/ ARTICLE: The Geology of Britain - A Climber's Perspective: Part 2 - Sedimentary Rocks
Thought this would be a boring read but actually quite enjoying this mini series.
Great article, really enjoyed it, thanks
A good quick overview, but
"without a look at limestone; that which is found from the south coast of England, through Wales, the Dales and into the central belt of Scotland"
should go on to add
...and then on to Skye & the far NW (eg Durness limestone)
yes and via Ballachullish...
I was hoping this was going to be an exciting read, however I was dismayed by the lack of detail that the author conveyed about about the formation of these sedimentary rocks, the processes responsible for their deposition (ripple strata are not the same as cross strata), and what make the Millstone Grit different from the Fell sandstone (pictured in figure 1) or from some other (clastic) sedimentary rocks, like the Chester Formation.
What are these abrupt variations in sediment type? If you look at figure 1 in the article, both the cross strata and the underlying "stuff" are composed of sandstone, so why are these two layers different? Were both of these sandstones deposited in a river, under the same conditions? If you look at a modern river, and the surrounding flood plains, you will notice that the flood plains cover a much larger area than the river. Why don't we see any layers of these mudrocks at many crags? For example, Hen Cloud is ~35m tall, and exposes no mudstones. Why?
I appreciate that someone is making an attempt to describe the geology of Britain from a climbers perspective, but to do the subject justice would require much more detail, and explanation to the causes.
Maybe focus on questions which are universal to many sandstone crags like "What forms these diagonal layers in these sandstones which i am climbing on?", or "Why is the sandstone here coarse, gravelly and ripping my skin, while up there, the sand is finer, and doesn't hurt as much?", or "why is Stanage edge grey-ish, the roaches red/orange-ish, and Ousal crack in Churnet valley propper red?".
I think you need a geology degree to cover that lot, I should know I have one! That's just from your sandstone perspective, how about all the igneous, pyroclastic and metamorphic questions? That's getting to be a large article. Maybe your inspiration to do more research and maybe that degree?
....and a quick answer, you don't see the mudstones in the grit as they are soft and erode quickly - usually at the top and bottom. Also depends on how thick the grit/sandstone unit is. Go to Avon and you will see shale bands, where there is a lot of interbedded shale then you exclusive climbers won't climb there. Folks who dry tool often use these locations. Also quarries are usually in the more massively bedded shale free rocks. Go to the coast and you will see plenty of shale, think Mam Tor etc. Hope that helps.
The quick answer is the wrong answer. ;-). The mud was there, they were never preserved into the rock record. One of the reasons is lateral accretion and avulsion of rivers (meanders meandering, and then the river jumps to a new location during a flood), combined with a slowly subsiding basin (subsidence is important to preserve the sediments). The slow subsidence rate means that the river migrates and jumps many times at the same level, meaning that over-bank muds associated with with the flood plains are erroded and lost, and are replaced with sands sand bars (both within and at the edge of the channel). If that happens enough, you are just left with sand. Mam tor, as you mention is based with shale, but that's a different mechanism which caused the transition from shale at the base of the hill, into the Mam Tor grits (that was infilling of the basin by turbidites), and shouldn't be compared with the fell sandstone (deposited by rivers): they are two different environments.
OK, for igneous rocks generic questions such as "why are these crystals here coarse, and over there they are tiny", "why is granite light coloured, and gabbro/basalt dark coloured?", "why do igneous rocks tend to form big blocky outcrops", or "what can the type of igneous rock tell me about the history of the area?" would be good universal questions.
For metamorphic rocks, questions like "why are these rocks shiney!?!" or "why are these rocks folded" or "why have these rocks got layers and melted blebs in them?" or "what causes the sheety layering in slate, and why" would be good to ask.
These generic questions apply universally to most igneous/metamorphic/sedimentary outcrops across the world, as the processes governing the formation of the rocks are largely universal.
Oh, and questions for limestone: "when will the weather improve, so i can go back to climbing on the grit?" :-D
Seems like you know the answers to your questions anyway...
These articles have to be very generic- once you're dealing with tougher subjects like influence of the caldoCaled orogeny, Atlantic opening on the mountains of the UK things get tricky very fast
I thought both of these articles worked well as an on-line introduction to the topic, however as I posted in the first article a link to a map would be useful - eg http://mapapps.bgs.ac.uk/geologyofbritain/home.html?
or the 3D version here: http://mapapps.bgs.ac.uk/geologyofbritain3d/index.html?
I also found this book useful in understanding Geology from a climber's perspective 'Granite & Grit by Ronald Turnbull.'
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