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 19^th 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

@Northumbrianman

The post Snowdrops appeared first on WallCAP.

Bedtimes were strict when I was 11, so there was a going-on-holiday sense of excitement when we were woken in the early hours of the morning. Still drowsy, my older brother Steve and I were brought down to sit in front of the black and white television to watch the first man step out onto the moon’s surface. We had followed the whole mission on the BBC from the launch 5 days earlier and watched as the Lunar Excursion Module had separated from the Command Module on the evening of Sunday 20^th July 1969.   I can remember little of the BBC programme other than the air of excitement and the strange other worldly images that unfolded as Neil Armstrong made his famous small step speech as he bounced onto the moon’s surface. For an eleven-year-old me, the extraordinary achievement was merely what was happening in front of me, my normal. Now, I look back on the levels of danger involved, the near total vacuum and absolute zero emptiness of space, the solar radiation, the primitive computing technology, the nail-bitingly fine margin of fuel and the absolute hostility to life of the moon’s greyscale airless surface and I am amazed.

The rock samples brought back have been examined and analysed and we now know a great deal more about the ancient igneous surface of the moon and how it has been scarred by impacts. The favoured theory for its formation has the earth colliding with another smaller, Mars-sized planet named Theia about 4.5 billion years ago. Both planets had already accreted enough mass and heat to have differentiated into a metallic core and rocky mantle. The glancing impact resulted in a significant chunk of Theia’s core being dumped inside the earth with the remaining fragmented core and mantle material ejected to form a disc of material rotating around the earth. This fragments then accreted to form the moon with the multiple impacts generating so much heat that the surface of the moon down to a depth of maybe 500km became a magma ocean. The subsequent history of the moon is dominated by two processes. The first is a consequence of asteroid and comet impact, the results of which are an obvious feature of the cratered surface of the moon. The order in which impacts happen can be worked out from the way that ejecta from impacts overlap allowing for a timeline to be calculated for both the impacts and the features they interact with. The size and frequency of impacts has reduced significantly over time after a particularly intense phase known as the late heavy bombardment between 4 and 3.85 billion years ago. The second process is the progressive cooling of the moon with its magma ocean consequentially crystallising, with the heavier minerals (olivine, pyroxene) sinking and the lighter ones (plagioclase feldspar) floating, a process called fractionation. The paler grey uplands of the moon owe their colour to these lighter, feldspar rich fractionates. Volcanism is part of this process and vast amounts of magma (and some explosive pyroclastic material) flowed into lowlands on the nearside of the moon to form its “seas” cooling to form the dark basaltic plains of the moon’s Maria. Most of the Maria were formed between about 3 to 3.5 billion years ago but some volcanism may have carried on until about 1 billion years ago, 500 million years or so before multicellular life started to evolve on earth.

The moon is very different from the earth, it has no atmosphere, no magnetic field, no tectonics sufficient to rejuvenate its surface, very small amounts of water and no life. Yet there is a great deal we can learn from the moon’s geology to help understand the Earth’s pre-history. Not least is that early collision with Theia which tells us part of the story of the internal structure and geochemistry of our earth. The Moon’s collision-scarring tells another hidden part of the Earth’s past. The Earth’s tectonic activity means that the surface of our planet continues to be refreshed and reworked. The bombardment the moon suffered would have been equally inflicted on the Earth, however on Earth the scarring has since largely been erased. On a broader scale, our knowledge of the moon’s geology confirms and enrichens our understanding of the way that planets accrete and differentiate. The moon’s early history as a hot tectonically active planet before it cooled and died confirms and informs our knowledge of igneous processes.

50 years on from the Apollo moon landings, there have been no repeat performances. In the intervening time, humans have only ventured as far as the earth’s orbit. This underlines the achievement of the Apollo program not only technologically but also politically. It is only now with a much more robust baseline of technology, a firmer “foothold” in the Earth’s orbit and with the growing involvement of private enterprise that the prospect of manned exploration is in sight, this time to Mars.

Part of the reason for writing this blog now is the current attention on Mars as Perseverance, the  Mars rover and its sidekick the Insight helicopter send back yet more astonishing images of the planet. Mars has had quite a lot of attention from unmanned spacecraft in the years since Apollo, with orbiters and several landings with robotic exploring devices.

Mars has more affinity with Earth than the moon albeit doesn’t have the rather exotic collision and core-sharing co-history. Mars in common with the other rocky planets (Earth, Venus and Mercury) has a metallic core a rocky mantle and a crust. As it is larger than the moon its tectonic history stretches almost to the present albeit in its last throes. It still has an atmosphere, and this leads to the interaction between its atmosphere and Mars’ considerable topography. With lower gravity than the Earth, Mars has been unable to hang onto much of its atmosphere which has progressively spun-off into space, leaving a carbon dioxide remnant to kick up dust-devils. There was a time, however, when Mars’ atmosphere was more like that on Earth and there was free water and ice. What this means is that on Mars, we not only see the volcanic activity and the impact cratering but also an impressive range of sedimentary features.

One of the images for this month’s mystery rock for the WallCAP project is an example of this. We know from looking at terrestrial examples that when water or air transport sedimentary material, that they deposit that material in layers. Sometimes, when ripples or dunes are created these layers will be in the form of cross-lamination. The image on Mars was taken by the Curiosity rover and is of a rock outcrop named Mont Mercou which is located within Gale Crater on the slopes of Aeolis Mons (also called Mount Sharp). Aeolis Mons is thought to be the remnants of a large stack of layered sedimentary material. In the image of Mont Mercou you can see that it has two parts. The upper portion has cross-bedding in it, which is similar in form to that seen in the other image for this month’s WallCAP mystery rock. This is a picture of wind-blown, aeolian dune deposits from the Yellow Sands Formation at Tynemouth. The lower portion of Mont Mercou is a stack of horizontally laminated sediments which remind me of the layering which can be found in the sediments at Ladycross Quarry. The Martian deposits have been interpreted as lake deposits which progressively infilled Gale Crater after its formation between 3.8 and 3.5 billion years ago. To make these sorts of lake deposits would require the repeated, rhythmic influx of flowing water to bring the layers of coarser sediment into the lake. These coarser laminations are separated by thin bands of finer material which imply periods of limited water movement in which these finer grained sediments could settle out. There is an active debate about exactly what processes are reflected in the images being sent back from Mars and I am looking forward to following this story as more information becomes available and the debate continues.

The astonishing range of images and data which Curiosity, Ingenuity, Perseverance, Insight and their ilk are bringing us seems to have become my new normal. There is so much more that could and may well be explored not only within our nearest rocky neighbours but also out to the distant gas giants with their extraordinary moons as well as to the remote ice planets. Yet the long intermission of lockdown has also meant that my other new normal in the physical tangible world, the one that I can touch and see and smell and hear, has been focused down to what may be easily reached from North Northumberland. What is clear, is that I am not going to run out of interest between the local, tangible and touchable and the huge variety of what may be accessed via the low carbon format of the internet.

Acknowledgements and further information

You can follow Curiosity Rover on this interactive map here: https://mars.nasa.gov/msl/mission/where-is-the-rover/

Moon image by Gregory H. Revera – Own work, CC BY-SA 3.0, httpscommons.wikimedia.orgwindex.phpcurid=11901243

Vandelinus Crater: By James Stuby based on NASA image – Reprocessed Lunar Orbiter 4 image cropped in Adobe Photoshop to show Vandelinus crater and surrounding terrain. The original image should be public domain as it is a work of the U.S. Government (NASA). Immediate source: Lunar and Planetary Institute, Lunar Orbiter Photo GalleryLunar Orbiter 4, image 184, h1 [1], Public Domain, https://commons.wikimedia.org/w/index.php?curid=30497934

Anorthosite: a coarsely-crystalline anorthosite from Labrador from the Ten Mile Bay Quarry, near the town of Nain along the Labrador coast, eastern Canada. The quarry exploits the Nain Anorthosite (Nain Plutonic Suite), a mid-Mesoproterozoic intrusion (1.29 to 1.35 billion years) emplaced along the Abloviak Shear Zone. https://commons.wikimedia.org/wiki/File:Blue_Eyes_Granite_(anorthosite)_Labrador.jpg

Barringer Crater Arizona: By National Map Seamless Server – NASA Earth Observatory, Public Domain, https://commons.wikimedia.org/w/index.php?curid=7549781

Mont Mercou image edited by Kevin Gill: https://twitter.com/kevinmgill

Mars Image: By ESA & MPS for OSIRIS Team MPS/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA, CC BY-SA IGO 3.0, CC BY-SA 3.0 igo, https://commons.wikimedia.org/w/index.php?curid=56489423

@Northumbrianman

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