For a Named Coastline Examine the Effect of Rock Type and Structure on Coastal Landforms
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The coastline I have chosen is part of the world heritage site (the Jurassic Coast), from Old Harry Rocks to St Oswalds Bay
Discordant Coast – the structure and alignment of these rocks have a significant effect on the landforms produced. This is a discordant coast where the geological sequence has produced distinctive coastal landforms.
At Old Harry/Ballard Point and at Durlston Head are outcrops of rocks resistant to erosion. The chalk escarpment which bends from St Oswalds Bay eastwards to Old Harry increases in width to produce steep 60 metre cliffs. Erosion at the chalk headland of Ballard Point has produced stacks, wave cut platforms, notches, caves and arches.
The more resistant Portland limestone has also produced vertical cliffs and caves. It is more resistant to erosion than the chalk. Both outcrops produce pronounced headlands. The much softer Greensand and Wealden beds have been eroded more rapidly creating a large bay (Swanage Bay) between these headlands.
Concordant Coast – whilst the geology essentially remains the same, the structure here is different. This is a concordant coast with unique coastal landforms, The outer layer of the concordant structure is made up of a thin layer of Portland limestone.
At stair hole the Portland limestone had been punctured in several places. The resistant nature of this rock has resulted in caves/arches/blowholes and as waves have penetrated the outer barrier a small cave has formed as the softer Purbeck/Greensand rocks have been easily removed.
At Durdle Door, what was once part of a cave entrance remains as a large natural arch, because of variations in resistance of this section of Portland limestone.
At Lulworth Cove , a meltwater overflow carried a river into the cove removing the softer Purbeck/Greenland rocks and eroded a small aperture in the Portland limestone. The sea subsequently eroded the softer sediments widening the central part of the cove, but making much less progress at the entrance to the cove against the more resistant Portland Rock. At the back of the cove is the chalk escarpment (see map). Although thinner and parallel in alignment it creates significant vertical cliffs at Lulworth and St Oswalds Bay.