Geology
Stratigraphically these limestone beds lie towards the top of the Durlston Formation of the Portland Group. They were deposited during the Berriasian age of the Early 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' is properly classified as a biomicrudite, as it consists of 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 hematite or limonite, giving red or brown varieties, while other beds contain glauconite giving a green (or occasionally blue) colour.
Read more about this topic: Purbeck Marble