The Marks and Spencer's 300 Million Year Old Wall

The 300 Million year Old Wall.
The Crinoidal Limestone of Marks &Spencer's

Marks & Spencer's end wall is clad with a magnificent fossil display. It is of Crinoidal Limestone, called Dene Stone, and comes from the Matlock area. It is sometimes called Derbyshire Fossil Limestone. It was also used for the Festival Hall, built only a few years before this building.

Dene Stone is a Carboniferous Limestone full of the stems and ossicles of crinoids (sea lilies), brachiopod shells, and other fossils, neatly sectioned and polished for our enjoyment. The wall has weathered, cutting back the surrounding limestone and leaving the harder fossils to stand proud.

Crinoid fragments cut lengthwise and across.

Crinoidal Limestone

Crinoids, called sea lilies because of their plant-like appearance, are in fact animals and are the only ones of their kind (echinoderms) still to be found alive today. These however were living in the Carboniferous Period - from 345 million years ago (m.y.) to 280 m.y. ago - the period which also produced the Millstone Grits and the Coal Measures.

The following quotation describes another piece of crinoidal limestone:

The base of General Gordon's statue is made from a very old sea floor athwart what are now the hills and valleys of the Peak of Derbyshire in whose waters waved countless stalked crinoids - the sea lilies - but over whose surface no sea bird wheeled and cried, for this was so long ago that no bird had been created or evolved’

"Building Stones of London”,  by Elsden and Howe, 1923.

In the Early Carboniferous Period England was near the Equator. The country was a mass of tropical coastal swamps and seas, with clear, shallow and water like the present day Barrier Reef. These are the sort of conditions in which limestone is laid down.

A crinoid showing the stalk and branching 'arms'.

A Bivalve Shell, from the same M&S wall

The shell was filled, over time, with limestone deposits so that shell and sediment became one stone. Then it was cut through as we see it today, to reveal some of the old shell ridges. The shell was like a modern cockle.

Besides the crinoids in the limestone there are corals

A Coral from the wall surrounded by crinoid fragments

Another Coral with crinoid fragments

The presence of the corals tells us a lot about the shallow sea conditions under which this limestone was laid down.

Corals seem to live abundantly only in seas with an average temperature of 29° and a salinity slightly above the normal: in depths of less than 25 fathoms and situations where the sea floor, owing to the clarity of the water and its freedom from sediments, is bathed in sunshine. This last condition is necessary because the corals have an algae in  their tissues living symbiotically with them (coral and algae both gain from the living together and that algae needs strong sunshine to survive. (if the water is too deep the sunlight cannot reach the coral and it dies).  During Jurassic times such conditions must have prevailed over south and central England.

This coral rich rock is found in sheets only about three metres thick. These sheets are separated by normal limestones that obviously represent times when the depth of the sea was too great for the growth of coral reefs.

"Fossils", by H.H.Swinnerton, p.159, slightly adapted.

Thus the presence or absence of corals reveals the alternate rise and fall of sea levels over enormous periods of time.

The Types of Limestone

Limestone is a sedimentary rock, formed by layers of chalky material being deposited one above the other. It is formed in shallow water, warm and either fresh or marine, clear and free from mulls. Limestone is being deposited every day on coral reefs and being re-deposited in Yorkshire caves as stalactites. Here the older limestones are being dissolved, trickling down and reforming.

Page 5 Page 7
Updated: August 16, 2011