Limestone is an organic, sedimentary rock. This means it was formed from the remains of tiny shells and micro-skeletons deposited on the sea bed. Over the years this sediment was compressed to form solid rock. Limestone is formed in layers – called bedding planes. These bedding planes contain vertical cracks called joints. Joints and bedding planes make the rock permeable. The image below shows joints and bedding planes at Malham Cove, Yorkshire Dales.