and the air expands, it blasts a hole in the roof of the cave. Over time a
blow hole may develop which resembles a chimney with an opening on the
surface of the headland.
Solution is another process at work in forming these features. Sea water
contains chemicals such as carbonic acid which can dissolve certain types of rock
such as limestone and chalk. Salt from sea water and spray is also capable of
corroding several rock types. As the structure of the rock breaks down, it easily
erodes when attacked by other erosive processes.
As these 3 processes combine to widen the arch, the roof becomes too heavy and
finally collapses into the sea to form a sea stack. This is a column of rock cut off
from the headland and surrounded by water.
e.g. caves and stacks are found along the Cliffs of Moher
Sea Spit:
This is a feature of marine deposition.
A spit is a ridge of sand and shingle, always above sea level and attached to the
land at one end. Many spits curve at the end due to wave refraction.
Spits tend to develop where the coastline undergoes a change of direction e.g. a
river mouth, a sheltered bay or between an island and the shore where longshore
is interrupted.
Spits are formed by the processes of constructive waves and longshore drift.
Constructive waves have a powerful swash which carries much material onto
the beach and a weak backwash, as the water percolates through the beach
depositing much material.
Longshore drift occurs where the waves break onto the shore at an angle of
less than 90. The swash moves the material along the beach and as the backwash
returns some of this material, the next swash moves the material further down
the beach. Thus sand and pebbles are moved diagonally along the beach and
begin to pile up where the coastline changes direction.
Longshore drift is slowed and some of the larger particles are deposited in the
slacker water in the lee of the headland (A) or river mouth. As the spit continues
to grow into a ridge, storm waves throw larger material above the high water
mark (B) making the feature more permanent; while under normal conditions,
the finer sand is carried towards the end of the spit at (C). Waves swing around
the end of the spit causing it to curve towards the land. Low energy waves enter
the sheltered area behind the spit depositing fine silt and mud, creating an area
of salt marsh.
e.g. Bull Island in Dublin Bay
or
Wexford Harbour.
Beaches: This is a feature of marine deposition.
A beach is a build up of loose material such as sand , pebbles, boulders, shingle
mud and shells. It forms between low water mark and the highest point reached
by storm waves.
These materials are deposited by means of constructive waves.
Beaches are common where the coastline is broken by headlands and coves.
Incoming waves are slowed down by the calm water in the sheltered cove.
Deposition occurs and constructive waves carry mainly finer material such as
sand into the shore to form bay head beaches. When these waves break on the
shore, their powerful swash sweeps material up the beach. As the wave retreats,
the backwash is weak because much of the water percolates through the sand
and shingle and so is unable to bring much material back out to sea. The beach
begins to grow.
Shingle beaches have a steeper gradient than sandy beaches. This is because
water will pass through coarse grained shingle more rapidly than through fine
grained sand.
Beaches can be divided into 2 sections – the backshore and the foreshore.
Storm beaches occur in front of cliffs or sand dunes in the backshore. Large
boulders are thrown here during storms by waves driven on by strong winds,
high above the level of a normal high tide. The storm waves percolate back down
the beach without moving the stones. All these boulders are smooth and rounded
due to attrition(the load is constantly bashing off itself).
A berm will form at the spring high tide level. This is a long ridge of large
shingle running parallel to the shore. Several smaller ridges may be found below
this, which mark the height of successively lower high tides which follow the
maximum spring tide.
Beach cusps also occur in the backshore. These are sandy areas surrounded by
ridges of shingle.
The foreshore is composed of sand and shell particles. It has a gentle gradient
and is covered by the sea every day. Constructive waves lead to the formation of
ridges and runnels. Ridges are broad, gently sloping lines of sand. Pools of water
are trapped in shallow depressions called runnels between the ridges as the tide
goes out. Both these features are parallel to the shore.
e.g. Brittas Bay Co. Wicklow.