We often admire Cambridge’s historic buildings and beautiful views. But we should also look down at what lies beneath our feet, says Trinity Fellow in Earth Sciences, Professor Marian Holness.
And nowhere more so than the cobbles in front of Trinity’s Great Gate. To the untrained eye they might like boring brown stones, but they are a remarkable geological time capsule.
Laid out when Great Gate was built in 1547, the cobbles are a medley of flint, igneous rocks such as basalt and granite, and metamorphic rocks. Perhaps one of the most striking, to a geologist, is a reddish-brown rhomb porphyry.
The flints apart, all the rocks making up cobbles are from places hundreds of miles away from Cambridge, says Professor Holness.
The nearest metamorphic rocks like this are in Scotland, the nearest basalts are in the Lake District and the rhomb porphyry – a very rare rock type – is only found in Antarctica, East Africa and in Norway, near Oslo.’
So the big question is how material from Scotland and Norway made its way to the Cambridge region all jumbled together for easy collection by the road builders.
The answer lies deep in the past and began to be unearthed by Cambridge scientists in the nineteenth century.
When Henry VIII established ‘the Holy and Undivided Trinity’ in 1546, amalgamating two earlier foundations of King’s Hall and Michaelhouse, a road ran from what is now Trinity Street through the College to the commercial wharves on the river.
The carpet of cobbles in front of Great Gate was created by re-using the stones from that road.
Road builders of the sixteenth century wouldn’t have known their origins but by the nineteenth century Cambridge scientists had discovered where and how these stones ended up in Cambridge.
There are clues in the triangular shape of many of the cobbles and the presence of the rhomb porphyry.
‘Triangular rock fragments are characteristic of those transported by a glacier. The rhomb porphyry is so localised to the Oslo region that if we find a rhomb porphyry cobble elsewhere in Europe, we know the ice must have flowed from Oslo to near where the cobble was found,’ says Professor Holness.
So the cobbles at Great Gate date from the greatest of the recent Ice Ages, 480,000 years ago, when much of northern Europe was covered in ice and the sea level was 120 metres lower than today.
A continuous sheet of ice flowed from southern Norway, down through Denmark and northern Germany, and across where the North Sea is now, reaching the northern part of East Anglia. Somewhere along its route it was joined by an ice stream moving south from Scotland,’ says Professor Holness.
‘The vast ice sheet of half a million years ago ended up dumping materials, both brought down from Scotland and all the way from Norway, close to Cambridge.
The builders of that ancient road chose their materials carefully, says Professor Holness. ‘They collected the cobbles from the fields nearby, and used only the types of cobbles which could withstand the metalled hooves of horses and carts, unlike softer sandstone and limestone which they used for buildings.’
The deposits of glacial material in the Cambridge area, as typified by the Great Gate cobbles, have been very important in helping scientists to map the course and extent of ice sheets. But geology is also vital for the present and future, says Professor Holness.
Geology is vitally important to us right now, as it gives us the window back into Deep Time and tells us how the climate has changed over the last 4.55 billion years. This information is essential if we are to understand what humans are doing to the climate now.
Tressilian Charles Nicholas
Eagle-eyed visitors will spot a cobble inscribed TCN just in front of Great Gate. It commemorates Tressilian Charles Nicholas (Tress), former Senior Bursar and the first Research Fellow in Geology at Trinity College.
Following his degree at Trinity, Nicholas undertook fieldwork in Caernarfonshire and the resulting dissertation gained him a prize Fellowship in 1912.
During World War One he served with the Royal Engineers at Gallopoli, where he met TE Lawrence (of Arabia), as well as in Flanders and Egypt. Nicholas was awarded the OBE and MC for his war service and returned to Cambridge in 1919 where he was re-elected to the Fellowship he would hold for the next 70 years.
He gave up teaching geology to become Senior Bursar in 1929 and was instrumental in the purchase of the farm land on which Felixstowe Docks were built.
Nicholas retained his interest and enthusiasm for geology throughout his life. The Sedgwick Museum contains over 1000 specimens collected by him.
After the death of his wife in 1969, he moved into rooms in Great Court and died aged 101 in 1989. One of his last requests was that the cobble stone of red Rhomb Porphyry, which he had passed every day for 80 years, be inscribed with his initials.