 Durdle Door - Geology: Encyclopedia II - Durdle Door - GeologyThe arch has formed on a concordant coastline where bands of rock run parallel to the shoreline. Here the rock strata run near vertical, and the bands of rock are quite narrow. Originally a band of resistant Portland Limestone ran along the shore, the same band which can be seen two miles down the coast forming the narrow entrance to Lulworth Cove. Behind this is a 400 ft (120 metre) band of various weaker rocks which are easily eroded, and behind this is a stronger and much thicker band of chalk, which forms the Purbeck Hills. The limestone ...
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