Cavedale Lava Samples

[pwhole]

As part of a long-term private research project into the lava bodies around Castleton in the Peak District - essentially belonging to geologist Mark Harwood (though I've managed to elbow my way in over the last few years by doing underground work he can't do) - we've been logging the various outcrops, both above and below ground, to try and add a bit more caver-related information to the geological records that already exist. To that end, we did a lot of subsurface geophysics scanning of the Speedwell Vent a few years ago (documented in TSG 19, and also expanded for a MSc Thesis soon after), and we've been resurveying and sampling in mines known to contain outcrops - mainly the tuff in Pindale End mine, but also a bed of the Lower Millers Dale Lava seen 25m down Hazard engine shaft. We've recently found a new, completely undocumented occurrence of lava/tuff, which will be documented soon.

The Cavedale Lava is particularly interesting as it sits above a large chunk of Peak Cavern, and although the 'umbrella' effect of the lava is well-known, in terms of preventing formations from growing beneath it, less has been written about the formation of the cave itself beneath the lava. As the White River bedding is not far below the horizon of the lava, and the deep shaft of Victoria Aven and the top of George Cooper Aven both break into a lava choke at the very top, it's likely that the lava bed (or breaches in it) strongly influenced the development of the cave below - this may have occurred synchronously with the downcutting of Cave Dale itself. The lava outcrop as mapped by the BGS is an unusual shape, and as far as we can tell hasn't been verified by any underground testing from the surface. To that end we revisited the area in February to make a full inspection of all the visible outcrops, and I offered to gather underground samples wherever possible.

Echo Chamber, reached by climbing Victoria Aven and then descending George Cooper Aven from the Victoria Line trunk passage, has a large collection of lava pieces at the base of the first main slope - one is a cubic metre in dimension, and may be part of a hexagonal column - but there are many small pieces scattered around, and so on a recent trip I grabbed a few samples, which are shown below. Quite a variety of types, but my knowledge of this stuff is limited, so they'll be sent over to Mark for analysis soon. But three out of four are vesicular, whereas the fourth is more of a tuff, with some pretty green crystals scattered throughout. I'll reserve my judgement any further at the moment, but they look very nice

These have clearly been washed down the cave system from the outcrop high above, most likely in the vadose phase when the various breaches of the White River bedding were occurring, but that's as much as I know so far. Unless anyone else knows more! I've read all the geological memoirs.

A QGIS render below shows the Cavedale Lava outcrop in pink, with some of the Peak Cavern survey beneath in red.

 

[pwhole]

Probably the very top of the bed - a prominent ash band/clay wayboard beneath the Rodger cliff outcrop:

 

[langcliffe]

Fascinating!

 

[pwhole]

These large chunks can be found much further down the path, a good 50m or more vertically below the outcrop, though the first one has clearly rolled in floods. The other, larger one looks as though it may be a column, held in place by sediment. In which case it's been there a while.

 

[Pitlamp]

Excellent stuff pwhole.

In some ways this reminds me of the somewhat on and off efforts to trace porcellanous limestones by a few of us across the Dales, in relation to their hypothesised influence on speleogenesis. I mention this merely to flag up the existence of such beds; I can't remember to what extent these occur within the Peak District limestones; in the Peak the situation may not be quite as simple as "limestone or lava"; certain other horizons within the limestone may also act as aquicludes. It's worth bearing the possibility in mind, at least. (Knowing how thorough you are, you probably already have.)

 

[Andy Farrant]

The role of the lava beds and ash deposits, including the weathered ash deposits forming the clay wayboards are well known potential inception horizons and act in a very similar way to the shale beds in Yorkshire and marl seams in the Chalk. Other stratigraphical discontinuities within the limestone succession can also influence cave development, for example chert and dolomite beds, or depositional hiatuses forming bedding planes.. For the Peak-Cavedale area, what would be fascinating to see is a 3D geological model with the lava flows, ash beds and major speleogenetic bedding planes highlighted along with the cave surveys. The technology is there now.

 

[alastairgott]

would be fascinating to see is a 3D geological model with the lava flows, ash beds and major speleogenetic bedding planes highlighted along with the cave surveys. The technology is there now.

Hi Andy, do you know where I can get access to the data for lava flows, ash beds and bedding planes. Do you have any examples of integration of similar data within a 3D model I could have a look at survex, lox, (GIS?).

Or is it a case of pinpointing spots underground and meshing/layering/over(under)laying between them?

 

[Brains]

The redness at the top of this outcrop looks to me to be contemporary weathering in air of the top of the flow, developing a very rusty appearance.
Basalt has a tendency to spheroidal weathering, sometimes as layers giving the name onion skin weathering. Even so some areas of the photos seem to show pillow lava development. This would not exclude the formation of vesicles in the lava. This gives evidence of at surface eruption in shallow water bodies with the upper surface at least being above water before further sedimentation.
Looking forward to future updates

 

[pwhole]

Here's the lava/tuff bed in Hazard Mine shaft, 25m down, so around 403m AOD, almost 100m above the Cavedale Lava. Top wayboard, lava and then the bottom wayboard. I believe this correlates with the Lower Millers Dale Lava, but it looks much more tuff-like here, and appears to have faint bedding lines in places. The limestone above and below is relatively coarsely broken, but the miners have trimmed the lava bed to be perfectly circular. The total thickness was 3m, with the lava itself being 1.6m. The bottom wayboard had been cut back to form a ledge - possibly just to stop chunks falling down the shaft. Although the lava is meant to be vertically faulted here, I saw no evidence of that, with a perfectly congruent bed. There is a clay wayboard further down at approx. 55m that is definitely faulted vertically, but only by a metre or so.

From memory I'd thought the lava to be about 15m down, and so took a 25m rope, Using about 1.5m rigging at the top left it all a bit tight, and with the stretch in the rope, and by abseiling onto the stopper knot I could just hook my feet underneath the lava to stay in position to grab a few shots on my phone. Mark was holding the tape measure at the top, and with me yelling up the dimensions we just about managed it. He did look quite relieved when I eventually climbed out.

 

[ChrisJC]

I'm impressed that this sort of thing can be identified. When I am abseiling down a shaft, the geology literally passes me by!

Chris.

 

[LadyMud]

When I am abseiling down a shaft, the geology literally passes me by!
On the other hand, when I'm prussiking up a shaft, I have LOTS of time to gaze around . . .

 

[Pitlamp]

That shaft photo looks like a still image captured during a colonoscopy!

 

[pwhole]

Here's a few shots of the pale green Pindale Tuff, which outcrops in the floor of Pindale Quarry (now mostly buried in spoil - here seen in the quarry road), and runs underground past Black Rabbit cottages down to Pindale Mine, although the BGS map shows it running out before there, which is odd, as it's documented in the mine records, shaft section and in geological memoirs. In Pindale End Mine lower shaft, just a little to the south of the larger site, the tuff is overlain directly by shale, with no limestone present. The upper shaft workings have better exposures, with some very nice infilling of vertical cavities adjacent to the vein, and the vein is faulted, with some passages having a lava wall and the other wall limestone. Much of the tuff seen underground is soft clay with pellets - the 'cat dirt' described by miners, though there is plenty of solid too. It's also more of a turquoise colour than the pale green seen on surface, but that may just be weathering differences.

 

[Flotsam]

How would the vertical cavities have been created and filled? I would have thought it would have purely been a horizontally laid layer.

 

[RobinGriffiths]

Paleokarst infill??

 

[pwhole]

Sorry - been digging all day. But yes, infill of karst cavities must be the answer. The tuff is fundamentally ejected ash, so it would settle into existing voids naturally as it fell - either in air or through water I guess.

Incidentally, I found some lava chunks in the footpath behind Goosehill Hall yesterday, but they're so low down that they must have rolled down from the Cavedale Lava outcrop above at some point. They were too far away to have rolled from the Speedwell Vent, unless the topography was very different then.

 

[pwhole]

Here's a few more taken at depth in one of the Moss Rake mines - this small unit was found at the base of an internal climbing shaft, about 90m down from surface - although unsurveyed, it would be roughly around 325m AOD, so could correlate with the Cavedale Lava, though it's a variable deposit. The shaft bottom had a single passage in the base that immediately opened into lava, and then returned into limestone after only three metres and then another climbing shaft dropped down deeper - we didn't have time to drop that then, but others may have later. But even in those few metres the character varied from hard rock to tuff, with pink calcite stringers running through it and some bright green clay inclusions.

The first shot shows the shaft bottom from the lava unit, and the second shows the way ahead in the opposite direction, regaining the limestone and the deeper shaft down, with a windlass scrape-mark. The third shows the boundary between the lava and limestone. The rest show detail of the various types seen.

 

[pwhole]

Here's a few poor-quality phone shots of a previously-undocumented lava/tuff deposit, seen in a small lead mine near Castleton, and in an area not generally associated with igneous rock. Forgive me for not revealing the location at the present time, but it is being written up now for publication. But the deposit is unexpectedly met in a small level, and is roughly spherical in shape, though obviously irregular, and about 2.5m diameter. The miners don't appear to have scaled back the walls much, and it almost resembles a small 'lava cave'.

Several small veins of dark purple fluorite cut through the tuff in places, or fill the fractured boundary between it and the limestone, but as can be seen, the tuff itself is also stained bright yellow in patches, presumably following tiny fractures. A small (capped and buried) climbing shaft in the roof from surface (10m deep) drops straight into the centre of the deposit, and the mine level appears to have been driven into limestone from the shaft base, suggesting the miners may have known the deposit was there. Geologically, that's all I know! A sample of this tuff was retrieved and given to Mark Harwood for close comparison with a lump of the Pindale Tuff, its nearest known equivalent, but it's closer to the Cavedale Lava in stratigraphic altitude.

 

[Cantclimbtom]

View attachment 16342

View attachment 16343

For the curious but also ignorant and dimwitted folk like me, would you mind idiot proofing this by drawing some crude arrows on the above pics to label lava / Tuff / etc etc
(Also if the Tuff is yellow, is that iron coming later, because I *assume* the ash wouldn't have been yellow when originally deposited)
Thanks in advance for your patience

 

[pwhole]

I've labelled the tuff and limestone in two of them and reposted as I can't edit the post above now - and added another showing the tuff texture more - fundamentally the tuff is the blue-turquoise coloured material, and close-up it looks a lot more like ash than rock. The yellow patches are definitely staining after deposition, and probably iron-based, but I'm hoping someone more experienced may know. All of this would have been buried by shale, and it's not too far from the margin now, so it's possible that mineral leakage from that did the staining.