UKC

Conglomerate - geological question

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 Robert Durran 09 Jul 2021

Most conglomerate we climb on seems to consist of a sandstone medium with harder rounded "potato" pebbles embedded in it which either provide typically slopey holds or pockets where they have fallen out.

I climbed at Sarclet for the first time recently. There the pebbles have generally been sheared off flush with the sandstone  to provide a pretty much uniform surface more like conventional rock and the climbing is quite unlike on other conglomerate I have experienced.

My partner and I were baffled by what process could have sheared off the pebbles like this. Anyone got any knowledge or plausible idea?

 Rog Wilko 09 Jul 2021
In reply to Robert Durran:

I’ve come across something like this. In the west fringes of the north Pennines there occurs brockram which is strictly breccia (angular fragments) rather than conglomerate. I think it forms The Hoff near Appleby. The only mechanism I can think of for the splitting you observe is frost wedging.

 DaveHK 09 Jul 2021
In reply to Robert Durran:

Just speculating but the action of cliff collapse due to undermining by waves might give a smooth shear surface?

OP Robert Durran 09 Jul 2021
In reply to DaveHK:

> Just speculating but the action of cliff collapse due to undermining by waves might give a smooth shear surface?

I would have thought that would break the sandstone medium but leave the pebbles intact (?). In fact it being a sea cliff presumably regularly re-eroded by the sea action makes it particularly puzzling to me.

The frost splitting seems more likely to me, but again I would have thought that on a sea cliff, sea action would expose new surfaces more regularly.

Anyway, why would some conglomerate erode sheared flat surfaces but other conglomerate the "potato" surface?

 Cog 09 Jul 2021
In reply to Robert Durran:

Sounds like breccia rather than conglomerate, as stated above.

 wbo2 09 Jul 2021
In reply to Robert Durran:  I'd have to have a look at a picture, but as you describe you have the larger clasts in a matrix, and typically that matrix, even if cemented is going to be softer.   So...

Sarclet is a seacliff, and the orientation NNE/SSW is roughly perpendicular to some major tectonic stresses so I'd assume any fracture system 'grain' would be running parallel to the coast.  This will postdate the conglomerate, pass thro' clasts and matrix and provide a 'weakness' that erosion will exploit.

Looking at photo's I'm pretty comfortable with the above, but I'll discuss with a structural specialist. 

## Given that this is Devonian sst, and well cemented,, the matrix won't be soft, but the clasts will be mechanically VERY hard gneiss , granite etc. pebbles.  But there is a distinct tectonic fracture applied - look at various crag photos - postdating the deposition and cementation, and this is being exploited by erosion.  

It looks fantastic to climb   

Post edited at 10:23
In reply to Robert Durran:

Perhaps the relative difference in strength between the clasts and the matrix? If the matrix is very hard and well-cemented it could be harder or as hard as the (older, usually harder) pebbles/clasts. It can go either way. In some conglomerates the clasts leach or weather out ahead of the matrix - this happens in BVS volcanic lithologies sometimes. The brockram at the Hoff that Roger described is a sandstone matrix with mostly limestone angular clasts.

DC

 jkarran 09 Jul 2021
In reply to Robert Durran:

I suspect it requires the pebbles are better bonded into the matrix and of comparable mechanical performance, presumably by being chemically similar so there is some mineral growth across the pebble-matrix boundary or because the whole mass has undergone some degree of metamorphosis which will have made the material more homogeneous and may have introduced stresses/fractures. In that case it'll behave more like a massive block, cracking cleanly rather than around the weaker pebble boundaries as we see in more familiar conglomerates. Also by the matrix and inclusions being chemically and mechanically similar, subsequent erosion of the crag surface wont preferentially remove one or the other. 

jk

Post edited at 10:44
 heleno 09 Jul 2021
In reply to Robert Durran:

I don't know if it's a related phenomenon, but the pebbles at the Princess Cairn crag near Golspie are also sheared off, unlike the rounded pebbles at the other Golspie crags and Moy (and perhaps even more noticeably so than at Sarclet?) 

I'd assumed this was due to glaciation, as Princess Cairn is quite a bit higher - but perhaps instead its due to the same tectonic forces as at Sarclet?

I loved climbing at Sarclet BTW.  Wild, beautiful - and drier and more midge-free than the west coast.

 Toerag 09 Jul 2021
In reply to Robert Durran:

It looks very much like the breccia that exists on the isle of Sark.  As others have said above, if the matrix has bonded the chunks/pebbles sufficiently well to attain a similar strength to the chunks/pebbles then the whole mass will cleave because no part of it is weaker than the rest. 

Think about brick walls - if you make them with weak lime mortar the mortar fails before the bricks, if you make them with cement the bricks fail before the cement.

OP Robert Durran 09 Jul 2021
In reply to Cog:

> Sounds like breccia rather than conglomerate, as stated above.

Would the issue be any different for breccia than for conglomerate? Is the only difference the shape of the embedded lumps?

Funnily enough I only came across the the term breccia for the first time last week when climbing on the amazing pyroclastic breccia at Eshaness on Shetland. There the lower more sea battered rock seems to weather into pockets, but the higher or more shltered rocks tends to be more of the "potato" type.

Post edited at 11:43
 ChrisJD 09 Jul 2021
In reply to jkarran:

This would be my explanation as well.

Interesting little section of coastline!

Looks to be the Deveonian Langwell Conglomerate Member. ~ 400 million years old.

(used to known as the Sarclet Conglomerate)

Looks like it's a classic basal conglomerate overlying much older basement rocks (pre-Cambrian), from note on geo map, see below.

Part of the Lower Old Red Sandstone Group,  (Old Man of Hoy is Old Red Sandstone).

http://mapapps.bgs.ac.uk/geologyofbritain3d/

Then use postcode KW1 5TU to locate

The 1913 1:63K geo solid/drift map scan can be viewed here:

http://www.largeimages.bgs.ac.uk/iip/mapsportal.html?id=1002505

The 1985 1:50K geo bedrock-only map scan can be viewed here:

http://www.largeimages.bgs.ac.uk/iip/mapsportal.html?id=1002506

OP Robert Durran 09 Jul 2021

Thanks for the various fascinating replies.

So it looks like it is down to the relative hardness of the matrix and the embedded lumps and how well they are bonded. So maybe the matrix isn't actually sandstone (despite the other crags on that coast being sandstone)?

In reply to Robert Durran:

Remember “sandstones” come a variety of strengths dependent upon age, disposition all history, etc.  Compare gritstone to southern sandstone, or St Bees to the North York moors; all have varying strength and fabric.

OP Robert Durran 09 Jul 2021
In reply to HighChilternRidge:

But is any sandstone as strong as a metamorphic or igneous rock?

Post edited at 13:28
 ebdon 09 Jul 2021
In reply to Robert Durran:

definitely! lots of metamorphic rocks are very weak (think of  dyke or sill that has been eroded away, or slates and schists).  Many igneous rocks are weak and rapidly weather along joint plains.  in contrast, well cemented (with silica) sandstones can be very strong. Saying that in these conglomerates generally, only the stronger lithologies will be present as large clasts, but if the whole rock mass has been silicified then there can be little change in strength across it - the weakness will instead be jointing (due to tectonic stresses) or sedimentary bedding. 

 ebdon 09 Jul 2021
In reply to Robert Durran:

Not totally relevant to the question but fans of geological history and the geology of the area may find this interesting  (page 35 and 159 onwards specifically). Even non-geologists may recognize the names Peach and Horne, who were well ahead of their time and despite this document being over 100 years old still an amazing description of the area.  just goes to show the power of good scientific observation!

http://pubs.bgs.ac.uk/publications.html?pubID=B01956 

 wbo2 09 Jul 2021
In reply to Robert Durran: Some further comment... this stuff is indeed Devonian.. I haven't looked exactly at this but I have looked at comparable Dev. rocks at the Åmelfot glacier in Norway (and that is absolutely worth a visit for a very unique landscape).  It formed in an inland enviroment at the end of the Caledonian orogeny, after welding together older continents  Laurentia, Baltica and Avalonia, forming very large mountain ranges with deposition in subbasins between mountain ranges.  This stuff would be forming as the mountains eroded, filing valley floors as both flash floods and more moderate terrestial deposits.  These deposits are VERY thick - 10's of kilometres thickness are exposed at Åmelfot.

 That has relevance as the transport distance for the clasts won't be very far - they'll be of local rocks like Lewisian gneisses and some Moine schist, though as they're rounded you can assume softer  rocks will be reduced.  The matrix is pretty hard, but I doubt it's as hard as a rounded gneiss or metaquartzite boulder, but clearly the difference in hardness isn't influencing how it fractures much. (Breccia matrix can be very hard, very soft as there are many ways to create a breccia).

I think this https://www.ukclimbing.com/logbook/crags/sarclet-2960#photos&gid=1&... is a terrific photo and really shows the orientation of the dominant structural (and fracture grain) in the rock.  

 peter.herd 09 Jul 2021
In reply to Robert Durran:

I recall asking a few geologists this question about Sarclet and two things I recall coming away with a 're-aligned view' on were a) conglomerate can be very well bonded  b) shearing forces can be ruthless.

Now for some 'interested climber' pondering...

I'd assume the cement is made up of similar rock to the cobbles (sandstone, granite, schist, quartzite and basalt) and with high quartz content from those rocks plus a little heat and pressure the whole lot could well be pretty tough.  Perhaps tough enough that bonding cement-cobble could be as strong as bonding cement-cement? I've never pulled a cobble out at Sarclet but have Moy, the Camel, Riglos. 

On the NE coast of Skye you get these 'blebs' which are lava (dolerite) engulfing sedimentary cobbles. You can see in places where they've  split that the shear can be uniform throughout or at the interface. Perhaps an example of something inbetween in terms of bonding strength?   

When/where have the shearing events that give us smooth/steep cliffs at Sarclet occurred and what magnitude would they need to be? Not sure - glacial rebound, weathering, tectonics?  I'd bet Steven Andrews would know all the answers.

Pete 

 wbo2 09 Jul 2021
 Chris Craggs Global Crag Moderator 09 Jul 2021
In reply to Robert Durran:

I have come across the same phenomena at Montserrat (Spain) where cobbles - sometimes really large ones- are cleaved smooth with the rockface. I wondered if it linked to the jutting part being subject to heating/cooling and micro expansion/contraction over millennia,

Chris

 Harry Ellis 09 Jul 2021
In reply to peter.herd:

Yeah but what’s he ever done on grit?

 Steven Andrews 09 Jul 2021
In reply to Robert Durran:

Hi Robert, it is rather a simple answer, that folk seem to have broadly gotten to, but the whole of the Caithness coast along there is defined by the Helmsdale, and then Wick faults, which are a continuation  of the great Glen fault and such major fault systems cause significant fracture systems to which pebbles are of little significance.  The second factor at play is cementation (diagenesis), which for most of the Devonian up there is calcium carbonate, but this has been accompanied by significant, but not metamorphic, pressure and heat from burial and tectonic action and has created a pretty solidly bonded matrix to the conglomerate. Much as some of the Devonian sandstones are very sandy (old man of hoy) and some of them are very solid (yesnaby), the same can be true of conglomerates due to different levels of cementation/diagenesis (burrial/pressure/heat/fluid flow). There are other Devonian conglomerates that display this fracturing (lending themselves better to trad) such as Struie Crag).

Other points to pick up on:

 Much of the other rocks up there are interbedded sandstones, limestones and mudstones, which can all be hard and offer good climbing (And great fossil fish!!).  This is largely due to the calcium carbonate content.

Almost all rock types can be good or bad for climbing depending on their burial and tectonic history.  This comes back to gritstone, southern sandstone, NW quartzite, etc all being the same lithology (sandstone) but all having completely different climbing properties. Now consider that metamorphic rocks all started as another rock type. Metamorphosed mudstone will form Schist which can be pretty hard but not always (high organic content could make graphite which would be pretty crap climbing!).  But metamorphosed sandstone, or even sandy mudstones could provide really sound rock (sometimes still called schists).  And then there is metamorphosed granite, etc such can form superb climbing as in the gneisses of the NW.

So now I will stop as you will likely be either baffled or bored.  Always happy to talk further though!

 CurlyStevo 09 Jul 2021
In reply to peter.herd:

It does seem counter intuitive to when a pebble pops out when you stand on it? Its unusual also right as most conglomerate the pebbles are last to go.,  

 Steven Andrews 09 Jul 2021
In reply to peter.herd:

Sorry, didn't see your message there Pete.  The fault system was activated during the Caledonian orogeny, when multiple terrains/continents collided to piece together all the parts of Scotland.  Of course, the Devonian sediments were shed from the resulting Caledonian mountain chain and then had to be buried (a few km) and lithified before the faulting was reactivated.  The great glen fault system was largely strike slip, so either side slides by the other laterally, rather than up or down. Movement on the great Glen fault system is thought to only amount to around 30km after the Devonian sediments were deposited but it was still a site of significant stress focus.  It was then also likely active in more minor movements, including uplift of the sediments after burrial, right through to the present day.

 peter.herd 10 Jul 2021
In reply to Steven Andrews:

Thanks. My understanding was limited to  the early bits that produced the rock but hadn't realised the GGF was subsequently reactivated and propagated fractures (iv'e wrongly called that shearing). Makes sense as wbo2 also mentions the alignment of the coast with the Great Glen. Now I recall there have been relatively recent earthquakes recorded in Inverness!

OP Robert Durran 10 Jul 2021
In reply to Steven Andrews:

Thanks to all. What a wealth of knowledge there is on here to be tapped!

Can anyone recommend a good, accessible book on the geology of Scotland for a climber? I spent a few days last week exploring the coasts of Shetland for new crags and realised that I am decidedly ignorant and was at least in need of a decent geological map🙂.

 Jon Read 10 Jul 2021
In reply to Robert Durran:

Not sure if it's been linked before, you're probably already familiar with it (and obviously not a book you can take to the crag!) but this geological map is extremely good, and I use it all the time to counteract the FRCC guidebooks view that almost all rock in the Lakes is Borrowdale rhyolite:

https://mapapps.bgs.ac.uk/geologyofbritain/home.html

In reply to Jon Read:

> I use it all the time to counteract the FRCC guidebooks view that almost all rock in the Lakes is Borrowdale rhyolite:

I think you will find that the FRCC guidebooks describe all the varied geology that we climb on across the area from Ordovician to Triassic, and all the varying lithotypes therein.

DC

 ChrisJD 10 Jul 2021
In reply to Jon Read:

Yep, linked it further up.

This '3D' version of the BGS viewer is better though:

http://mapapps.bgs.ac.uk/geologyofbritain3d/

There is also a phone app.

You can also view all the hi res scans of the 1:50k / 1:63K BGS maps online here:

https://webapps.bgs.ac.uk/data/maps/

Use this page to work out the map you need: http://mapapps2.bgs.ac.uk/geoindex/home.html?layer=BGSPap50k

BGS has fully grasped the open license approach to their data, unlike OS who are being dragged kicking and screaming.

 ChrisJD 10 Jul 2021
In reply to Robert Durran:

> Can anyone recommend a good, accessible book on the geology of Scotland for a climber?

Not sure how 'accessible' they will be:

https://shop.bgs.ac.uk/Shop/Category/RegionalGeology

 Jon Read 10 Jul 2021
In reply to Dave Cumberland:

Apologies, it's "borrowdale volcanic" throughout the select guide. 

Edit: which is equally uninformative.

Post edited at 10:40
 wbo2 10 Jul 2021
In reply to peter.herd:But the movement on the GGF would be a shearing movement in the greater scale, setting up a fracture that will pass thro' affected rocks.  I was not so sure of the Great Glen Fault timing.

Pedantry corner - the Caledonian orogeny was caused by the collision of ancient continents I suppose you could call terranes, not to be confused with terrain.  Terranes can be best thought of as little microcontinents that were splodged (technically accreted) together, mainly in the PreCambrian, to make larger continents.  Examples would be , across Scotland, the Lewisian Gneiss, the Moine Schist and Dalradian Sands that were mini continents, then stuck together in Proterozoic to form part of Laurentia, that then got mixed up in the Cal. orogeny.  The sutures between the various terranes are still manifested as the Great Glen fault , Highland Boundary fault etc.    I'm personally interested in different terranes in Baltica in Southern Norway I'm sad to admit.

 ChrisJD 10 Jul 2021
In reply to wbo2:

This is quite neat for a big picture view of plate tectonics (though they need a reverse option to help you track the UK bits!)

youtube.com/watch?v=gQqQhZp4uG8&

Post edited at 10:58
 Doug 10 Jul 2021
In reply to Robert Durran:

I found 'Geology and Scenery in Scotland' by JB Whittow a good & clear introduction for a non geologist  when I was a student studying ecology but its both long out of print & probably a bit dated now although I'm sure it can be found second hand with a bit of searching. More recent is 'Land of Mountain and Flood: The Geology and Landforms of Scotland' by Alan McKirdy, Roger Crofts & John E. Gordon.

 timparkin 10 Jul 2021
In reply to peter.herd:

> Thanks. My understanding was limited to  the early bits that produced the rock but hadn't realised the GGF was subsequently reactivated and propagated fractures (iv'e wrongly called that shearing). Makes sense as wbo2 also mentions the alignment of the coast with the Great Glen. Now I recall there have been relatively recent earthquakes recorded in Inverness!

and around Lochaber!

OP Robert Durran 10 Jul 2021
In reply to Doug:

> More recent is 'Land of Mountain and Flood: The Geology and Landforms of Scotland' by Alan McKirdy, Roger Crofts & John E. Gordon.

Thanks. Ordered a copy.


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