Lulworth Cove is known internationally as the classic form of a near-circular bay resulting from the differential erosion of weaker strata behind a resistant and protective outer wall of harder rock.

Lulworth Cove and its surrounding geomorphology is internationally renowned. This is due in large part to the natural, dynamic, erosional processes that have taken place and which continue to shape the coast. The southern coast of the Isle of Purbeck and the coast west from Lulworth Cove are well known both for their geological structures, and because they are the most important locations in Britain for showing how coastal landforms are made as a result of the effects of the sea working on differential rock structures and strengths.

The effects of this differential erosion of the rocks are well illustrated here, as well as at the scale of bays at nearby
Worbarrow Bay and St Oswald’s Bay.

 

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Lulworth Cove. © Steve Belasco - stevebelasco.net

The rocks around Lulworth have been lifted and folded into their current position. The oldest of these rocks are the Portland Limestone. These formed during the late Jurassic /early Cretaceous, 145 Million years ago in a warm, shallow sea environment. The Portland limestone rocks are hard, resistant rocks that make up the coastline here, the fortress holding back the sea.

Yet when the sea breaks through cracks and faults within these harder rocks, the softer rocks behind are more vulnerable to erosion.

Because of the rock formation process and how the rocks have subsequently tilted and folded (as a result of tectonic movement around 25 Million years ago), combined with the influx of increased water from melting ice at the end of the last ice age (around 11.5 thousand years ago), ancient streams from the land side and harsh waves from the sea side were able to break through tiny weaknesses in the hard Portland limestone.

Once through, the power of the water (and the energy caught up in the waves) were able to begin eroding the rocks. When the younger, softer clay rocks behind the Portland limestone were reached by the sea, erosion was able to occur much more rapidly.

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The area around Lulworth Cove as seen from the sea. © Steve Belasco - stevebelasco.net

Lulworth Cove is a classic example of a ‘concordant coast’ – i.e. the same rock types are parallel to the coastline, with alternating bands of different rock types running parallel to the coastline behind it (as opposed to a ‘discordant coastline’ where the rock types are perpendicular to the coastline, presenting differing rock types to the sea along the coastline).

The Cove is formed due to the alternating rocks being differentially resistant to erosion. Portland Limestone (most resistant); Purbeck Limestone (not as resistant but more resistant than next rocks); Greensands/Weald Clays/Upper Greensand /Gault (least resistant and rapidly eroded); Chalk (resistant but softer than Portland Limestone). The distinctive Clam or horseshoe-shaped cove formed roughly 10,000 years ago by the power of the sea due to wave diffraction.

The narrow cove entrance (between the resistant pillars of Portland limestone) causes waves to bend in an arc shape around the rocks. When a straight wave hits a barrier with a hole in it, the resulting wave pattern on the other side is semi-circular . Eventually this wave pattern wore away the rocks to form this perfectly semi-circular cove loved by tourists and locals alike.

Once the water broke through the hard limestone defences, erosion wore the rocks away in a fan-like action. Wave energy is then dissipated (transferred or spread out) by the protruding parts (the original coastline) of the cove.

See the video below for more.

Uninterrupted offshore ledges across the mouth of Lulworth Cove show that no major fault-aligned submerged valley exists in the mouth of Lulworth Cove.

This suggests that water did not flow out of the cove in a distinct channel when sea level was below the depth of the present seabed (-5 m), otherwise a channel would have been incised here. Lulworth Cove formed once sea level rose above this –5 m level to erode and remove material behind the Portland Stone entrance.

Once sea level falls below –5 m, coastal processes no longer have an effect within Lulworth Cove. Thus, Lulworth Cove probably formed when sea levels were at or very near to their present level.