This earthcache takes you on a tour of some of the older buildings in Cambridge city centre and covers about a mile. It can be completed in around an hour on foot, not including the time required to properly admire the history and architecture of this incredible place. Much of the route is not accessible to vehicular traffic but is suitable for bicycles, strollers and wheelchairs, although some task points may not be reachable by someone seated in a wheelchair. Paid parking is available nearby. No point on the tour is more than 10 mins walk from market square. Refreshments are available along most of the route. The walk takes you past or near several other caches on your way around.
You will need an accurate length measuring device in order to complete the tasks. This cache can be combined with Cambridge Stones – Magma to form a loop. You can do them in either order but this one is chronologically first.
The History of Stone Use in Cambridge
There is a clear historical pattern to the usage of the various building stones, reflecting a combination of availability in the source quarries and feasibility and cost of transport to Cambridge. Local clunch was the first to be used, followed by oolite rubble and the very resistant Barnack oolite. Jurassic rocks were usually transported to Cambridge by water along the Rivers Nene, Welland or Ouse and the Fenland dykes. Oolites from Weldon, Clipsham, King’s Cliffe and Ketton were successively deployed when the Barnack deposit was worked out (1460). Use of Magnesian Limestone from Yorkshire (1446-61) in King’s College was only economic because of royal subsidy. When coastal shipping improved in the early eighteenth century, the Portland limestone from the south coast became an economic and fashionable alternative to the yellow oolites. With the nineteenth-century railways came Bath Stone and the Ancaster oolite, the latter much favoured by Victorian builders and restorers. The railways also brought slates from Wales and the Lake District, supplementing the Stonesfield or Collyweston "slates" and tiles.
The Tour
The stones that you will see along the way were all formed in some way that involved water. They represent geological activity over a period of more than 420 million years. That staggering span of time makes the entire history of human existance seem insignificant in comparison. Water is still doing its part in the lifecycle of the stones as it erodes them steadily and returns them to their constituent particles. The stones seen during the walk are part of some of Cambridge's oldest buildings, magnificent in themselves. Don't forget to take the time to admire them. Let's begin.
Walk SE along Downing St. and turn left into Tennis Court Rd. Proceed along Tennis Court Rd for 200m. On your left as you proceed is Downing College.
The distinctive "white" bricks in the 19th century terraced houses on your right were built using bricks made locally from Gault Clay, which underlies most of Cambridge. The clay was formed from muddy sediments during the Cretaceous Period, from 65.5 to 145.5 million years ago. The roof tiles are Welsh slate which was also formed from sediments but compressed into sheets by tectonic forces, over many millions of years. You may notice that some of the tiles have a slight purple tinge, caused by the presence of iron in the mud. Slate is a waterproof, metamorphic stone formed when the same forces that shape the earth itself stretch and compress the rock and cause some of the minerals within to change into flat, sheet-like minerals.
Proceed along Tennis Court Rd, turn right into Fitzwilliam St and walk to the end, where it meets Trumpington Rd. As you walk, notice the plaque to the right of the top of the door of number 22, commemorating where Charles Darwin lived on his return from his voyage on the HMS Beagle. The houses here also contain "white" bricks.
Opposite you to the right on Trumpington St. is the Fitzwilliam Museum, built between 1837 and 1848. It is mainly constructed from white Portland Limestone from the Isle of Portland in Dorset but around the doorway you will see a yellow limestone, called Ketton Oolite, has been used for decoration. Ketton is about 40 miles NW of Cambridge. These oolites were formed during the Jurassic Period, from 145.5 to 199.6 million years ago, in warm, tropical waters when the land that now forms Britain was near the equator and part of it was under the sea!
Oolites are very popular with architects because the ooids that form them are very evenly sized, causing the rock to split in a predicatable way. This means that a stonemason can carve intricate details into the stone without the stone crumbling.
Proceed NW along Trumpington St. for 150m to Little St. Mary's Church, passing Peterhouse, the oldest college in Cambridge, on your left.
The church was originally built from Barnack Stone, a Jurassic oolite from Barnack, near Peterborough but the wall facing Little St. Mary's Lane has been resurfaced with Stamford Oolite from Licolnshire. This oolite was formed in a shallow sea and in places the ripples formed by the movement of the water have been preserved. These stones have been "face-bedded", which means that the ripples, which were formed horizontally, have been laid vertically and are therefore visible in the wall. The ripples are identifiable by their smooth crests and troughs and should not be confused with the rougher saw marks on some stones.
Task 1: Look at the section of wall under the third window left of the church entrance. Find the stone which has the greatest distance between the ripple troughs and crests. What is that distance? Identify the stone's location by number of rows up from the bottom and number of stones in from the left e.g. third row up, fourth stone from the left.
Continue 220m along Trumpington St, crossing Mill Lane and Silver St. Opposite Silver St. is St. Botolph's Church, built around 1350. The building has been much repaired but it contains a range of materials including flint, limestone, Barnack Oolite, Clunch, a local type of chalk, and Carboniferous Sandstone probably quarried from around Cambridge.
Continue past St. Catherine's College on your left and Corpus Christi College on your right to the much more modern looking Keynes Building, at King's College.
The Keynes Building was built between 1965 and 1967. The white stone used to face this building is Portland Roach, an Upper Jurassic limestone also from the Isle of Dorset, which is characterised by the presence of large fossils. These fossils include bivalve and gastropod mollusc shells, as well as other marine invertebrates. Many of these are preserved as moldic fossils i.e. the impressions left when the shells dissolved after the rock had formed around them.
Task 2: Look at the horizontal raised bands of stone at the SE end of the building i.e. the left-hand side as you face it. Measure the length of the large (mussel-shaped) moldic mollusc fossil in the bottom left-hand corner of the second stone segment from the left in the lowest raised band of the wall.
Trumpington St. now becomes King's Parade. Continue 200m northwards past King's College and marvel at the grandeur that is King's College Chapel. Work on the chapel began in 1446 and was not completed until 1515. The lower part of the building is Magnesian Limestone from the Permian Period, 251 to 299 million years ago. King Henry VI, founder of the college, chose this stone because it came from quarries that he owned. After his death, work was stopped but resumed in 1476 using more local oolitic limestones. The change in stone is visible. If you look at the NE spire, around the height of the top of the main window, you can see that the stone changes in colour from grey to white.
Continue north along King's Parade for another 50m. On your right is Great St. Mary's Church, built using oolite limestone, and opposite it on your left Senate House, which was built using the finer Portland Limestone. The street furniture is modern, erected in 2000, and built from a Jurassic limestone from Lyon, in France. The stone contains calcite crystals and traces of purple colour caused by haematite, a mineral form of iron oxide.
Beyond Senate House, turn left into Senate House Passage. At the end of Senate House Passage is Trinity Hall Lane and facing you is Trinity Hall. The front of this building was refaced in 1852 in Ancaster Stone, a Jurassic oolite from Lincolnshire, after it was damaged by fire. The oolite displays distinctive cross-bedding, which formed when ripples in the original sediment moved on top of each other over time, visible as sloping parallel bands in the stone. Occasionally, faster moving water eroded the cross-bedding, leaving a series of horizontal bands visible on top of the sloping cross-bedded bands.
Task 3: Look at the three layers of blocks that run along the bottom of the wall to the left of the doorway, above the base layer. One of these blocks has been laid upside down. We know this because the horizontal bands left by the faster, eroding water are below the sloping bands of the cross-bedded sediment, rather than above as they were when they formed. Identify the stone's location by number of rows up from the bottom and number of stones in from the left e.g. third row up, fourth stone from the left.
Proceed northwards along Trinity Hall Lane and turn right along Trinity Lane. On your left is Trinity College and on your right, Gonville and Gaius College. Turn left into Trinity St. and walk 50m or so to Trinity Gate. This magnificent gate was built between 1490 and 1535. Above it is a statue of King Henry VIII, carved from Cretaceous Clunch, quarried at nearby Barrington. The clunch was deposited in the Upper Cretaceous Period, between 65.5 and 99.6 million years ago, and is formed from the fossil remains of single-celled planktonic algae called coccolithophores. It forms the core of many of the limestone-clad walls around Cambridge. Clunch is an unusual choice of stone for a statue due to its relative softness and you can see that this one is quite weathered.
Continue another 50m along Trinity St. as it becomes St John's St. On your left is St. John's College Chapel. The chapel is built from Ancaster Oolite with decorative red sandstone columns around the windows. The roof is Lake District Slate, which has a distinctly green colour. Contrast this with the slate you saw in Tennis Court Rd. This slate was formed in the Ordovician Period, from 443.7 to 488.3 million years ago, when layers of volcanic ash were heated and compressed. The ash contains the mineral chlorite, which gives this slate its green colour.
Continue along St. John's St to the intersection with Bridge St.. On the eastern side of the intersection, opposite right, is The Holy Sepulchre or Round Church. This famous church was built around 1130, making it one of the oldest buildings in Cambridge, and is one of only five round churches in England. The church is mainly Barnack Limestone and the roof is another Jurassic limestone known as Stonesfield Slate, from Oxfordshire. The limestone has a short lifespan compared to true slate, around 200 years.
Task 4: Explain why the roof of The Holy Sepulchre is tiled with Stonesfield Slate rather than the more durable Welsh Slate seen in Tennis Court Rd. or the Lake District Slate seen at St. John's College.
This walk ends here. From here you can continue along Bridge St. to the river (left) or the town centre (right), retrace your steps to a suitable point or continue your journey with Cambridge Stones – Magma.
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Terminology
Clunch is a local type of chalk.
Metamorphic Rocks are changed by heat and pressure deep underground.
Oolite is a type of rock, usually a limestone, that is composed mainly of ooids. Ooids are spherical grains that look like very round grains of sand. Today these grains form in places with warm seas, like the Bahamas.
Ooids are formed from very thin concentric layers of calcium carbonate which grow around a tiny rock or shell fragment. These form in a similar way to the thin layers of limescale (also calcium carbonate) that can build up in kettles. Ooids are very rounded because they are rolled by currents and waves on the seabed as they grow. Many of the limestones you will see along the way are formed from ooids.
Sedimentary Rocks are formed by the accumulation of sediment on land or in water. The sediment is buried and compacted by the weight of new sediment building up above it.
References
The Sedgwick Museum of Earth Sciences. Cambridge Geology Trail. Retrieved 6 August, 2012.
Nigel Woodcock; David Norman (20 Aug 2010). Building Stones of Cambridge: A walking tour around the historic city centre. Retrieved 6 August, 2012.