Broadfield Down

[twiglet]

Just musing, but heres a question for anyone with a bit of geology knowledge out there. What are the
chances of there being any cave systems under Broadfield Down (The limestone plateau north of Mendip
where Bristol Airport is) ?  I know there are a few rock shelters and mineshafts, but looking at the classic
karst dry valley of Goblin Coombe, it feels like there should be a cave there somewhere.

 

[Les W]

There ought to be caves there and there are a couple of good resurgences in the Avon Gorge that may well be draining that area, however, my understanding is that for caves of any reasonable size to form there is a need for local drainage to be concentrated, normally by some sort of impervious capstone, that causes the runoff to form distinct streams which form discrete sinks they cross onto the limestone will form decent sized caves (i.e. enterable by man). Without this concentrating effect the runoff will sink in many very small fissures and not enlarge them very much at all.
Broadfield down does not have such a "caprock", although it may have in the past so it shouldn't be ruled out entirely.
The underground streams will form bigger passage as they unite so although discrete surface sinks don't seem to be apparent. If access could be achieved from the resurgence end though there may well be a large cave system.

 

[Chocolate fireguard]

Following on from what Les W said, when I was a member of the Limestone Caving Club in Derbyshire back in the 60s we spent a lot of time digging out shakeholes on Bradwell Moor, a couple of km west of Bradwell,  in the hope of finding a "top entrance" to Bagshawe Cavern which is the resurgence for the area.
The moor is a big flattish area of limestone.
One dig got down about 8m but mostly we  found impenetrable fissures.
We eventually decided that the area never had an impervious cap so for the reason Les spelled out  above we weren't going to find a negotiable entrance.
I believe that old lead mines in the area did find larger natural passages at depth.

 

[Thomas Tom-Thomas]

Good question.  Was thinking about this today while I was climbing at Goblin Combe (and loads of other times too).  I have no answers, but a couple of potentially half formed observations.

It's been interesting running around the area during the recent floods - the bottom of Bourton Combe was carrying a pretty big stream at one point which seemed to appear from nowhere (one moment running shin deep in water, the next step was on a dry path), and I've seen a decent sized stream that ponded and disappeared in a gruff by the side of Barrow Court lane.  Went back for a closer look at the Bourton Combe one and it seemed to be coming out of small cracks in the path itself.

Suppose this fits with the "Not a big enough channel for a proper cave" theory.

...there are a couple of good resurgences in the Avon Gorge that may well be draining that area,

I'm geologically pretty clueless, but wouldn't the very low lying valley of Ashton Vale/Flax Bourton get in the way of anything sinking on Broadfield Down from resurging in the Avon Gorge?

 

[Les W]

I'm geologically pretty clueless, but wouldn't the very low lying valley of Ashton Vale/Flax Bourton get in the way of anything sinking on Broadfield Down from resurging in the Avon Gorge?

You are probably correct. I completely forgot about Ashton Vale.  :-[

It is of course possible that there is drainage below Ashton Vale from Broadfield Down, but I think unlikely.
Ashton Vale is Mercia Mud Group sediments although the limeston might continue beneath it at an elevation that will allow groundwater movement beneath the valley. After all, both the Cheddar and Wookey drainages go very deep before resurging.

The resurgences in the Avon Gorge will almost certainly drain the plateau around Failand and Wraxall so this begs the question, where is all the water that falls onto Broadfield Down? If it doesn't cross to Failand then it must return to the surface somewhere else. A good project for someone I think. 

 

[twiglet]

Despit being a relatively small area of limestone, Broadfield Down has six dry valleys.
Bourton Combe,Cheston Combe,Taps Combe,Cleeve Combe,Goblin Combe & Brockley
Combe. The latter has a site "Pots Hole" marked on the OS map. Water is said to emerge
from this after heavy prolonged rain. Has anyone ever seen this ? I never have, but it does
beg the question, where is the water that formed these valleys now ?

 

[twiglet]

Oops, typo error, make that "despite"  :-[  Maybe the active quarries in the area like
Stancombe or Freemans will break into something then ?

 

[Thomas Tom-Thomas]

...but it does beg the question, where is the water that formed these valleys now ?

Hi Twiglet.  As I've said above, I'm no expert, but I think the volumes of water that formed these Combes was a result of radically different climatic conditions (desert/Glacial melt).  The Environment centre people summarised it nicely here:

http://www.goblincombe.org.uk/about/history/

It's a nice and interesting area, wouldn't be surprised if someone found a cave system hidden in there.  For what it's worth, my money for a possible resurgence would be on Barrow Gurney.  Just something about the shape of the ground and the springs around there...<end of wild speculation>

 

[bograt]

Thinking about this, has anyone checked out the mine shafts you mentioned?, comparing it with the southern limestone plateau in the Peak District, most of the natural stuff has been found through intersection by miners, think Bung series in Knotlow, the crawls at the bottom of Tearsall, Water Icicle etc.--
Also, any knoll reefs - always a good place to look.

Must admit, whenever I see programs about the Avon Gorge, I wonder about caves.

 

[exsumper]

I'm geologically pretty clueless, but wouldn't the very low lying valley of Ashton Vale/Flax Bourton get in the way of anything sinking on Broadfield Down from resurging in the Avon Gorge?

You are probably correct. I completely forgot about Ashton Vale.  :-[

It is of course possible that there is drainage below Ashton Vale from Broadfield Down, but I think unlikely.
Ashton Vale is Mercia Mud Group sediments although the limeston might continue beneath it at an elevation that will allow groundwater movement beneath the valley. After all, both the Cheddar and Wookey drainages go very deep before resurging.

The resurgences in the Avon Gorge will almost certainly drain the plateau around Failand and Wraxall so this begs the question, where is all the water that falls onto Broadfield Down? If it doesn't cross to Failand then it must return to the surface somewhere else. A good project for someone I think. 

The most plausible explanation, would be that the Broadfield down plateau has not received the same level of scrutiny that the Mendip Plateau has! After all, speleogenesis of caves of the Mendip plateau appear to have managed quite well without a cap rock?

My own opinion is that a glacier of the Anglian Glaciation had considerable impact on the speleogenesis and development of the caves on Broadfield Down, Western Mendip, Burrington, and the southern flanks of Blackdown; as well as St Cuthberts Swallet and older caves above Wookey Hole!

 

[twiglet]

Hi Bograt,I have laddered one of the more solid looking shafts (20ft down to a muddy choke with old digging kit in situ) but most
of them are horrifically loose, bristling with piles of red ochreous choss just waiting to avalanche onto the keen explorer! The area
is heavily forested, but there are loads of gruffs/rakes lurking amongst the trees.

 

[Les W]

After all, speleogenesis of caves of the Mendip plateau appear to have managed quite well without a cap rock?

Alex,
although Mendip doesn't have a "cap rock" in the true sense of a rock overlying the limestone, it does have an effective concentrator of drainage with the streams running off the central Old Red Sandstone core onto the impervious shales before it sinks at the limestone boundary. In this case the shales, even though they are beneath the limestone effectively work in a similar fashion to a cap rock due to the steep bedding of the limestone.

 

[bagpuss]

Thinking about this, has anyone checked out the mine shafts you mentioned?, comparing it with the southern limestone plateau in the Peak District, most of the natural stuff has been found through intersection by miners, think Bung series in Knotlow, the crawls at the bottom of Tearsall, Water Icicle etc.--
Also, any knoll reefs - always a good place to look.

Must admit, whenever I see programs about the Avon Gorge, I wonder about caves.

I believe someone has a book...there are a few caves but nothing that big. There is a rather pretty small cave close to underneath the bridge (not the one you pay to go in) .

 

[rhychydwr1]

Avon Gorge? Yes there is a book....

 

[mikem]

There is Yorkhouse Cave in Brockley Combe, all 9m of it & Downside Garage Cave (8m) by the airport,which was apparently used as an air raid shelter. There are also several small holes entered by the quarries behind Backwell, the biggest of which appears to be Dial Quarry Cave, north of Felton, which had 37m of passage,although much has been quarried away.

The only major cave known in Bristol is of course Pen Park Hole, which was formed by acidic water from hotsprings rather than a stream.

Mike