Sea caves and sea arches – it's all about the rock. Hard granitic rock weathers uniformly, making sea arches and sea caves rare. But layered metamorphic rocks, now there's potential for arches and caves. The layers have different degrees of hardness and shear. Sea water pounds and eats away the softer areas. Occasionally the rock type, the coastline configuration and persistent sea conditions combine to produce a fantastic cave or arch. But sea arches don't usually last long – the same erosion processes that creates them also tears them down. Photographer: Bill Eichenlaub