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What is limestone?

Limestone is a sedimentary rock made up of the remains of sea creatures and plants that lived in a tropical sea that approximately 345 million years ago covered the area of the Peak District now known as the White Peak.

The sea was warm and nutrient-rich, providing ideal conditions for many marine species.  Corals grew to form reefs, which encircled clear-blue lagoons of shallow sea and provided sheltered conditions for a rich diversity of life including:

• Brachiopods, bivalves and the snail-like goniatites
• Trilobites, similar in appearance to the woodlouse
• Crinoids or sea-lilies, which were a type of star fish with five long food-gathering tentacles that was attached to the sea floor by a long stalk made up of rings of calcium carbonate.

Each of these species used calcium carbonate from the nutrient-rich sea water to form a hard structure that protected or supported its softer parts (a skeleton).  For example, brachiopods had 2 shells that completely covered the softer feeding and digestive organs.  The crinoids grew stalks made of calcium carbonate plates stacked one on top of the other like a packet of Polo mints.

Line drawing of a sea-lily. The stalk is commonly found among limestone in the Peak District.	This specimen is packed with the fossilised cylindrical stalks of the sea-lilies

As each trilobite, crinoid, bivalve or other sea creature died, the soft parts of its body decomposed, whilst the harder, calcium carbonate shell, stalk, etc. fell to the bottom of the sea floor.  Over 40 million years, it is estimated that 2000 metres of shell debris built up at a rate of approximately less than 1 cm per 100 years!  Eventually, this layer of shell debris was compacted to form limestone.  Many of the original shell and stalk structures can still be seen as fossils within the rock.

In the Peak District today there are three main types of limestone:

Standard limestone is thickly bedded with many joints.  Fossils are often difficult to see as they were broken up by wave action, although whole beds of crinoids can be found.

Notice the layers in this cliff face at Lathkill dale - these are the bedding plane highlighted by erosion.

Reef limestone are found in separate mounds or hills and may have lens-like shapes to their bedding or no obvious structure at all.  Fossils are much easier to find all.

Chrome Hill is made of reef limestone.

Muddy limestone formed in the deeper basins around the coral reefs and lagoons.  These areas were less hospitable and could not provide the light and clean water conditions needed by corals.  The muddy limestones are full of impurities and when polished have an appearance that has given them the name black marble.

Black marble veins at Ashford

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