whitelackington
New member
Mendip Geology, I am confused.
On Mendip is there more than one layer of Limestone :doubt:
On Mendip is there more than one layer of Limestone :doubt:
Mendip Geology
- Thread starter whitelackington
- Start date
Brains
Well-known member
There are indeed many layers of rock, some of which are limestone. and some of which are flat. Most are all wrinkled up and cut about, cracked, broken, folded and overlaid with other stuff. The only real surprise is that there is enough to make even a half decent sized chamber...
cap n chris
Well-known member
Brains said:
So it must be very surprising indeed to discover that UK's largest known chamber is located within it!....
cap n chris
Well-known member
graham said:whitelackington said:
Yes (but it can depend on what you mean by layer
.... along the lines of "is there more than one layer of sand in the Sahara?".....
whitelackington
New member
graham said:whitelackington said:
Yes (but it can depend on what you mean by layer
What I am asking,
is there a band of Limestone, layed down over a period of time, followed by a band of different stuff,
followed by another layer of limestone?
Brains
Well-known member
Yes, sort of!
A BED of rock can be of variable thickness from a few mm to a few m thick. It would generally be present over a large geographical area. One BED is seperated fron another by a change in rock type - different rocks (limestone / sandstone / shale etc.), or by a change in the same rock type (silt/sand/gravel etc), a change that is widespread and easily recogniseable. The sequence of sediments will build by like a huge layer cake, and certain groupings of BEDS will be generally similar but have other beds within them, so the Lower Limestone Shales (LLS) would contain mainly shaley limestone beds, but could - will also contain shell beds, coal seams, sandstones, seat earths, clay layers, ash layers...... but generally the limestone shale will predominate. The Great Scar Limestone will generally contain massive beds of limestone (several m thick), but will also have clay, coal, shale layers as well.
Individual beds can be hard to trace from one area to another, but fossils are a great help. Formations like the LLS are seen over a wide area and help to sort out regional geology.
Once layed down, the rocks are squeezed, folded, cooked, twisted and broken; eroded away and recovered, and generally messed about with.
It has been said that most of geological time is found in the bedding planes rather than in the beds themselves, which tend to form relatively quickly.
Does that help at all?
A BED of rock can be of variable thickness from a few mm to a few m thick. It would generally be present over a large geographical area. One BED is seperated fron another by a change in rock type - different rocks (limestone / sandstone / shale etc.), or by a change in the same rock type (silt/sand/gravel etc), a change that is widespread and easily recogniseable. The sequence of sediments will build by like a huge layer cake, and certain groupings of BEDS will be generally similar but have other beds within them, so the Lower Limestone Shales (LLS) would contain mainly shaley limestone beds, but could - will also contain shell beds, coal seams, sandstones, seat earths, clay layers, ash layers...... but generally the limestone shale will predominate. The Great Scar Limestone will generally contain massive beds of limestone (several m thick), but will also have clay, coal, shale layers as well.
Individual beds can be hard to trace from one area to another, but fossils are a great help. Formations like the LLS are seen over a wide area and help to sort out regional geology.
Once layed down, the rocks are squeezed, folded, cooked, twisted and broken; eroded away and recovered, and generally messed about with.
It has been said that most of geological time is found in the bedding planes rather than in the beds themselves, which tend to form relatively quickly.
Does that help at all?
Hughie
Active member
An important thing to remember is that a change in the rock type denotes a change in the situation at the time. If it is just the 'type' of limestone that has changed then there has been a change in the type of sediment that has settled at the bottom - maybe different eebie jeebies living in the water, or a different proportion of lime mud to fossils etc. If the rock has changed to coal then it means that the relative sea level had dropped and was no longer within the sea. A change to shale usually means that the relative sea level had risen and the water was a little deeper than that at which limestone usually forms. It is quite common to get series of shales through limestones to beach type deposits and finally coal which represent a period of overall relative sea level fall. This sequence can be repeated many times. Wonderful examples of this (I think it is called Yordale series?) is visible along parts of the coast in the North East of England.
Peter Burgess
New member
Many years ago, I stumbled across a wonderful map in a Uni bookshop, which I immediately bought, knowing it might 'come in handy'. It's Geological Sheet ST45 (Cheddar) from the IGS, at a scale of 1:25000, and in a series entitled "Classical Areas of British Geology".
It shows more details that the normal series of geological maps, and is very useful when explaining the geology of Mendip to new cavers. It has a good section of the area, and a detailed list of beds. Oldest first, (from central Mendip outwards, north and south), these are the beds detailed on the map:
Lower Limestone Shale, - Grey-green shale with local developments of crinoidal and oolitic limestone
Black Rock Limestone - Dark grey crinoidal limestone with two prominent horizons of chert (850-950 ft)
Dolomite - up to 300 ft - not present everywhere
Burrington Oolite - Light grey oolite with bands of crinoidal debris in lower parts (550-700 ft)
Cheddar Limestone, Cheddar Oolite, Calcite Mudstone - collectively called Clifton Down Limestone, 520-700 ft
Hotwells Limestone - mainly granular grey limestone - about 700 ft.
Overlying uncomformably on part of the above series of beds, the Triassic Dolomitic Conglomerate deposit.
The map has a liberal sprinkling of dip angles on it, mineral veins, faults, and superficial deposits.
Something I noticed for the first time the other day, from this map, is that there is a significant overthrust fold (I think that's what it's called) near the UBSS HQ, just west of Burrington Combe, resulting in inversion of the beds (overturned strata). I wonder what effect this might have on cave formation there?
It shows more details that the normal series of geological maps, and is very useful when explaining the geology of Mendip to new cavers. It has a good section of the area, and a detailed list of beds. Oldest first, (from central Mendip outwards, north and south), these are the beds detailed on the map:
Lower Limestone Shale, - Grey-green shale with local developments of crinoidal and oolitic limestone
Black Rock Limestone - Dark grey crinoidal limestone with two prominent horizons of chert (850-950 ft)
Dolomite - up to 300 ft - not present everywhere
Burrington Oolite - Light grey oolite with bands of crinoidal debris in lower parts (550-700 ft)
Cheddar Limestone, Cheddar Oolite, Calcite Mudstone - collectively called Clifton Down Limestone, 520-700 ft
Hotwells Limestone - mainly granular grey limestone - about 700 ft.
Overlying uncomformably on part of the above series of beds, the Triassic Dolomitic Conglomerate deposit.
The map has a liberal sprinkling of dip angles on it, mineral veins, faults, and superficial deposits.
Something I noticed for the first time the other day, from this map, is that there is a significant overthrust fold (I think that's what it's called) near the UBSS HQ, just west of Burrington Combe, resulting in inversion of the beds (overturned strata). I wonder what effect this might have on cave formation there?
cap n chris
Well-known member
Overthrust folding is also termed a "nappe" if my memory serves.
I think you meant to type uncomformity not uncomforably!
Mendip geology is much more complicated than a straightforward layering model might have suggested!
I think you meant to type uncomformity not uncomforably!
Mendip geology is much more complicated than a straightforward layering model might have suggested!
Peter Burgess
New member
No, I actually typed 'uncomformably', which I thought was the adverb to describe how the younger stratum in an unconformity lies on top of the older strata. Is this wrong, any geologists out there to advise?
cap n chris
Well-known member
Righto, Peter.
I'm not a geologist so am out of my depth on this nomenclature/adverb malarkey.
I'm not a geologist so am out of my depth on this nomenclature/adverb malarkey.
Peter Burgess
New member
Neither am I. Perhaps there's a petro-geek out there to make us feel nice and warm inside for using some long words correctly.
Peter Burgess
New member
http://www.thefreedictionary.com/unconformably
http://www.thefreedictionary.com/nappe
:chair:
http://simple.wikipedia.org/wiki/Fold_(geology)
Overfold, not overthrust fold
http://www.thefreedictionary.com/nappe
:chair:
http://simple.wikipedia.org/wiki/Fold_(geology)
Overfold, not overthrust fold
L
Limestone_Cowboy
Guest
Unfortunately I am a petro geek as you put it. I disagree with some of the Wikipedia definitions though, unconformably is perfectly acceptable as the adverb to describe the overlying rocks in an unconformity.
A nappe is a term usually reserved for large mountain ranges such as the Alps where vast sheets of rock have been transported, although they are usually also folded over at the front edge of the nappe.
I've personally never heard of an overfold as wikipedia suggests, we would normally just call what you describe as an overthrust fold, a fold. Not all folds have to be in the vertical plane, it is quite common to get folds in all sorts of orientations.
Hope this helps?
A nappe is a term usually reserved for large mountain ranges such as the Alps where vast sheets of rock have been transported, although they are usually also folded over at the front edge of the nappe.
I've personally never heard of an overfold as wikipedia suggests, we would normally just call what you describe as an overthrust fold, a fold. Not all folds have to be in the vertical plane, it is quite common to get folds in all sorts of orientations.
Hope this helps?
Peter Burgess
New member
Nothing unfortunate about it, old boy. I like geeks - after all I am one myself. I can tell, because when I get going on a subject that I think is really interesting, everyone else's eyes glaze over.
Hughie
Active member
Peter Burgess said:
'Unconformably' is indeed the correct word to use in this case!
An unconformity is a break in the sequence of strata in an area that represents a period of time during which no sediment was deposited. Unconformities are more obvious when the older underlying beds have been tilted or folded before the overlying beds are laid down. I would have thought that the unconformity almost always represented a period when the rock was no longer under water (and therefore probably suffered erosion too)
Hughie (or rather, Elaine) is available for any further technical geological questions!
Peter Burgess
New member
So what is the dolomitic conglomerate?
From the map I have, it seems to lie at the base of the hills, and in some of the valleys that cut through the hills. Was it deposited under water, or was it like 'scree' off the hillsides, which consolidated into a rock? I always imagined the Mendips in Triassic times as a range of desert-like hills standing above a desert plain - rather 'Martian' in appearance, not that I've been to Mars, you understand.
From the map I have, it seems to lie at the base of the hills, and in some of the valleys that cut through the hills. Was it deposited under water, or was it like 'scree' off the hillsides, which consolidated into a rock? I always imagined the Mendips in Triassic times as a range of desert-like hills standing above a desert plain - rather 'Martian' in appearance, not that I've been to Mars, you understand.
Brains
Well-known member
Conglomerate is a jumble of stuff of varying grain size from boulders down, and can be formed as talus / scree in a desert type enviromment. The bits tend to be less agular than breccia, which is also made of lumps of all sizes. Breccia is perhaps more commonly found in fault zones or volcanic debris. Dolomite is a Ca/Mg carbonate, which effectively is formed as a replacement for Ca carbonate rather than as a primary deposit, by the action of Mg rich groundwaters. Traditionally, dolomitic rock is not supposed to host caves, but can be a good source of mineral veins as the ground water circulation and alteration implies. Dolomitic Conglomerate is therefore a jumble of what was limestone debris...
Peter Burgess
New member
Please, someone, correct me if I am wrong, but does not Wookey Hole emerge through the DC?
Also, my map shows a location near Rickford, where 'Conglomerate with fish remains' can be found. Suggests that the area was a bit wetter than a desert when the DC was deposited.
