This formation consists of alternating layers of limestone and shale , laid down as sediment on a shallow seabed early in the Jurassic period ( about 210-195 million years ago ) . The cliffs are one of the richest fossil locations in Britain . The formation consists of alternating layers of limestone and shale , laid down as sediment on a shallow seabed early in the Jurassic period ( about 210 â `` 195 million years ago ) , and is one of the richest fossil locations in Britain .