Distant origin of cobbles at Cambridge’s Trinity College revealed
As well as admiring Cambridge’s historic buildings and beautiful views, we should also look down at what lies beneath our feet, according to Trinity College fellow in Earth sciences, Professor Marian Holness.
She became tired of tourists only looking up at the statue of the University of Cambridge college’s founder, Henry VIII above the Trinity’s Great Gate, when in her opinion the real highlight was the remarkable cobbles below, which have turned out to be a remarkable geological time capsule. Now they have inspired her to make a film about their story.
Laid out by sixteenth-century builders, and worn flat by cartwheels and horses hooves across the centuries, some of the Trinity cobbles are reddish brown – rare rhomb porphyry rock which is found only in Antarctica, East Africa and Norway.
Professor Holness says: “There is a great story to be told about the cobbles outside the Great Gate, which are a mix of hard rocks. 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 roadbuilding in medieval times, would involve the use of material that was cheap and ready to hand. These cobbles must have come from close by. 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, it turns out, is that the rocks were transported from Norway by a glacier during the greatest of the ‘recent’ Ice Ages – 480,000 years ago.
Then, much of northern Europe was covered in ice, the sea level was 120 metres lower than today and the North Sea did not exist.
She adds that the cobbles’ shape is natural. “Unlike more modern areas of cobbling and Cambridge, which use trimmed stones with a square or rectangular shape, these cobbles are variable in shape and size. We're looking at natural materials that haven't been modified by the road builders, but just used as they found them.”
Henry VIII established Trinity College 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.
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 stones were found along the edge of fields that had been cleared by workers. This stopped after the Black Death in 1340 as there were no longer enough workers alive to do the field clearances.”
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.
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.”