The Cambridge church was founded by the Norman fraternity of the Holy Sepulchre, who arrived in Britain with William the Conqueror. Impressed by the round style of the early Romanesque churches they saw in the Holy Land during the first crusade in 1097, the Normans brought the design to Europe and even as far as Cambridge. The Cambridge Round church is now just one of four surviving medieval round churches of this antiquity in Britain.
Following substantial additions, especially a heavy 15th century tower, which subsequently threatened to collapse, the church was extensively restored to its earlier Romanesque style in the 19th century. Central to the original form is the conical roof with its beautiful stone ‘slates’, whose slightly irregular texture and colour enhances the attractive picturesque look of the building.
Today, the slating of a roof generally implies the use of grey Welsh slates, which in geological terms are true slates, that is they are ancient seabed muds, which have been compressed by deep Earth processes into slates. They became prevalent in the 19th century, once Welsh slates could easily be transported around the country by boat, train and lorry. However, in earlier 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 slates in the true geological sense but limestones, which could be split into slabs thin enough to be used on a roof.
In Early Mediaeval times, the nearest building and roofing stones had to be brought from limestone quarries in the Jurassic age hills of Oxfordshire, Lincolnshire and Northamptonshire.
Barnack stone from Northamptonshire was used to build Peterborough cathedral (from A.D. 664) and subsequently became the main material for the construction of the abbeys and cathedrals of East Anglia, such as Ely Cathedral (1174-89), early Cambridge colleges such as Corpus Christi and the main fabric of the Round Church.
But the roof of the Round Church required the use of Collyweston Slate from near Stamford in Lincolnshire. Collyweston slate is again another Jurassic age limestone and has been quarried since Roman times. Once the surface outcrops of the rock were worked out, the quarrymen followed the seams of strata underground, despite the extra cost and risk involved. Collyweston limestone can be split into remarkably thin ‘slaty’ sheets that are only around 10mm thick and are ideal for roofing.
However, the Collyweston limestone did not originally form as thin sheets but has 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 our grimy and corrosive atmosphere, the slate deteriorates and needs to be replaced. Lack of new supply of Collyweston slate and the high costs of working it have resulted in the replacement of Collyweston slate by the nearest equivalent limestone slate, which has the same visual appeal.
This turns out to be Stonesfield ‘slate’, another Jurassic limestone of considerable geological interest, which also had to be extracted from pits and underground workings. It was in the 1960s that the Round Church was re-roofed with Stonesfield ‘slate’.
Stonesfield Slate is of great geological interest as it was the source of many historically important fossils, including some of the first dinosaur remains and oldest mammal fossils. But again Stonesfield Slate does not last for ever, weathering causes it to continue to split (delaminate), especially around the exposed edges and now after 50 years has had to be replaced.
Unfortunately, like the Collyweston Slate, the original Stonesfield ‘slaty’ limestone is now no longer available. Modern Stonesfield slate is slightly different but has the advantage of being accessible from surface outcrops and is extracted from opencast quarries.
The modern quarried Stonesfield ‘slate’ is over twice as thick as the Collyweston slate, which makes the roof much heavier but still produces the same visual effect. The repair and restoration of the Round Church roof has required the replacement of more than 80% of the original Stonesfield slates with the new Stonesfield slate. The work has been carried out by a firm of specialist roofers still based in Collyweston, near Stamford in Lincolnshire.
The Sedgwick Museum has extensive collections of the original Stonesfield Slate and its fossils, along with samples of the Barnack and Collyweston rocks in the Watson collection of building stones.
Cambridge Geology Trail, published by and obtainable from the Sedgwick Museum, Cambridge