We’re back to earth (more specifically the Himalayas) for this month’s blog from our Community Geologist, Dr Ian Kille. Read on to find out more about the effect of ice on our landscape and Mystery Rock 14 from last month’s newsletter. If you’d like to receive our monthly newsletter and get involved with our Stone Sourcing activities, sign up as a volunteer here.
The taxi drivers at Arughat Bazar wished to proclaim their faith with slogans emblazoned on their battered cars like “Jesus Dead For You” (sic). It could also be taken as a warning of what lay ahead on the narrow winding dirt roads with vertiginous drops of hundreds of feet to rocks strewn in the powder blue Budhi Gandaki River in the gorge below. This heart-stopping drive took us to the start of a 14-day hike around Manaslu in the Nepalese Himalayas. The Himalayas are a good place to learn new perspectives and to let go of preconceptions. The sheer scale of the place provides a cold and implacable sense of one’s place in the world.
This is a story I have already told in an earlier blog (“Scratching the Surface”) but it seems to me worth re-telling to set the scene for explaining this month’s WallCAP mystery rock. One of the (many) preconceptions I brought with me to Nepal was about glaciers. From the pictures I had seen I assumed that they would be snow-strewn and that in the crevasses you would be able to see icy layers turning deeper shades of blue. In preparation for the summit of the Manaslu Circuit at the Larkye La pass I had asked my guide, Roshan, about crampons, but he had said they wouldn’t be necessary. This puzzled me at the time, but he was oh so right. There was some snow and some ice as we reached the pass just after sunrise, but the way was mostly covered in rock debris ranging in size from boulders to sand. This was in part because the bits of the glaciers we were walking on were at relatively low 
level in the zone where the argument between snowfall and melting is played out. It is however also to do with ice’s phenomenal ability to break up and move rock around.
Not only were the glacier surfaces we walked on covered in rock of all grain sizes, but around the margin of the glaciers were huge ridges of sediment. These marginal moraines were more prominent as the surface of the glacier was much lower than the moraine ridges. Each year the glacial tongue will extend during the colder months as more snow, compacting to ice, flows down the mountain side. As the seasons turn and the sun fights back against the ice, melting becomes faster than the ice flow. This 
annual argument is usually balanced, but now it is turning in favour of the warming climate and these glaciers are clearly in retreat, leaving behind massive piles of rock debris.
Mountain glaciers are impressive, not least for their mountainous setting, but they are not on the same scale as the polar ice sheets. Where the mountain glaciers measure in the tens of kilometers, even the relatively small ice sheets of Iceland measure in the hundreds. Flying to San Francisco many years ago the view down to Greenland was clear and cloudless and we flew for hours across unbroken ice.
When thinking about the effect of ice on our landscape in the Hadrian’s Wall corridor, it is this sort of ice that would have been the cause of many of the landscape features we now see. The margins of Iceland’s 
icesheets as they retreat provide a good analogue for what we see here.
Ice is a powerful tool, and this is because of two things. The first, that within the normal temperature range on our planet it passes between a solid, a liquid and a gas. The second is that when it solidifies from a liquid, it expands. This is very unusual and has consequences. The first is that it operates like a natural crowbar. Water that infiltrates rock, when it freezes, will happily break open even the hardest of rock types. The second is that it floats on top of its own liquid, water, rather than sinking. Without this feature ice-skating would never have become a thing and not only because of the skating-surface that ice forms as it floats. There is yet another feature, which is the way that ice responds to pressure. Usually, pressure will compact a solid so that it becomes more difficult to melt, however ice under pressure will tend to melt. This 
is what happens at the knife edge of skate-blades, where the intense pressure this causes against the ice results in the formation of a thin layer of water, lubricating the skater’s path. Without this, the steel blade would stick to the ice, and the skater’s glide would become a skater’s face-plant.
A similar principle applies to icesheets. There is a balance of pressure and temperature where the advance of the icesheet is enabled by a layer of water at its base. It is not only the ice-fractured rock which falls onto the ice which creates rock debris, but the water and ice underbelly of the glacier which grinds and flushes out the rock. We can see examples of this within the Hadrian’s Wall landscape, for example in one of the early WallCAP Mystery Rocks from near to Heavenfield. Here the surface of sandstone has been flattened and smoothed, and linear grooves have been raked into its surface, all by the action of ice.
What happens, then, when the ice melts? At the poles where ice flows out into the sea, as the ice melts, the rocks contained in the ice fall to the sea floor. These random stones are referred to as dropstones and can be seen in the arctic deep. They can also be found in the geological record and are one of the pieces of evidence used to show that there have been a few periods of the earth’s history where the entire planet has been glaciated – popularly named as Snowball Earth.
On land we can see the systematic remains of glacial action and retreat. Ground moraines and the drumlins that are created from them by the continued movement of the ice, scatter the landscape. Other features like eskers, and subglacial channels which have formed as a result of water-movement under the ice leave ridges and furrows in the landscape. As the ice retreats, the meltwater from the ice is added to the prevailing precipitation creating huge volumes of water. Sometimes this may be trapped by retreating ice to create lakes and consequent flat-lying lake 
deposits. This water also creates large high energy river channels which rework the glacial debris leaving behind meandering deposits, which often border contemporary rivers now much shrunk in size. Around Hadrian’s Wall this means that most of the landscape has a covering of one sort of glacial deposit or another.
Ice’s rock-moving super-power is manifested not only in the massive volumes of rock debris it creates, but also in the distance it moves this material and the incredible size of some of the boulders it drops into the landscape. One such 
boulder is the Loch Maben Stone which features as this month’s Mystery Rock for the WallCAP project. This huge, rounded stone is made of granite, a rock type which is foreign to the bedrock it overlies (the Triassic St Bees Sandstone). The nearest granite to the Loch Maben Stone is approximately 30km away, beyond Dumfries. Glacial erratics like this come in all sizes, but they have all wandered from their place of origin within a carpet of ice, hence the name erratic. Erratics weighing many thousands of tons have been recorded as well as pieces that have wandered for thousands of kilometers. On the Northumberland Coast It is possible to find a particularly characteristic volcanic rock, with elegant, elongated phenocrysts, which have travelled from Norway. This means that the glacial ice not only walked 500 miles and walked 500 more but also walked on 
water for this stone to fall down on the Northumberland shore.
These erratics were one of the pieces of evidence that convinced Louis Agssiz, a Swiss geologist of the 19th century, that there had been a past ice-age during which glaciers had extended all over Switzerland from the Alps and out into the plains of Europe. Working with William Buckland, the only person in Britain he could convince of his ideas, they looked for evidence of glaciation in Britain and concluded that all of Scotland and Ireland had been covered in ice. These findings were published in 1840 in a 2-volume work “Études sur les glaciers”. This was a moment in the history of geology where pre-conceptions about past climate took a radical turn, providing a crucial piece of understanding to build our contemporary picture of climate change.
Back to more local concerns, it has been fascinating spending time at the recent Heritage at Risk dig undertaken by WallCAP at the Cambeck Crossing, the first this year with volunteers. Near to the trace of the wall, an area was uncovered containing cobbles which were probably sourced from the nearby riverbed in which there are sizable banks of water-worn cobbles. Examination of these cobbles shows them to be made of sandstones (including some distinctive poorly sorted grey sandstones which are likely to be turbidites), low grade metamorphosed sediments and granites. These are all erratics and likely to have been derived from the Ordovician and Silurian terranes and Devonian granites to the north west. I discussed the sequence of events that brought this about with the volunteers: the coming of the ice and how it levelled the landscape, leaving behind a rich variety of glacial erratics and how this material would have been re-worked by the rivers. We also discussed how lifting the weight of the ice allowed the land surface to rise, 
like a super-tanker being unloaded, so that the Cambeck cut down through the glacial material and the soft sandstone underneath to create the gorge we now see. That our landscape, which seems set and permanent, can change so much in a relatively short time is remarkable and the role which climate change plays in this important. With the rate of man-made climate change exceeding that of the last ten thousand years what will be the consequent changes that we will see? It seems we have reached another moment in geological history where there is a need to revise our preconceptions and take action.
Attributions
Example of a glacial dropstone from Namibia, in rocks that date to the second Snowball Earth. The stone was likely carried and dropped by a floating ice shelf, and when it plunked into seafloor sediment below, that sediment folded around it. (Penny shown for scale.) Image by Paul Hoffman in: https://astronomy.com/news/2019/04/the-story-of-snowball-earth