Author Topic: Limestone Pavement  (Read 1158 times)

Offline twiglet

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Limestone Pavement
« on: August 12, 2020, 09:31:42 am »
Can I ask a question about geology and the formation of karst pavements please ? I was under the impression that they only form when the surface soil was scraped away by an ice sheet. On a walk on Sunday, I found an extensive limestone pavement at Cheston Combe, near Backwell in Somerset ! I remember from Geography lessons at school that the Pleistocene glaciation only reached as far south as Gloucester & the Brecon Beacons, so how can there be a limestone pavement near Mendip ?
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Offline mikem

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Re: Limestone Pavement
« Reply #1 on: August 12, 2020, 10:18:10 am »
Other erosion can result in the smoothing off of surfaces, but glaciation is the most common (& it removes the covering rock, soil comes later).
https://www.backwellenvironmenttrust.org/index.php/2014-04-11-20-09-53/features/121-limestone-pavement

Also (probably hydrothermal):
https://www.backwellenvironmenttrust.org/index.php/features/20-backwell-cave
« Last Edit: August 12, 2020, 10:27:18 am by mikem »

Offline Mr Dinwiddy

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Re: Limestone Pavement
« Reply #2 on: August 12, 2020, 07:49:19 pm »
Hello Twiglet
Have you got any photos you can post? I am interested in the distribution of pavements in the UK. I have seen a couple of tiny pavements in the Forest of Dean and there are karren/karst features on some of the Jurassic coastal limestone platforms on the Somerset coast.

I think it is difficult to define when a limestone platform, which is fissured by erosion, becomes a proper limestone pavement. The definition I usually use is does it show a recognisable pattern of clints and grikes?

Mikem is right in that any geomorphological process that smooths a surface (usually to a level or dipping bedding plane) is the staring point for a pavement. Most of the erosion that forms the clints and grikes occurs sub-soil so there needs to be a burial stage too and then subsequent exposure.
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Offline Pitlamp

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Re: Limestone Pavement
« Reply #3 on: August 12, 2020, 08:02:19 pm »
Twiglet - you wrote: "I remember from Geography lessons at school that the Pleistocene glaciation only reached as far south as Gloucester & the Brecon Beacons, so how can there be a limestone pavement near Mendip ?".

There has been a number of glaciations throughout the Pleistocene. You're right that the most recent (Devensian) ice advance probably didn't make it quite as far as Mendip. But one before it (the Anglian glaciation) certainly did; it even crossed the English Channel & put the wind up the French for a time.

Maybe your pavement is an old one, not related to the most recent ice advance?

Offline langcliffe

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Re: Limestone Pavement
« Reply #4 on: August 12, 2020, 08:04:56 pm »
But one before it (the Anglian glaciation) certainly did; it even crossed the English Channel & put the wind up the French for a time.

 :lol:

Offline Andy Farrant

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Re: Limestone Pavement
« Reply #5 on: August 12, 2020, 09:10:20 pm »
Limestone pavements can occur where ever the soil has been removed to reveal the weathered limestone bedrock beneath. Dissolution of limestone beneath a soil cover will enlarge the joints and fissures to create the classic clints and grikes, but they are only revealed when the soil is removed. The most common cause for soil loss is glacial activity, but any soil erosion can do the trick.  In the example at Backwell, it is the local volunteer group who removed the soil cover! If you removed the soil from many limestone areas you would see much the same thing, especially in flat or gently dipping limestones. In southern England, exposed limestone pavements such as these will rapidly be covered over by vegetation, and the fissures infilled with soil, but in upland areas of northern England, the climate is less forgiving and that, coupled with overgrazing will keep them exposed.

The ice never got as far as Mendip, even in the Anglian. It just about impinged on what is now the coast near Clevedon, but there is no evidence for any glacial activity on Mendip. Plenty of examples of superimposed drainage and periglacial activity, but no ice.


Offline Andy Farrant

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Re: Limestone Pavement
« Reply #7 on: August 12, 2020, 10:13:00 pm »
Don't believe that crap about glaciations reaching Stonehenge... Some people refuse to accept that the Stonehenge bluestones were transported by people, and instead suggest the ice sheets extended across Salisbury Plain, transporting a bunch of igneous rocks from the Preseli Mountains but nothing else! I mapped much of Salisbury plain and there is no evidence whatsoever of any glacial activity, no till, no erratics, nothing, nor is there any on Mendip.  See also James Scourse's article http://publications.thebritishacademy.ac.uk/pubs/proc/files/92p271.pdf

Offline mikem

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Re: Limestone Pavement
« Reply #8 on: August 12, 2020, 11:19:04 pm »
There are photos in my first link above.

Offline twiglet

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Re: Limestone Pavement
« Reply #9 on: August 13, 2020, 09:59:06 am »
Thanks Andy, & everyone else for your understandable replies. Yes I did take some pics but they are not as good as the slightly elevated pics linked to mikem first reply (a drone perhaps ?) I have found the East Wood limestone pavement in the Forest of Dean, is that the one you referred to Mr Dinwiddy ?  :thumbsup:
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Offline Mr Dinwiddy

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Re: Limestone Pavement
« Reply #10 on: August 13, 2020, 07:02:24 pm »
Thanks Twiglet and MikeM- Sorry, I did not realise that the impressive photos on the link from Mike were the same area of pavement as Twiglet was posting.
Yes the pavement I saw in FoD was at East Wood but there was a second site and I am not sure whether they were both part of the same or were separate. I will have to dig out the maps.
I will try to get a look at the pavement at Cheston when I am next Somerset way
thanks
Mr D

Offline twiglet

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Re: Limestone Pavement
« Reply #11 on: August 19, 2020, 03:36:21 pm »
The Pavement is within the Badgers Wood nature reserve, on the west side of Cheston Coombe about halfway up the hill. There is a layby for one small car actually in the coombe but the road is quite narrow, so probably safer to park near the church and walk up the footpath. :)
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Offline Mr Dinwiddy

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Re: Limestone Pavement
« Reply #12 on: August 20, 2020, 09:43:59 pm »
Thanks Twiglet, I hope to get a look sometime soon.

 

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