Sea Caves

[whitelackington]

Caves such as Gough's Cave and Wookey Hole have phreatic loops that go very deep. I don't do sumps but I have been told they can go deeper than present sea level, so this presumably happened when the sea level was lower than it is now, ie: the ice age.
Does this mean that there is a lot more cave down stream but full of sea mud :?:

 

[Peter Burgess]

so this presumably happened when the sea level was lower than it is now

Not necessarily, but I can't think of an easy way to describe why. I might came back to this if someone else doesn't post up a good reason.

 

[gus horsley]

Caves such as Gough's Cave and Wookey Hole have phreatic loops that go very deep. I don't do sumps but I have been told they can go deeper than present sea level, so this presumably happened when the sea level was lower than it is now, ie: the ice age.
Does this mean that there is a lot more cave down stream but full of sea mud :?:

The reason the phreatic loops go deep isn't due to a change in sea level (although a sea level change could alter the character of the cave). It's due to hydrostatic pressure. In a phreas the water is under pressure so it is free to flow in any direction, even upwards (like in a u-tube), following an appropriate line of weakness (such as a fault or joint). This effect is heightened by the fact that the water is dissolving the rock rather than eroding it. In the Mendips the strata is tilted, at about 45 degrees in places, so you get phreatic switchbacks as the water alternately dissolves a route along joints and beddings. And consequently you also get a lot of airbells as well.

If this doesn't make sense then I'll try to come up with a diagram of some description.

 

[Peter Burgess]

Yeah, that's it.

 

[gus horsley]

Peter's avatar shows the effect quite well.

 

[Peter Burgess]

Check out page 10 of

http://www.jncc.gov.uk/pdf/V12chap1.pdf

The first couple of diagrams show it a bit better than my Wealden anticline avatar.

 

[gus horsley]

I know your avatar isn't the Mendips, but it still illustrates the idea of an anticline with steep bedding. You don't get such a pronounced vertical phreatic range in other areas such as the Yorkshire Dales because the limestone there is bedded nearly horizontally.

 

[Peter Burgess]

The diagrams in that paper I posted a link for, are supposed to illustrate the development of somewhere like Ogof Ffynnon Ddu, where the dip is quite gentle. I think the principle can work in shallow-dipping beds as well as steep ones, though steep bedding planes are more likely to result in an active phreas well below current sea levels.

This reminds me - didn't the Severn Tunnel (Great Western Railway) intersect an active phreas underneath the estuary? It required a whole bank of huge Cornish engines to keep the water out of the tunnel. Where did the water emerge before the tunnel was built?

 

[gus horsley]

I believe the Severn Tunnel had been completed for several years before the water started to appear, which was derived from sinks close to Chepstow. They may originally have drained into Otter Hole but when the tunnel was driven it intersected the same band of limestone and artificially depressed the water table. Being into mines you can probably think of examples where wells have dried up, etc due to the effects of mining.

You're also quite correct about deep phreas not being restricted to areas of steeply-dipping limestone (Bar Pot shows evidence of being phreatic in origin). However, the phreatic systems in the Mendips could go below sea level because the limestone has a 45 degree bedding. Interestingly, there's evidence that if the bedding gets steeper than 45 degrees the phreatic systems tend to level out again, since the faults act like beddings and vice versa.

I haven't got into a debate about cave geomorphology for ages. Could be another ruddy duck in the making here.

 

[Peter Burgess]

I thought the Severn Tunnel construction was abandoned for a time after the works were flooded out. It was only when a bottom walking diver went down and closed a sluice in a set of doors that it was possible to pump it out and complete the work. Which sort of implies that the springs were intercepted while the tunnel was being built and not after completion. Its a classic example of a freshwater watercourse below sea level though.

http://www.divernet.com/history/severn999.htm

 

[gus horsley]

I just read the account and I think the six months or so from the start of construction of the tunnel to the water finding ingress is actually a reasonable time for capture of a stream to take place, especially if there was a pre-existing fault. This fault could have been present in the original phreas and, like many faults, been filled with loosely-consolidated material. However, there is a lower limit to which hydrostatic pressure operates, so although the fault existed, it would have taken an artificial lowering of the phreas for it have become operative. If you see what I'm getting at.

 

[Peter Burgess]

Being into mines you can probably think of examples where wells have dried up, etc due to the effects of mining.

The Kent Underground Research Group reports:

"Early Geological Survey publications and natural history journals contain several interesting references to similar caves in Kent. In 1879 workmen digging a tunnel from a well at Strood Waterworks broke into a large natural cavern containing running water, which was eventually explored for about 200ft. Another reference in 1907 described a cave in a well at Knockholt which measured 30ft long, 18ft high and 12ft wide with a stream flowing through it. A similar cave and watercourse is said to have been entered at a well near Chatham and three separate caves were discovered at Blackheath during the construction of a sewer tunnel. Unfortunately, none of these early finds are accessible today."

Examples of wells intersecting active cave systems.

 

[gus horsley]

I was thinking more of the opposite effect. For example, when the County Adit was driven in the late 18th century one of the immediate effects was that the wells which supplied Redruth with water (a town of approximately 50,000 inhabitants at the time) dried up. Mines which had no artificial connection to the adit suddenly found they didn't have to pump anymore. Because of the unexpected increase in water the dimensions of the adit had to be increased threefold. This effect happened over a matter of several weeks, so I'm applying the same sort of idea to the Severn Tunnel.

 

[Peter Burgess]

When a drainage level was driven to permit the extraction of stone beds at Merstham between 1807 and 1809, the local lowering of the water table caused a spring about half a mile away to dry up. Unfortunately this spring fed the village watermill, which thus had no power supply, and the miller successfully sued the quarry owners for damages. When the drainage level was blocked in 1820, and the quarries abandoned, the spring returned.

 

[Brains]

The Milwr Tunnel intersected many watercourses, including one rather moist one that caused the spring at Holywell to run dry... Would quote the ref but I have packed the book away somewhere (Cris Ebbs: Milwr Tunnel)

 

[gus horsley]

Peter. Why are our discussions turning into epics? I'm supposed to be working. I'm giving Civil Servants up and down the land a bad reputation. :twisted:

 

[Peter Burgess]

I'm giving Civil Servants up and down the land a bad reputation.

Single-handedly?

 

[Peter Burgess]

including one rather moist one that caused the spring at Holywell to run dry

Which reminds me of a well sunk close to the sea at Eastbourne, at a place called Holywell, to supply the town with water, which worked fine for a while until the water turned saline.

 

[gus horsley]

The Milwr Tunnel intersected many watercourses, including one rather moist one that caused the spring at Holywell to run dry... Would quote the ref but I have packed the book away somewhere (Cris Ebbs: Milwr Tunnel)

True. And a mine at Cathole in the same area drained the phreas which is now represented by Ogof Hesp Alyn, giving an arduous but interesting look at a "recently" drained phreas:

www.ukcaves.co.uk/window.php?cave=238

 

[Deeply Mendippy]

When we did Beachy Head Cave we were advised by the Coastguard at Birling Gap to phone the coastguard's south coast control centre to avoid such a callout.
They can be contacted on 023 9255 2100 and were most courteous, thanking me for notifying them and asking that we called them back when we had finished.
This is actually quite a good idea for Beachy Head Cave as you are right opposite the lighthouse which (when I was there) is manned, they are quite likely to spot you and possibly call out the coastguard.

Also you have a built in callout system which ensures that the coastguard are the first to know about the cave rescue (like most cave rescues )

The other advice the Coastguard at Birling Gap gave us was to approach th cave from Eastbourne. This is a BAD idea. You have no idea how many clift falls there are to cross and when you get to Beachy Head you look towards Birling Gap longingly, knowing that it is the wrong way and you have to traverse all the falls again to get back to the car.

Is Beachy Head cave worth a trip?

I live not too far away and once went to have a look for it. We eventually spotted the entrance (i think) some 20 foot up a vertical cliff. We did not have the neccessary kit to scale the cliff and am wondering if it is worth the effort of going back with more equipment to look around it.