A fine-grained sediment, usually a mud stone, develops progressively to shale and then to slate through the action of burial, compression and heat. The physical characteristics of slate enable it to be split into parallel sheets – either riven or cleft. English and Welsh slate are among those that originated as volcanic ash deposits, which tend to be harder and more durable. Slate is non-porous because of its laminated structure, although some slates are quite soft and scratch easily. A riven or cleft finish is ideal for flooring as it provides a non-slip surface, even when wet.