This month’s post comes from WallCAP’s Community Geologist, Dr Ian Kille.
Below Ian gives a fascinating explanation of the mystery rock picture posted in last month’s Hadrian’s Wall Community Archaeology Project newsletter. If you’d like to receive our monthly newsletter and get involved with our Stone Sourcing activities, sign up as a volunteer here.
Another mystery rock appeared for the Hadrian’s Wall Volunteers in the last Newsletter from the WallCAP team. It featured rock at the surface in a field with a pavement-like surface. A closer look at the surface shows that there are striations, fine grooves covering the surface of the pavement, all aligned in a south easterly direction and all parallel with each other. The field in question is at the top of a small ridge which runs parallel to the Tyne Valley.
As an experiment, take a lump of ice out of the freezer, and rather than placing it in the waiting G&T (or indeed placing one piece in the G&T and then proceeding anyway) try rubbing it over a piece of painted wood….nothing much happens apart from the wood getting a bit wet? Try again…same result except a bit wetter?
It’s looking like ice is pretty rubbish at eroding things, if it can’t even take off a thin layer of paint?
Try the experiment once more (additional G&T optional), but this time sprinkle a bit of sand on the surface of the painted wood…and this time the paint (and maybe some of the wood) gets lightly shredded?
About 10 years ago I was lucky enough to travel to Nepal. During my trip there I was able to do some trekking. One of the treks took me along a route around Manaslu, the eighth highest mountain in the world which has the fearsome reputation of having the highest percentage of deaths per attempted ascents. Not surprising looking at it. Part of the trek was to take me over the Larkye La Pass at a little over 5000m and amongst glaciers. I eagerly anticipated that I would see brilliant blue ice reflecting the indigo of the high-altitude sky and that crampons and ice-axe would be deployed in style. Disappointingly, though there were many other compensations, there was scant ice and snow. You will already have noticed the picture below which shows that the glaciers are a complete mess of rocks, stones, gravel and sand. The glacier is acting as a conveyor belt for this massive burden of rock luggage being transported down the mountain.
This tells us many things about how powerful freezing water is at breaking up rock and how ice becomes fluid in glaciers. It also tells us that glaciers and ice-sheets are not just ice, they carry sediment too. Going back to our ice-cube experiment, it is not surprising, when you consider the weight of 500m plus thickness of ice and the payload of assorted sediment, that ice-movement is such a powerful force in shaping landscapes.
The ice-groves – which is what the mystery rock has engraved in its surface – tell us the direction the ice travelled in. When this is mixed with other landscape forms created by ice and meltwater as well as analysis of the types of rocks found in the sediments deposited by glaciers, it is possible to build up a picture of where ice-sheets used to flow across the landscape in and around Hadrian’s Wall (and elsewhere!).
The mystery rock field is located just to the south of the Wall near Heavenfield, not far from a low-lying cliff-face named the Written Crag. This location is wrapped around by a large bend in the River North Tyne to the west, just before it joins the River Tyne. This marks the point where ice-flow from the west flowing along the Tyne Valley met the ice-flow from the north and west which flowed down the River North Tyne. Here at Written Crag, formed from a ridge of harder rock, the ice was forced up over with ridge where the ice is at its most erosive.
The sub-surface geology and its history of formation and subsequent folding, faulting, uplift and down-warping are fundamental to understanding a landscape. However, the final shaping of our landscape, sometimes in a radical way, is largely a function of ice. The combination of its awesome power of erosion along with its phenomenal ability to transport and deposit vast amounts of sediment, make it an extremely important geological process.