Complex sentence: The arch has formed on a concordant coastline where bands of resistant Portland limestone ran along the shore, the same band appears one mile along the coast forming the narrow entrance to Lulworth Cove.
Split into simpler sentences: The arch has formed on a concordant coastline where bands of rock run parallel to the shoreline. Originally a band of resistant Portland limestone ran along the shore, the same band appears one mile along the coast forming the narrow entrance to Lulworth Cove.