This weekend I’ve been involved in a bit of geoconservation work a.k.a. extreme gardening. Working with the Staffordshire RIGS group, we were cleaning up a couple of outcrops along the Hamps and Manifold Geotrail at a site called Ladyside Wood (Location 19 on the map below). The parent body of Staffordshire RIGS, UKRIGS (UK Regionally Important Geological / Geomorphological Sites) has just changed its name to GeoconservationUK. Personally I think that this is a poor choice as the RIGS groups do so much more than geoconservation. I would have preferred GeodiversityUK which would have given a feel for the range of things that we do.
But anyway, back to the Manifold Valley …
The map shows that the Ladyside Wood locality should display the transition from the reef knoll facies of the Milldale Limestone (purple), through the bedded facies of the Milldale (dark blue) to the overlying bedded Ecton Limestones (pale blue). Unfortunately the sections have become somewhat overgrown.
Often in geoconservation we find that there is a certain tension between the geoconservationists who want to expose the outcrops and the bioconservationists who want to preserve certain plants. Before starting clearance on this section we consulted with the National Trust who own the land. Can you spot what was the only plant they asked us to leave? OK, I admit it is difficult from the photos but the answer is the yellowish shrub on the right of the lower image. The bizarre thing is that is a flowering red currant – a garden shrub and not even native to Britain. Apparently it looks pretty when it flowers!
Anyway, after five hours of pruning, scraping, sawing and scrubbing, we managed to get the outcrops returned to some of their former glory.
This is the first of what I hope to be many geoconservation sessions to be run by Staffordshire RIGS.
I’ve just finished preparing a presentation I’m giving at the 15th European Meeting of Environmental and Engineering Geophysics of the Near Surface Geoscience Division of EAGE (or Near Surface 2009). I’m so used to talking for a micro-century (~50 minutes) in lectures, it is hard to discipline one’s self to a 15 minute slot.
But, before I head of to Dublin, there is just time for the latest instalment of “Wrong Rock“. Last Time I discussed a church that had a new extension being built in a rock type that didn’t match the original. I also noted that the disused quarry that had supplied the original building was less that a kilometre away and it would have been much better from both an aesthetic and ‘stone miles’ point of view if a way could have been found to re-open that quarry to supply the stone for the church. This week I have uncovered what I consider to be an even bigger crime against stone.
Like most old churches it was originally built from the local rock, in this case the Lower Triassic Kidderminster Formation (in old nomenclature Bunter Pebble Beds) which as the old name suggests is a coarse, pebbly sandstone. Again, the original quarry is close by, less than 500 metres from the church. This time it isn’t the modern extension (which is round the back) that I’m railing against, but the repair work on the original. These repairs have been done with Triassic Hollington Stone (formerly Lower Keuper Sandstone) which I’ve discussed before. It might not look too bad from a distance, but close up it really clashes.
What is worse, is the nature of the repair. Rather than being flush with the existing stones, the steps on the top of the replaced blocks will collect water and cause the old stone block above it to weather even faster. And this is on a Grade One Listed Building, “a building of outstanding architectural or historic interest”. Special permission is required for repairs and particular materials are usually specified (although churches with current worship do have some exemptions). How much better the repairs would have been if the local stone had been specified and the replacement stone fitted flush with the original.
Recently I attended a geological walk lead by Mike Allen from the South Peak Estate of the National Trust to have a look at a new geological section that has been opened upon the side of Ossum’s Hill above the Manifold Valley in the Staffordshire part of the Peak District. The geology of the area is shown in Figure 1.
The walk departed from the car park opposite Wetton Mill which was the site of the shaft, dressing floor and smelter for the Botstone Lead Mine that operated until around 1850. Machinery was driven by a waterwheel from the River Manifold. The assent of Ossum’s Hill was via the footpath that follows the path of the Hoo Brook, a small misfit river in a wide valley carved by meltwater at the end of the last Ice Age. Part way up, a large Ash tree on the right marks the site of one of the entrances to the Botstone Mine (Figure 2).
The Hoo Brook valley sides show evidence of landslipage, probably the result of over-steepening of slopes by meltwater undercutting them. On the climb up the valley, many of the pauses for breath were used to discuss different facets of global climate change, both during the Ice Age and during the Carboniferous Period. The route then took us up a side valley to the left up beyond Ossum’s Hill Farm and then left again to the top of Ossum’s Hill where the depressions from numerous old mine shafts can be seen. The local farmer estimates the shaft of one of them to be at over 50 metres deep.
Over the hillcrest, the path then follows a farm track slanting down the hillside back towards Wetton Mill. It is here that the farmer, in widening the track, has opened up some new exposures of the Carboniferous Limestone. The upper part of the section displays the mid-grey coarse bioclastic crinoidal Ecton Limestone (Figure 3). This was deposited on the lower part of the shelf slope as turbidites from the shallower shelf areas.
As the path descends the hillside the transition can be seen to the underlying finer bedded and laminated micritic Milldale Limestone with thin beds of chert deposited under quieter conditions. Here is also displayed some tectonic deformation with a small fault and some local small-scale folding, probably related to the fault (Figure 4).
The view across the Manifold Valley clearly displays the reef-knolls within the Carboniferous Limestones as upstanding mounds in the topography (Figure 5), but, with the light fading fast, we didn’t linger too long and descended the hillside back to the Manifold Valley floor and returned to Wetton Mill.
Thanks go to the National Trust for organising the walk and Mike Allen leading it.
I’d like to introduce the concept of ‘stone miles’. A bit like ‘food miles’, building stones are often transported great distances (in some cases halfway around the world) when local ones will do, often much better. David Williams from ‘Stories in Stone‘ recently posted about the slate used for the new café at the top of Mt. Snowdon in Wales being made not of Welsh slate but of Portuguese. The problem is that British planning laws make quarrying difficult and expensive. Yesterday I came across an example of where this is having repercussions even on a small county scale.
Here is a lovely old church in the Staffordshire Moorlands having an extension built – can you spot the problem?
The main church is made of the local bedrock, the Rough Rock (Carboniferous, Upper Namurian [OK, Lower Bashkirian if you insist] in age). It is not hard to track down the original quarry that supplied the stone. It is only ~500 metres away, now overgrown but otherwise OK. This is what the Rough Rock looks like …
The problem is the extension is being built from Hollington Mottled Stone. I’ve discussed the Hollington Stone before, it is a Lower Triassic sandstone and looks like this – spot the difference?
Now, I’m not criticising the church. They have used the most locally available currently quarried building stone, it even comes from the same county, but it is the wrong rock and it doesn’t match. When it weathers it will become a closer match in terms of colour, but in terms of grain size and texture it simply isn’t right.
The Rough Rock is no longer quarried in Staffordshire for building stone. It is quarried where it is less well cemented, for glass sand, but that is another story. It is a shame that planning laws are such that it is simply uneconomic to reopen the original quarry to take a limited amount of stone for a heritage project that will be a perfect match to the church stone.
Britain is rapidly loosing it’s geodiversity. In Staffordshire about 25 different rock types were locally quarried for buildings. We are now down to two, and one of those isn’t really suited to much beyond drystone walling repairs. It is very sad to see local historical heritage buildings being added to with the wrong rock simply because it is the closest available match going.