For February’s blog, our Community Geologist, Dr Ian Kille, ventures far beyond the Wall to tell us more about Mystery Rock 11 from January’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.
Coming down the track were some local villagers. They were not the porters I had seen many times, often wearing flip-flops or bright purple wellies carrying cast iron pipes, water tanks, chickens and more. The porters tripped lightly with their massive loads, along the mountain trails which lead through the villages and monasteries around Manaslu up to the high passes of the Nepalese Himalayas and the 
border with China. These villagers were carrying slabs of stone on their backs, each slab many feet square, inches thick, made of the local metamorphic schists and used as roof tiles in the local houses. A little further up the track a cluster of villagers gathered around large stacks of the roof slabs taking their turn to pick up a load to be moved down the valley. I enquired of my guide, Roshan, what the reason for the mass movement of roofing material was. He told me that the villagers had found that the mountainside above their village was unstable and was going to collapse 
imminently in a landslide that would bury the village. The villagers, without any external help, had simply taken their stone-built houses apart, moved them down the valley to a safer place and re-built the village. What I was witnessing was the last step in moving the remaining roofing slates to the new village. I carried on walking up the track contemplating the stoic, hard-working approach of the villagers and the challenges of living in this remote mountain landscape. A few hundred yards up the track we came to the edge of the landslide. A new track had already been trampled, snaking its way precariously through the morass of sand, mud and rock and showing us our route on up the valley. A tongue of loose material thousands of feet long had spewed down the side of the mountain, with rock shards of all sizes, smothering all before it including the previous location of the village.
The mountain landscape that I had been walking through on this trek around Manaslu, up to the high pass at Larkye La at 5106m was epic in many ways. As a geologist it constantly contrasted the seemingly static geology of the UK with a landscape that was active in all too human timescales and on a scale that was sometimes hard to comprehend. Just recently in Uttarakhand, a part of the Indian Himalayas, a probable glacial collapse on the Nanda Devi glacier caused a flood which ripped down the Rishiganga River wiping out a hydro-electric dam and killing some 150 people. 
In 2015 an earthquake centered at Gorkha, close to my starting point for the Manaslu trek, killed some 9,000 people with another 22,000 sustaining injuries and over 600,000 homes destroyed. There was frequent evidence on my trek for recent landslides as the still rising Himalayas are cut deep by boulder-choked rivers, cracked by ice and shaken by earthquakes. There was no question that this was a landscape which was actively being formed.
It is not all destruction though. The massive scale of erosion 
in the mountains creates massive amounts of sedimentary material. Within the Budhi Gandaki River, which provided my route in around the east of Manaslu, there were places where piles of sediment many hundreds of meters thick had accumulated. These in turn had been cut into by the river exposing the extent of these recent accumulations in 
crumbling cliff-faces by the river. Further down-stream where the valley spreads out from the mountain front, enormous volumes of coarse sand and gravel accumulates providing aggregate for concrete, made visible in the form of piles of locally made concrete bricks drying in the sun.
Further east in Nepal lies Kathmandu and the Bagmati River. This river is holy to both Buddhists and Hindus and winds its way around the east and south of Nepal’s capital city. The Bagmati river basin is sandwiched between the massive Gandaki basin to the west and the equally large 
Kosi basin to the east. Unlike its neighbours which count the highest Himalayan peaks within their patch (Everest is part of the Kosi basin and Annapurna is within the Gandaki basin) the Bagmati river is rain fed rather than gorged on glacial meltwater. In the Kathmandu valley it is a slow river running across the top of the pile of sand and clay that marks where a large lake has silted up, filling in and levelling the valley. This ancient lake not only gave the valley its fertility but provides the area with another valuable resource, clay. The town of Thime to the east of Kathmandu is the pottery making centre of the district, though clay work can be seen throughout the Kathmandu Valley. Just to the north of Bhaktapur (the city of temples 8 miles east of Kathmandu) there are brick making chimneys surrounded by vast piles 
of bricks, like a Nepalese version of Bedfordshire. The clay isn’t just used for pots, bricks and tiles. Part of the extraordinary architecture of Bhaktapur and the other towns of the Kathmandu valley includes intricate relief patterns made from moulds to create patterned bricks in terracotta-red. These are combined with wood and stone to produce beautiful relief panels, architraves and other features. This work which reached its apogee between 1500 and 1800 AD is part of the reason that the Kathmandu Valley was listed as a World Heritage Site. The citation includes this statement “These monuments were defined by the outstanding cultural traditions of the Newars, manifested in their unique urban settlements, buildings and structures with intricate ornamentation displaying outstanding craftsmanship in brick, stone, timber and bronze that are some of the most highly developed in the world”.
The sedimentary picture painted in the Himalayas has huge brush strokes illustrating in bold the different types of sedimentary process. It shows the power in the forces of wind, water, ice and earth movement to erode. It demonstrates in extremes the relationship between energy and sediment grain-size from the house sized boulders moved by the raging meltwater in mountain rivers to the finest particles of clay filling the still waters of ancient Lake Kathmandu. It also gives a vivid rendering of the karmic nature of sedimentary environments, the give and take that erodes and deposits, creating both destruction and valuable resources.
In a place of extremes like the Himalayas this is all made obvious, though these principles apply just as much in other environments. If we travel the many thousands of miles in distance back to the landscape around Hadrian’s Wall and back in time for 300 million years or so to the Carboniferous period, the same things were happening. The layered stratigraphy of the rocks, varying from sandstone through siltstone to shale and coal speak of the different energies of the environments in which they were laid down. The estimated 2km thickness of Carboniferous sediments preserved in the Northumberland basin points to the huge volume of sediments eroded from the Himalayan scale mountains to the north, the eroded roots of which can now be seen in the Highlands of Scotland. In the same way that the Newars of Kathmandu benefited from the lacustrine resources of the Valley, we too have benefitted from the stone, coal, clay, iron and limestone found in the Carboniferous sedimentary sequences.
The give and take of the Carboniferous sediments is more subtly drawn in the mystery rock in this month’s WallCAP newsletter. Imagine a river going into the dry season, such that the river dwindles into still 
discontinuous pools in which the remaining sand and clay settle out. The pools then dry out producing a crust of partially hardened silt. Sometime later the weather breaks, a massive storm lashes down soaking the landscape and creating flash floods which create fast flowing turbid rivers. When these rivers overwhelm the old dried out pools, the power of the flow rips up the partially hardened muddy sand breaking it into fragments and carrying them along in the flood. Further down-stream as the current wanes these muddy clasts are redeposited mixed in with sand carried easily in the fast-flowing water. This is this sort of deposit that we are observing in this month’s mystery rock. It is what geologists call a lag deposit and a nice example of the way that sedimentary rocks don’t just record a sequence of deposition but also one of erosion and redeposition too.
https://www.youtube.com/watch?v=I0o1AhGuwMg&feature=youtu.be