When wave energy is high coastal erosion takes place resulting in

a variety of coastal features. The rate of erosion is moderated by the mixed

geology of the coastline. Hard rocks such as basalt are more resistant and thus

erode much slower than soft sedimentary rocks such as limestone. We call this

differential erosion. Coastlines that have an alternating geology of harder and

softer rocks results in the formation of headlands and bays as can be seen here

in Port Macquarie. The places where the rock is harder has headlands jutting out.

Where the rock is softer more erosion takes place and bays form. Over time the

headlands become more exposed to the erosive powers of waves due to rain

refraction. This occurs because the waves slow down first in shallower water

forcing the refracting parts of the wave to break first at the headland,

while the other part of the wave is traveling faster towards the beach in

deeper water. The erosion at headlands results in the formation of spectacular

coastal features such as cliffs, wave cut platforms, arches,

caves, blowholes

and stacks.

When large amounts of water are constantly hammering away at the coastline the

processes of hydraulic action and abrasion can cause cliffs to retreat and

create a feature called a wave cut platform. Wave cut platforms are broad, flat

areas of rock that are often found that the base of cliffs and headlands.

At the edge of the platform there is an escarpment. Hydraulic action is when waves

smashing against cracks and joints in the rock compress the air those cracks.

The pressure created by this can eventually cause fragments of rock to

break off. Abrasion occurs when wave action transports sediment which hits the

cliff and gradually wears it away. Attrition also takes place where the

waves cause rocks to rub against each other. As this happens the original rocks

reduce in size to small and rounder fragments. As these processes take place

a wave cut notch can form at the base of the cliff. The notch forms because the

force of waves is only eroding the rock between the high and low water

mark. This causes the rock below the low-water mark to remain. Overtime this

notch gets progressively bigger until the cliff eventually collapses. As a

result of collapsing cliff faces there is a lot of debris at the base made up

of large boulders. In these places the debris from collapsed cliffs is broken

down by waves. The backwash carries the debris towards the sea forming the wave

cut platform. Then the whole process repeats and the cliff continues to

retreat. Understanding the workings of erosion processes such as cliff retreat

is very important for people as they impact heavily on any human coastal

development. For caves to forum there must be a

weakness in the headland's rock, such as a soft rock type or an existing crack. Over

time hydraulic action makes these cracks bigger and bigger until a cave forms.

Sometimes a cave roof will partially collapse to form as a blow hole such as

this one here. Other times the waves will erode the cave

right through to the other side of a headland

to form an arch. Eventually the ceiling of the cave will completely collapse

causing part of the headland to detach and form a vertical column of rock in

the sea known as a stack. Another peculiar type of erosion feature found

on rocky shores around the world is called honeycomb weathering. This

formation is caused by salt crystals getting into cracks in the rock. Sea

spray and wind are the usual ways for salt water to make its way into these

places. When the water content evaporates salt crystals are left behind. The rocks

go through a wetting and drying process as they are often found in intertidal

zones. When they dry from the Sun the crystals heat up and expand which puts

pressure on the confining rock.