This is an urban EarthCache in which geocachers are invited to examine the roofing of a church in the centre of Cambridge. The learning point of this EarthCache is to become familiar with slate and limestone.
Everything you need to answer the questions is available by attending the co-ordinates and reading this lesson. I don’t anticipate you will have to research anything extra online, although you’re welcome to do so if you wish to.
There are roads close to GZ, so please be careful around them. There’s no need to enter the church, (although it is lovely inside!) If you choose to go in there will be a small charge for most visitors to cover the cost of maintaining of this beautiful and ancient building.
Welcome to the Round Church!
The Round Church, or more formally titled the Church of the Holy Sepulchre, s an ancient landmark in the centre of Cambridge. It is located on the corner of Round Church Street and Bridge Street and has been a Grade I building since 1950. It is famous for its unusual circular form. Dating from around 1130, the Round Church is one of the oldest buildings in Cambridge and is one of only four surviving medieval round churches that survive in Britain. (The others are Temple Church in London, St. John the Baptist Church in Little Maplestead, Essex, and the Holy Sepulchre, Northampton.)
The Round Church of Cambridge was founded by the Norman Fraternity of the Holy Sepulchre, who arrived in England at the same time as William the Conqueror. The Crusades had begun a couple of decades earlier, and as the Normans returned from the Holy Land, they copied the building style. The Round Church shares features with the 4th century Church of the Holy Sepulchre in Jerusalem.
Over the centuries, the church has been extensively extended, repaired and rebuilt. In the 15th century a square tower was added that was so heavy, it almost collapsed. In the 19th century, the church was extensively restored to its earlier Romanesque style including the replacement of the conical tower that you can see today. The roof of this conical tower features some old weathered ‘slates,’ which are the subject of this EarthCache.
Minerals make up rocks. Rocks are formed in many different types of environment. These can be on, or within the Earth's crust. There are three types of rock, and each is formed in a different way.
Sedimentary rocks are formed on the Earth’s surface from the products of weathering which then becomes cemented or deposited. Limestone is an example of a sedimentary rock.
Igneous rock is formed within the Earth’s crust, or on it’s surface. It is formed by the cooling of magma (molten rock.)
Metamorphic rocks are formed inside the Earth by temperature and pressure changes that affect existing rocks. Slate is an example of a metamorphic rock.
Metamorphic rocks are igneous, sedimentary, or preexisting metamorphic rocks that are now inside the Earth. Sometimes they are within the crust and sometimes the upper mantle. At that depth, rocks are subject to great pressure and temperature. Although very great, the temperature is still not high enough to melt the rocks, because otherwise igneous rock would form. The pressure is much greater than that required solely to break the rocks up. In fact, the pressure is so high that it changes the chemical make up of the rocks by forcing the elements in the minerals to change position. This may take thousands or millions of years.
Metamorphic rock needs either pressure or temperature, or both of these occurring together, to form.
Different grades of temperature and pressure will cause the same original rock to form very different metamorphic rocks.
Slate, which forms from a sedimentary rock like shale or mudstone, appears as a very dense and smooth rock. It is a hard rock with a well-developed slaty cleavage. This means it has extremely closely spaced, parallel planes of weakness that give slate its ability to split into very thin, platy layers. Slate is a product of low grade metamorphism, which means that it can form in temperatures and pressures less than other types of metamorphic rocks. Low grade metamorphic rocks typically are very fine grained. Slate can be many different colours, often black, blue, green, red and brown. It is smooth to the touch.
However, in medieval times the cost of transporting stone building materials was so high that any appropriate local stone was used. The nearest ‘slates’ to Cambridge were not ‘true’ slates in the true geological sense but in fact various kinds of limestone, which could be split into slabs thin enough to be used on a roof.
Many limestones formed when sediment gathered on the floor of ancient warm, shallow seabeds. Limestone principally consists of clean-washed sand-sized grains of calcium carbonate. Some of these grains came from the shells and calcified skeletons of sea creatures. Others grew there, on the seabed, as the mineral calcite was precipitated around smaller particles (in the same way that limescale might build up on the inside of a kettle.)
In early medieval times, the roof of the Round Church was made from a rock called Collyweston ‘Slate,’ which as discussed, is actually a fissile (easily split) limestone. It was quarried near the town of Stamford in Lincolnshire. Nevertheless, like true slate, Collyweston 'Slate' can be split into remarkably thin ‘slaty’ sheets that are only around 10mm thick and are ideal for roofing.
Collyweston limestone does not naturally have the slaty cleavage that ‘true’ slate has, so to get the cleavage desirable for use as a roofing material, the quarried stone needs to be matured and split by natural processes. The rock was recovered in large and relatively thick slabs, which were left outside and exposed to overwinter freezing and thawing. The development and expansion of ice crystals along the natural layering of this ancient sedimentary rock, split it into thin slabs, ideal for use as roofing ‘slates’.
However, even Collyweston ‘Slate’ does not last forever and after a few centuries, exposed to physical and chemical erosion from the atmosphere, the slate deteriorates and needs to be replaced. Production of Collyweston ‘Slate’ was in serious decline by the 19th century because the advent of the railways meant that true slate from Wales was now available at a competitive rate. Lack of new supply of Collyweston ‘Slate’ and the high costs of working it meant that the church guardians needed to find an alternative rock. (Welsh slate would change the church’s visual appeal.)
Much of the roof you see today is made of the rock the guardians chose. It was in the 1960s that the Round Church was re-roofed with Stonesfield ‘Slate’. Stonesfield ‘Slate’ is another Jurassic limestone, but one that is quarried underground in Oxfordshire. The town of Stonesfield sits on the Taynton Limestone Formation and the rock quarried there included some of the earliest dinosaur fossils to be found in Britain. In 1824 a Megalosaurus skeleton was found, and later remains to be found included pterosaurs, theropods and prehistoric crocodiles
But as with Collyweston ‘Slate,’ the Stonesfield ‘Slate’ that was installed in the 1960s is subject to chemical and physical weathering. And again, like Collyweston ‘Slate,’ the Stonesfield ‘Slate’ that replaced it is no longer available, forcing the church guardians into another dilemma.
Rock is still quarried in the Stonesfield area, but in modern times it tends to be from surface outcrops and is extracted from opencast quarries. It keeps the visual appeal of the Collyweston and first Stonesfield ‘Slates.’ Better still, it is more than twice as thick as the rock installed in the 1960s. However, it is much heavier. The long term effect of putting another heavy roof on the Round Church is still not known. To try to compensate some of the stress, not all of the roof has been replaced with the heavier rock.
To log this cache, please visit the published co-ordinates and answer the questions below. Once you have obtained the answers, please send them to me via email or through the Message Centre. You are free to log your find once you have contacted me. You don't have to wait for a reply. If there are any questions about your answers, I’ll contact you.
Logs without answers will be deleted. Please don’t include close up pictures in your logs that may answer the questions.
Please remember to be careful around the roads. The best view of the roofing ‘slates’ might be from the pavement outside the church, which is slightly raised from the door level of the church.
- Please look at the roofing ‘slates’ of the Round Church. What colours of stone can you see?
- What evidence of weathering can you see on the ‘slates’? Where on the roof is the weathering?
- These ‘slates’ have been hand cut and laid out by a master roofer and are not uniform in size. Please describe the general dimensions and layout of the ‘slates.’
- Please identify any new ‘slates’ that have replaced the 1960s renovation. Remember, these 'slates' are twice as thick as the earlier stone. What per cent of the roof ‘slates’ are new?
- Optional, take a photo of yourself and/or your GPS in the general area of this EarthCache.
Good luck, and thanks for visiting this EarthCache!
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For full information on how you can expand the Church Micro series by sadexploration please read the Place your own Church Micro page before you contact him at churchmicro.co.uk
See also the Church Micro Statistics and Home pages for further information about the series.
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