Nanga Parbat is one of only 14 mountains over 8,000m in height. It’s summit is a mere 25km from the Indus, a major river flowing off the Tibetan Plateau and then along the length of Pakistan. The mountain face between the two (the Rakhiot Face, above) is often called the world’s highest as there is a drop of 7,000m, from peak to river. Nanga Parbat sits at the Western syntaxis of the Himalaya, this is the point where the structural grain of the Himalaya changes from East-West to North-South. It corresponds roughly to the corner of the Indian plate (buried somewhere below), where the plate boundary swings around.

Nanga Parbat contains granites at the surface that cooled only a million years ago. Not lava but granite that was intruded at depth and yet is now at the surface. A active major thrust puts gneisses on top of river gravels. It’s estimated that 25km of crust has been eroded away in the last 10 million years (or 12-15km over the last 3Ma, take your pick).

A hard rain falling

In order to remove 25km of crust, you need a lot of erosion. That sort of erosion requires high rainfall and a major river to take all the bits away. Both of these things are present at Nanga Parbat. What is mind-expanding is the idea that the erosion and the tectonics are linked. 

The “tectonic aneurysm” model (defined in Zeitler et. al 2001) suggests that the tectonics of the area are caused by the high rainfall and presence of the Indus river valley. Consider a river valley cutting deep into a mountainous area – it’s made a kilometres deep hole in the crust and this weakens the rocks below and either side of it.

To state the obvious, remove rock from the surface and the material below moves closer to the surface. This does two interesting things – firstly the reduction in pressure makes brittle rocks weaker. Secondly, the valley will change the thermal structure of the crust. Cutting a hole into the crust will bring hotter rocks closer to the surface (because the surface is lower). Then if the rocks beneath the valley start moving up (a process called advection) then they may move up so fast that they can’t cool down on the way. There’s more. Releasing pressure in very hot rocks (decompression) can trigger melting and granite production. The presence of melt reduces the strength of the crust by an order of magnitude. This means that the crustal flow will become easier as the rocks become weaker.

Erosion causes uplift which causes crustal flow which causes more uplift which causes more erosion which….. This positive feedback loop turns the initial weakness into a much bigger structure that affects the entire crust. The term ‘tectonic aneurysm’ refers to a medical condition where a weakness in an artery wall can cause serious medical problems as it gets bigger. My analogy of the overfilled sandwich with a cut in the top is more cheerful, but ‘tectonic aneurysm’ sounds much more sciency.

Egg or chicken?

The tectonic aneurysm model has also been applied to the Eastern syntaxis, at Namche Barwa in Tibet, where the Tsangpo river forms a deep gorge. These syntaxes are tectonic features, related to the corners of the Indian indentor, yet the aneurysm model regards a river gorge as thing that initiates extrusion. Why is there a connection between the two?

For me this is the most satisfying part of the model - the rivers flow across the syntaxes because of the geometry of the mountains – tectonics controls the location of the rivers which in turn influences the tectonics. Processes that affect the surface and those involving the entire crust are intertwined in a dance that lasts millions of years.

Consider the growth of the Himalayas. Rivers that used to drain off proto-Tibet into the Tethys ocean are now blocked by a mountain range, where India is pushing into Asia. These rivers start to flow parallel to and behind the mountains. Over time syntaxes develop and news rivers start to cut into them. Eventually these new rivers reach the old mountain-parallel ones and ‘capture’ them. The precipitation that has been stuck to the North of the Himalayas can now get through and the major river systems of the Indus and the Tsangpo-Bhramaputra can finally reach the Indian Ocean. At the ‘knick-point’ where these rivers drop sharply in altitude major gorges are formed. These ‘cut the sandwich’ and allow the weak hot middle crust to extrude out to the surface, forming some of the most fantastic scenery on earth.

The idea that patterns of erosion affect tectonics is a lovely illustration of the interconnectedness of the geosciences and is a current topic. A  very recent paper in GSA Today by Paul Kapp and co-workers looks at wind erosion in the North of Tibet. They show significant rates of erosion and speculate a link with the (small-scale) tectonics of the area. It seems you don’t need big rivers (or indeed water) for the atmosphere to influence the way rocks deform.

The processes of crustal thickening that created the hot middle crust now extruding to the surface at Nanga Parbat affects all of Tibet. What happens to this soft material where it can’t reach the surface? Also are the syntaxes the only places the sandwich has leaked? Interesting questions. Someone should write a blog-post about them…

This post is part of my journey into the geology of mountains.

Further reading

All of the papers listed here are publicly available right now. Click on the links and you get the entire paper no matter who you are. There are links in the text above, but here’s a list of the good stuff.

The good folk of the Geological Society of America make GSA Today available to all.

The GSA Today paper Zeitler et. al (Erosion, Himalayan Geodynamics, and the Geomorphology of Metamorphism) is the place to start for the tectonic aneurysm model.

Peter K. Zeitler, Anne S. Meltzer, Peter O. Koons, David Craw, Bernard Hallet, C. Page Chamberlain, William S.F. Kidd, Stephen K. Park, Leonardo Seeber, Michael Bishop, & John Shroder (2001). Erosion, Himalayan Geodynamics, and the Geomorphology of Metamorphism GSA Today

There is also a great set of papers on the Lehigh University website.

The  Kapp et. al paper is in GSA Today, link to html version if you didn’t like PDF link above.  DOI: 10.1130/GSATG99A.1

Kapp, P., Pelletier, J., Rohrmann, A., Heermance, R., Russell, J., & Ding, L. (2011). Wind erosion in the Qaidam basin, central Asia: Implications for tectonics, paleoclimate, and the source of the Loess Plateau GSA Today, 21 (4), 4-10 DOI: 10.1130/GSATG99A.1

For more information on Nanga Parbat geology, the 2006 paper from Jones et al.  gives a good overview, plus a taster of what I’m writing about next.

That’s me, standing in front of chorten (Tibetan Buddhist religious structure). The rather pretty cliffs are Tethyan sediments, now a long way from the sea. Note that the red layers are above the green, yet there are big red blocks sitting quite below the green cliffs. The blocks suggest there has been a big landslide at some point, but no doubt fluvial processes also move material down.

A Google Earth image shows that this is a common pattern along the river valley. Most of the big ‘streams’ of material that reach the valley floor are red, even though the green outcrops are nearer.

Here’s one last image.

Here, my glamorous assistant points out what is unmistakeably a landslide. The link between the scar on the red cliffs and the big pile of boulders is very clear indeed.

No.

Thrusting is important, of course, but as always real rocks are more complicated. For example, across the whole of the Himalayan chain, coincident with the highest peaks, is a structure called the South Tibetan Detachment system. It has a normal sense of shear, so it moves in the opposite sense to a thrust. Normal faults are associated with extension, not compression. To find something like this in the heart of a mountain belt caused by two plates colliding is extremely counter-intuitive.

So, a simple explanation (“its all thrusting”) is woefully inadequate. What more sophisticated models are there and how do they explain the South Tibetan Detachment?

Critical taper theory is a model that was originally developed for accretionary wedges.

Image from Wikipedia

I like to think of it in terms of a bulldozer pushing a pile of sand up a slope. The bulldozer (e.g. Japan) is scraping sand off the floor (Pacific plate). The sand deforms into a wedge, and reaches a stable shape with a particular angle between the slope and the surface of the wedge – the critical taper. This angle is controlled by the balance between movement of the sand on the floor (the ‘basal décollement’ at the top of the oceanic plate) and deformation within the wedge itself.

The wedge is a dynamic system. If something changes (like material being eroded off the top) then the wedge will deform internally until the critical taper is reached.

"Call this a pun? Fool" - a critical tapir.

Critical taper theory works well for accretionary wedges and has been has been modified and applied to mountains, usually using the name of orogenic wedge. Talking about the Himalayas once more, there is a structure that acts like a basal décollement called the Main Frontal Thrust. This is a feature found in many mountains belts so the concept of an orogenic wedge is a useful one.

There are many papers on this subject, but a classic of the genre is John Platt’s 1986 paper “Dynamics of wedges and uplift of high-pressure rocks” (available to us all via the man himself). It extends critical taper concepts to mountain belts such as the Alps or the Franciscan complex in California and addresses the more complex rheology of orogenic wedges (they are made up of viscous metamorphic rocks, not just cold rigid sediments). Its purpose is to explain how high-pressure rocks come to be at the surface within these wedges.

An orogenic wedge will thicken by thrusting, until it reaches its a stable geometry. If new material is added at the front of the wedge, then the wedge will shorten internally to regain a stable geometry. If new material is added underneath, to thicken the wedge, then it will extend internally. This extension within the wedge provides a mechanism to exhume high pressure rocks, to bring them to the surface via extension, even within a compressional orogen. Look at the John Platt paper for more explanation and some nice diagrams.

This is a neat trick and gives us a way to explain the South Tibetan Detachment system. The Himalayas are made-up of weak hot rocks piled up in a wedge between Tibet and the Main Frontal Thrust. They want to collapse down again, but they are kept up by the continuous movement of the Indian plate. This delicate balance has been disturbed in the past, meaning that there has been extension within the wedge, forming our normal faults. Problem solved?

For the sake of this post, perhaps yes, but I’ll leave you with a cliff-hanger. Extensional movement on the South Tibetan Detachment can in places be shown to the happening at the same time as thrusting structurally below. Wedge dynamics can’t explain this, it can only explain  thrusting and extension at different times (when the wedge is too thin or too thick). Also think of the geometry of this. If you have an extensional fault above a thrust then the material in between is being squeezed out, like a soft filling in squashed sandwich. What fresh madness is this?

This post is part of my journey into the geology of mountains.

For me, and dare I say it for any British educated hard-rock geologist the answer has to the Assynt district, in Sutherland, Scotland.

Denise asks for somewhere relatively remote and hard to get to. In British terms, Assynt is  as far away from ‘civilisation’ as it gets. It is almost at the very northerly point of Britain. From the bits of Scotland where most people live (the Midland Valley) it takes most of a day to drive there. The final part of the journey is on single-track roads that weave their way through an amazing undulating landscape. It is in the bit of Scotland that is more Scandinavian than British – Sutherland means ‘south-land’ in the language of the Vikings, but it is far enough North that I’ve seen the aurora borealis there.

To the Geology. This photo encapsulates the main geological features of the western half of Assynt.

This is the a view of the amazing mountain of Suilven. You’ll have spotted that it is made up of sedimentary rocks – Proterozoic rocks called the Torridonian. These sit on Lewisian Gneiss, Archaean and Proterozoic basement gneisses, visible in the foreground hillock. If you home sits on the the Canadian or Scandinavian shields then these are familiar rocks, but for the British Isles these are exotically old and exotically high-grade.

Side view of a sheath fold, Lewisian Gneiss

Intense glacial scouring makes this an amazing landscape. The gneisses form a rugged craggy ‘cnoc and lochan’ landform with lots of little lakes and rock hillocks. The Torridonian forms spectacular mountains (well, hills really) that rise above. In Assynt the Torridonian also includes a layer of suevite, recording a major meteorite impact somewhere nearby.

Why should you care about this area? Well, it is a UNESCO Geopark, for one and perhaps the main reason for this is that it contains the Moine Thrust Zone. This is a major thrust structure, marking the edge of the Ordovician/Silurian Caledonian orogeny. It puts the Moine Schists (lateral equivalents of the Torridonian, now strongly deformed and metamorphosed) over undeformed ‘foreland’ rocks. The foreland contains Cambro-Ordovician sediments which sit unconformably over both the Lewisian and the Torridonian. Assynt is a particularly interesting part of the Moine thrust zone as it is a culmination – there is a big package of thrust slices between the undisturbed foreland and the Moine schists. The Cambro-Ordovician sediments are an important part of the picture as they have a regular stratigraphy made up of varied distinctive rock types. This makes the wild structure of this ‘zone of complexity’ much easier to map.

In the early 20th Century, Peach and Horne of the (state-funded) British Geological Survey produced a classic report on the area. This proved without any doubt the reality of thrust faulting and is a significant event in the history of geology as well as a classic of fieldwork. The rock type mylonite (characteristically formed at thrust contacts) was first identified and named (by others) in the Moine thrust zone just north of Assynt.

As well as showing the complex geometry of thrusting in three dimensions via detailed mapping and copious cross-sections, Peach and Horne also had influential thoughts on the broader tectonic implications of what they found.

In Assynt the ‘zone of complication’ is awesomely complicated. The structure is truly three-dimensional. Well four-dimensional really, as structures cross-cut and debate rages over the sequence of thrusting over time.

Here’s an picture of a relatively simple area (the Glencoul thrust), to give you a taste.

Starting from the base moving up, first you see a hummocky area of Lewisian Gneiss. Then there is a crag of slightly-pinkish layered Cambrian quartzites. There is an unconformity at the base of the crag – it was once a rocky seafloor that became covered in sand. There is a step in the slope and the final rock unit appears – hummocky Lewisian Gneiss again!

The step in the slope marks the position of the Glencoul Thrust, one of the thrust planes that make up the Moine thrust zone. Think about this – all of the upper part of the hillside has been pushed on top of the lower part. If you trace the fault you realise that this would mean 10s of kilometres of horizontal movement. This is an amazing thing. Peach and Horne’s work is important as it settled the matter, proving the reality of thrust faulting, for the first time.

When geologists first starting debating the Moine Thrust, the way Gneiss formed wasn’t understood. Geologists argued that the picture above showed a conformable set of sedimentary rocks. Imagine, if you didn’t know that gneisses aren’t formed on the sea-floor then this makes sense, more sense than the idea of hillsides climbing on top of each other, anyway. Charles Lapworth, who first named mylonite and identified thrusting in Assynt, at one point had nightmares of being bodily caught up in the Moine Thrust, being crushed under what he called the great Earth engine.

You should visit Assynt, if you get the chance, it will give you good dreams, not nightmares.  The site of the North West Highlands Geopark will help you plan your visit. While you are waiting, here are some ways to visit virtually. You can get to geological maps of the area via the BGS geology of Britain viewer.  There’s an introduction to the Geology on Leeds University’s Assynt Geology website.

If you are as obsessed with Assynt as me, you should buy the centenary Special Publication from the London Geol Soc. It’s not cheap, but it is very large and very good. Portions of it are available on the Internet via the BGS open access repository. Other books include Exploring the landscape of Assynt from the BGS which is an informal guide for walkers and ”A geological Excursion guide to the North-West Highlands of Scotland” for a more geology focused guide.

To get a sense of the structural complexity, the paper on the Traligill Transverse Zone by Maarten Krabbendam and Graham Leslie is hard to beat. Here’s a taster of some modern BGS cross-sections to close out with.

Image sources

If you don’t, welcome! You have found a fabulous way of wasting time whilst marvelling at the beauty of the earth’s surface. Your mission is to look at the picture below, find the same picture in Google Earth and claim your prize! You do this by logging the location in the comments, along with some remarks on the geology of the area. Your prize is to choose and host the next picture (if you don’t have a blog, not a problem). There is more information about the game available. No Schott rule, so old lions can start tucking into the challenge right away.

I’ve picked a large image so I hope this won’t be too difficult. A hint: this image is not of England (even though we are currently suffering a drought).

Time for a clue, I think. Here’s a view of the same area from another angle.

I’m building up a list of sites as I find them, as I will use them in my blogging. I’m focussing on Geosciences but many sites are not subject specific. Please make use and if you know of any others, let me know via the comments. I’ll add them to the main text over time.

Caveats – A lot of these sites don’t have a massive amount of content. Some list papers but have no publicly available copy. Often the copies are ‘pre-publication drafts’ with odd formatting. All sites have terms of use that you probably read, definitely if you want to do anything but read them.

Usage – Most of these are repositories for organisations. If the primary author of a paper you know of is at one of these institutions then its worth a look. At least some of them are Google searched, in which case that is your best route if you are looking for a specific paper. Either way, don’t assume a recent paper can only be found via the journal.

In descending order of usefullness:

http://nora.nerc.ac.uk/ is a repository for papers produced by UK government agencies, for my purposes the British Geological Survey and British Antarctic Survey. Coverage sketchy before 2010 but good thereafter.

http://www.geosociety.org/gsatoday/ All of the GSA’s “GSA Today” are available online and they are keen for bloggers to refer to them.

Open source journals may be the future? EGU has a selection: http://www.egu.eu/publications/open-access-journals.html (thanks Bill).  The Solid Earth journal is the one that most caught my eye.

http://oro.open.ac.uk is a repository for the UK Open University. Seems to list all papers since 2009 and about 30-40% have text.

http://dspace.mit.edu MIT seems to have very good coverage.

http://eprints.whiterose.ac.uk is pretty good coverage.

Poikiloblastic in the comments has useful stuff about planetary science articles and lists of Open Access Journals.

John Stevenson, aka volcan01010 my blog-neighbour, points me to http://www.pubvolc.net/ which is a rather smart idea. It is a volcanology literature database that allows you to contact the author for a reprint.

http://wiredspace.wits.ac.za/ appears to be just MSc/PhD theses at Wits University in South Africa.

http://earth.usc.edu/~jplatt/Publications.html  only papers by John Platt, but each one a gem!

http://ora.ouls.ox.ac.uk/ is a repository for Oxford University and http://www.dspace.cam.ac.uk/ the same for Cambridge. Coverage of geoscience is pretty poor. Oxford is apparently digitising all of its theses, even the old paper ones, which is rather pleasing to me since I wrote one of them.

Rocks can behave rigidly, or under some conditions, they are ductile and can flow. This is a remarkable thing, which helps explain why the theory of Continental Drift was not more widely accepted (prior to the plate tectonic revolution of the 1950s). Seismic evidence from the deep earth shows that the crust and mantle rocks behave elastically (on short timescales). This was taken as evidence that they could never flow, meaning the continents couldn’t drift. We now know that hot rocks in the mantle can flow (over long timescales) and that this is a necessary part of plate tectonics.

What about other geological implications? What other things can be explained by the ‘Jekyll and Hyde’ way in which rocks deal with stress?

Small scale structures: Most rocks under surface conditions behave rigidly – they only change their shape by fracturing or by being dissolved. Rocks deformed near the surface are therefore typically fractured or faulted. Slightly hotter rocks may be deformed by dissolution, where material is dissolved into solution and redistributed into veins. Stylolites and cleavage are structures formed in this way that are common in deformed sediments. Salt and mud and are two substances that can flow in cold shallow rocks. Salt diapirs and mud volcanoes are structures where these light substances flow towards the surface.

Stylolite from Ron Schott on Flickr (http://www.flickr.com/photos/22644739@N00/4468019786)

To deform typical rocks in a ductile way, they need to be under conditions associated with metamorphism – below depths of about 15-25 km and above temperatures of ca. 300 ^oC.

Under these conditions, various forms of sold-state creep begin to work and rocks can ‘flow’. Typical structures here are fabrics, such as schistosity or gneissosity – entire rocks are deformed as shown by the flattened minerals within them. Often there are areas of very high strain called shear zones, typically containing mylonites. A process called strain softening, plus perhaps a focussing of water or heat flow makes these zones softer than the surrounding rocks, meaning that they get more and more deformed.

Shear zones are analogous to faults, indeed a major structure may be a fault near the surface but a shear-zone at depth. A shear-zone is likely to be wider than the equivalent fault – the deformation is distributed through a large volume of rock.

Shear zone in amphibolite from Ian Stimpson @hypocentre (http://www.flickr.com/photos/17907935@N00/6723121739)

What controls the brittle-ductile transition? Major controls on the rheology of rocks are  composition, temperature, pressure, the presence of liquid, and the presence of melt. Composition is quite hard to change, but if you increase any of the other things, the rock is more likely to behave in a ductile fashion.

Large scale features, large scale evidence. There are some big features that can  be explained in terms of their rheology, which means they can be used to directly measure the rheological properties of the crust.

Foreland basins are sedimentary basins where the accommodation space (the hole in the ground filling up with sediment) is created by the flexure of the rigid upper crust.  Think of the Indian continent being pushed under the Himalayas and Tibet.  The weight of Tibet is pushing down on the Indian plate and this bends it downwards. The space created fills with sediment being eroded from the Himalayas. The Ganges basin is formed.

The Indian crust is cold and rigid. Clever folk can do the maths on the shape of the crust as it bends down. This confirms that the pattern matches the model for rigid, elastic deformation. It also allows then to calculate the plate’s flexural rigidity, which is a measure of its strength. This means quantifying the rheology of real bits of the earth, which is a very useful trick.

If you read up more on foreland basins, the picture gets more complicated as, over time the crust will further deform in a viscous way. I want to take you away to a different example of viscous behaviour.

There are raised beaches all over high-latitude areas of the Northern hemisphere. These are fossil beach deposits now found metres above sea-level. They can’t be explained by global sea-level changes as equivalent features are not found further south.

Raised beach from Scotland (in foreground). From SAGT @http://www.flickr.com/photos/sagt/4304338029/

These raised beaches formed when global sea-level was similar to today’s level: they are raised because the earth’s surface has risen up since they were formed. This isn’t due to tectonics – these areas are far from any plate boundary. What these areas have in common is that they were covered with kilometres of ice during the last ice age. This ice weighed a lot and this force pushed the crust down. The upper crust bended rigidly, pushing underlying viscous rocks (probably in the mantle) out of the way. The ice melted relatively quickly, removing the downward force. The viscous mantle rocks are slowly recovering, flowing back into glaciated areas and lifting the earth’s surface, leaving fossil beaches high and dry. The overlying rocks are being passively pushed up, with occasional post-glacial earthquakes occurring in otherwise tectonically quiescent areas. Once again, these natural phenomena are an excellent opportunity to directly measure the physical properties of the earth’s interior: the rate of rise lets you measure the viscosity.

Crème brûlée versus jelly sandwich

So the earth is strong near the top and weaker at deeper levels? If so, it would be like a crème brûlée. A giant spoon digging into the earth would crack through an upper layer (like burnt sugar in a crème brûlée) and then scoop gently through the soft mantle beneath (creamy custard).

If the earth were boringly simple, this would be accurate, but as you know, life is more interesting than that. The earth gets hotter with depth, so rock is more likely to be weaker with depth, but the composition of the earth is not constant. It’s long been known that the surface layer that moves as part of plate tectonics (the lithosphere) is not just the crust. There is a layer of mantle material (lithospheric mantle) stuck to the bottom of the crust meaning that within the lithosphere, there is a dramatic compositional change which complicates matters.

An interesting paper by E.B. Burov and Tony Watts (available here, thanks GSA!) discusses the crème brûlée model and compares it with a ‘jelly sandwich’ model. A jelly (jam) sandwich has a soft layer between two stronger layers. The sandwich model supposes a strong rigid upper crust, a weak hot lower crust  and then a strong layer of lithospheric mantle.

You’d have thought that, knowing the composition of average crust and mantle and the conditions with depth, we could resolve this debate easily. As it turns out, although we know that olivine-rich mantle is much stronger than quartz-rich crust we don’t know by how much. It is very hard to run experiments with low strain rates and high temperatures comparable to real-life conditions so we can’t measure directly.

Burov and Watts combine numerical modelling and indirect observations to settle the issue. Think back to the Indian crust subducting under Tibet. One of the things you can infer from geophysical studies of these situations is the effective elastic thickness of the lithosphere. This is the cumulative thickness of rocks that are behaving elastically. For India, this is 70km, which is greater than the thickness of the crust. This is hard to explain, unless the lithospheric mantle is strong, suggesting you have a thick sandwich, not a thin layer of burnt sugar. They also show numerical modelling that suggests that without a strong lithospheric mantle, subduction and mountain building wouldn’t be possible.

Note that for this last section I’ve switched to talking about weak/strong rather than rigid/ductile. If you a simple sort like me, you can think of the two as synonymous, but they are not really which is why Watts and Burov talk about long-term strength. Complications arise because most rocks are viscoelastic – they transmit earthquake waves elastically (over short timescales) but behave viscously over longer timescales. The paper gives you a taste of this, but I’m not the man to take you any further into that world.

Rocks behave in complicated ways when placed under stress. I hope I’ve given a taste of the complications that arise from this in the wider world. I leave you with one thought: if you squeeze an over-filled sandwich – what happens to the filling? I will return to this and how it relates to Tibet and the Himalayas, in another post.

We’ll start with Southern Geologist’s first contribution to an accretionary wedge, on a new blog. The post describes a inspirational introductory course in Geology, which for variety and interest put other sciences in the shade.

Short Geologist picks an anonymous college chemistry teacher whose good teaching cured a phobia of ‘hard’ equation-rich subjects and laid the foundations for successful geological studies.

Denise Tang over at Life as a Geologist owes a debt to Prof. LS Chan, who introduced her to Geology and so “passed the flame”.

My post on John Dewey describes how I learnt the importance of breadth from a remarkable man.

John Adams (the Geologist) was taught geology by not one, not two, but three Ulstermen called Reid and is interested in knowing if anyone else remembers them.

Over at Life in Plane Light, Elli Goeke tells us about three ‘runner-up’ teachers  but settles for Kim Hannula as her most important, someone who is a mentor as well as a teacher.

Hollis over at Plants and Rocks gives us a tribute describing the life and work of Dr Brainerd “Nip” Mears Jr, a man who contributed to our understanding of the Geomorphology of the American west, but who put his students first.

Following a common theme, Casey at Gioscience lists the teachers (at University of North Carolina at Wilmington) who led him to a love of Geology: Dr. W. Burleigh Harris, Dr. David Blake and Dr. Michael Smith.

Moving away from formal education, Dana Hunter offers a characteristically engaging and generously-illustrated story  (also here) about how Lockwood DeWitt fed her geology addiction with a first opportunity to “see some stuff with an actual geologist”. There are some great descriptions of what being taught by a great teacher is like and how they build confidence as well as impart knowledge.

The man himself, Lockwood DeWitt submits a touching eulogy to Harold “Sharkey” Enlows the College teacher “who made me work the hardest, and from whom I learned the most”.

Ann over solved the difficult problem of picking between her university geology teachers by talking instead about an important school teacher, Miss Relic who through belief and encouragement changed for the better the way Ann thought about learning and her own abilities.

Ryan Jackson over at Educated Erosion had no problem choosing Coach Ford, an inspiring High School teacher who set him off on a rocky road.

The next wedge is hosted by Denise Tang and is “Geological Pilgrimage – the sacred geological place that you must visit at least once in your lifetime “. Get thinking…

I’ve some pictures from the Indian Himalaya. I’ve put a satellite image of the area up in WoGE #322 and talked about growing a beard while here already.

Trekking in the Himalaya, you get used to amazing views. But this was a taste of something special – Gumbaranjon peak, approached from the south.

The main buff-coloured cliff face is granite, with dark layers of xenolith. The green/purple rocks on the right skyline are behind, on the other side of a major extensional detachment (in the heart of a compressional orogeny).

Walking closer and around the base, we get a better view. The dark xenoliths and light granite make it a bit like looking at a negative photo. The eye tries to trace the pattern of granite intrusion by putting patterns into the places where the xenoliths aren’t. There are hints here that the intrusion process wasn’t simple.

That’s a fold of granite in a xenolith! It looks like there must be deformation active during the intrusion process. An early dyke has been folded and now sits in a xenolith surrounded by granite. It is very unwise to infer strain from a fold (if you don’t know the initial orientation), but its interesting that the nearly extensional structure is top to the north, which corresponds to a top to the left sense of shear in this photo.

Looking at the rocks at the base on the cliff gives more evidence of syntectonic intrusion.

The central block shows undeformed tourmaline muscovite granite sitting in a triangular space within metamorphic rocks. The shape looks like a ‘strain shadow’ around porphyroclasts in a mylonite.

All good things come to an end, but as we walked away from Gumbarajon it showed us its most majestic profile.

Click for more information on the Geology of this area.

Here’s an end on view, showing why I thought it was a grub hunched over itself. Note the ridged appearance.

It was a fossil echinoid, or sea-urchin to use its every day name. It is quite small (1 cm across) and quite lovely. I was quite excited as I never expected to find a fossil in my garden. The underlying geology is recent (post-Cretaceous) sediments but my garden sits on an old Thames river terrace. This is mostly gravel and you don’t get fossils in gravel. Or so I thought.

I was also excited as I’d just got a new camera with a macro setting for close-up pictures. I can take a hint from coincidence, so here we are.

A view of its underside. Note how the ridges we saw above curve down under. Towards the bottom of the image you can see little dots in an areas between two ridges. These are pore-pairs and are where the echinoid’s tube-feet were attached.  If you look carefully, you can just about make out a five-fold symmetry that is very distinctive of this group of animals (echinoids) and is rather more obvious in starfish.

Here’s a view of the other (top) side.

Again there is a ragged lump in the middle, which is a clue to how this fossil ended up in my garden. Echinoderms have a strong shell, but the middle of the top and bottom surfaces is open or covered by fragile bits of shell. This is so it can get food in and then out again. These more fragile bits are rarely preserved. This fossil is made of flint which will have replaced or infilled the actual shell. The impression of the shell will have been within a larger piece of flint – the raggedy bits are where the large piece broke up, revealing the fossil within.

Picture yourself in the Cretaceous. There’s a warm sea, which is shallow but weirdly there is no land in sight. A baby sea-urchin is burrowing in the chalk mud living a full and happy life – the white mud it frolics in suits it just fine. Eventually death comes and our hero ends up in a mass grave (that nice white mud is actually billions of algae corpses, or coccolithophores). More corpses lie on top, and things warm up a little. Silica and water zips around and our hero ends up entombed in flint. Fade to black.

A new scene, a chalk outcrop in a world that looks much like ours, only there are no people. A lump of flint is eroded out of the chalk and slides downhill. Below lies a river that will one day be called the Thames.

Now the Thames is flowing hard. It’s hard bouncing and banging along the river bottom and something has to give. With a muffled clonk that no-one hears, the flint pebble fractures and something beautiful appears  - our hero. It’s now exposed to the jostling and banging, but soon reaches quiet water and is covered by more flint.

We reach the present day. The gravel terrace has been mixed up by frost-thaw action during periods of near Arctic conditions (ice ages) and the top layer is now mixed in with dead plant matter to form soil.  A spade turns the soil  and our hero finally reaches the surface again, where someone can find him and tell his story.