Purbeck marble

Purbeck marble is a fossiliferous limestone found in the Isle of Purbeck, a peninsula in south-east Dorset, England. It has been quarried since at least Roman times as a decorative building stone. This industry is no longer active.
Geology.
These limestone beds were deposited during Lower Cretaceous epoch. Purbeck Marble is not a metamorphic rock, like a true marble, but is so-called because it can take a fine polish.
Its characteristic appearance comes from densely-packed shells of the freshwater snail "Viviparus". Sussex Marble is similar in type. The 'marble' large clasts (the snail shells) in a fine-grained limestone mud matrix.
The individual marble beds (also known as 'seams'), lie between layers of softer marine clays and mudstone, laid down during repeated marine ingressions. Some of the beds contain iron oxide/hydroxide minerals, such as haematite or limonite, giving red or brown varieties, while other beds contain glauconite giving a green (or occasionally blue) colour.
Occurrence.
Purbeck marble is found at outcrop, or beneath superficial cover, all the way across the Isle of Purbeck. The marble beds are never more than 1.2 m thick and are often much thinner. The outcrops lie within the Purbeck Monocline, with the beds dipping moderately steeply to the north.