Lost Your Marble?

An EarthCache about the Purbeck Marble of Lincoln Cathedral
 

Purbeck Marble is not a true marble, but rather a hard, dense limestone that can be polished to a smooth, shiny finish. The Romans used it for inscription plaques, architectural mouldings, and mortars and pestles. There seems to have been little use of it after then until the 12th century.

(Please note that there is a fee (Mon-Sat £11, Sun: £7) to enter the Cathedral. Children (0-16) enter for free, as do carers and also students of Bishop Grosseteste University, Univerity of Lincoln and Lincoln College. For full details and admission times visit this link)

Its use as a building stone

The unique shelly appearance and subtle colour variations of Purbeck ‘Marble’ made it the foremost decorative stone of the English medieval period, perhaps due to its distinctive rich, dark colour and lustre finish. It was also used as a cheaper alternative to actual marble, often imported from Belgium.  It was used extensively for architectural features, including columns and pillars, string courses and sculpture in high-status buildings such as cathedrals, abbeys and palaces. Over the centuries, layers of waxes and coatings have obscured much of the characteristic splendour of Purbeck Marble, but originally its polished surface would have had an opulence that mimicked true marbles and contrasted with surrounding stonework.  Some of the greatest examples of works in Purbeck Marble are St Edward’s shrine (1269) at Westminster Abbey and the abundant columns within the interiors of Ely, Exeter and Salisbury cathedrals, as well as here in Lincoln.

Deterioration of the stone

Although Purbeck Marble is a hard stone, it is not a durable one and is particularly susceptible to the environmental conditions in which it is used, including humidity. Since the 19th century, this problem has been commonly acknowledged: remedial works were often carried out with varying success and generally at a cost to the aesthetic values of the stone. After a survey of the Purbeck Marble at Lincoln Cathedral in 2014, Nicholas Rank (Surveyor of the Fabric) wrote of the need ‘to consider how best to implement a conservation policy to arrest the gradual loss of surface of the stone which is becoming increasingly a disfigurement to the building. This will require a nationwide investigation into the problems of conservation since this problem is not unique to Lincoln but can be found in a number of medieval cathedrals.’
 

The Geology of Purbeck Marble

Purbeck Stone is the general name given to limestones of the late Jurassic/ early Cretaceous Purbeck Group (152 to 140 million years old), quarried as building stones on the Isle of Purbeck in Dorset. The climate during this period was relatively warm and southern England was covered with freshwater and brackish deltas and lagoons adjoining a shallow inland sea. These environments supported a rich marine and land flora and fauna, consequently Purbeck Stone often contains abundant fossils, especially bivalves, gastropods and ostracods. Purbeck Stone varies in colour (from blue, green, reddish-brown to shades of grey) and there are more than 20 different named beds of it, each with its particular characteristics, qualities and uses.

Purbeck Marble itself is found near the top of the Purbeck Group and typically occurs as thin beds in outcrops at or near the ground surface. Individual Purbeck Marble beds range from around 300mm to 600mm thick and are interspersed with layers of shale and clay. The outcrops all occur within the Dorset Area of Outstanding Natural Beauty. The only true, metamorphic marbles produced in Britain were quarried in north-west Scotland on the isles of Iona, Tiree and Skye. Purbeck Marble contains a distinctive fossil remains characterised by pale-coloured, closely-packed freshwater snails set within a darker coloured, fine-grained limestone mud matrix. The stone is predominantly calcite with significant amounts of clay minerals and smaller amounts of other minerals, such as feldspar and pyrite.
 

Sedimentary or Metamorphic?

Sedimentary rocks are formed by the accumulation of other eroded substances. Rocks are broken up in to small pieces by wind, water, etc. Those small particles are known as sediments. These sediments get deposited by various mechanisms, forming very thin layers. Then these layers become harder over a long period of time, forming sedimentary rocks. The texture of sedimentary rocks reflects the way in which the sediments were deposited and by their subsequent weathering. Sedimentary rocks are easy to identify as layers are often visible. Most sedimentary rocks are formed under the water. Shale, sandstone and limestone are examples of sedimentary rock. These rocks are usually rich in fossils, the remains of the animals and plants, being preserved in rocks.

Metamorphic rocks are formed from existing igneous or sedimentary rocks or even from existing metamorphic rocks. When existing rocks undergo changes due to high pressure and/or high temperature and/or high shearing stresses, metamorphic rocks are formed. Usually metamorphic rocks are formed deep in the earth. Heat comes from magma, while pressure comes from the layer of rocks on top of the other layers. Metamorphic rocks are classified based on foliation as foliated rocks and non-foliated rocks. Foliation means the existence of series of parallel surface. These rocks usually contain crystal. Gneiss, slate, marble, and quartzite are some of the metamorphic rocks.

Where is it?

There are a number of places within Lincoln Cathedral where you can see examples of Purbeck Limestone.

1. The Font

By contrast, this black font is not made of Purbeck Marble, but rather was imported from Tournai in Belgium in the 12th century. It stands on a base of Derbyshire Carboniferous limestone called ‘Ashford Black Marble’, also not an actual marble but not from Tournai either! Tournai fonts were very common in the 12^th and early 123th centuries. You can find the font in the south-west corner of the nave.

2. The Nave

Many of the large pillars supporting the building are in part made from Purbeck Marble. Again, this can be easily distinguished from the surrounding stone by its black, lustrous sheen. There are some good example towards the east end of the nave.

3. The Choir

If you stand in the main choir and look up to the upper arches of the clerestory and triforium, you will see many examples of shafts of Purbeck Marble, again standing out against the creamy-gold of the surrounding stone.

While you’re here, see if you can find the pre-14^th century dumb-bell shaped Purbeck marble slab set into the floor inscribed ‘Cantate Hic’ (‘Sing here’) where the singer of responses in the choir stands to use the acoustics to best advantage. This stone is known as The Bone- can you see why?

4. Chapter House

The large, central, decagonal pillar is surrounded with shafts of Purbeck marble, their dark, polished surface in stark contrast to the main structure of oolitic limestone

The Tasks

To claim this EarthCache you must answer the following questions and send me your answers either by email or, preferably, using the geocaching.com Message Centre. You do not need to wait for a response before you log the cache. I aim to respond to all messages but sometimes it might take a couple of days so please be patient. ALl that is required is a good attempt, so please don't worry if you're not a confident geologist. The whole point of earthcaches is to learn somethiong new. I may respond with a few clarifications but will only remove logs where no real attempt has been made.
 

Q1. After having looked at the examples of Purbeck stone above, describe the differences between this stone and the main strcuture of the Cathedral in terms of colour, texture and composition.

Q2. Look for any fossil remains in the Purbeck limestone. Describe where you found it and how big it is. What creature do you think it was?

Q3 (a) After reading the information above, tell me whether Purbeck Limestone is metamorphic or sedimentary?

(b) Considering your answers to Qs 2 and 3a, in what environment do you think Purbeck Limestone was formed?

Q4. (Optional) Without posting spoilers, attach a photo of yourself to your log which also features an example of Purbeck marble