The trench runs across the crest of the ‘Malvern Axis’, a major monoclinal fold trending north-south through central England that brings up Precambrian (~677 Ma; Cryogenian) to the surface. The Malvern line separates the two Precambrian terranes of the Midlands Microcraton, Wrekin Terrane to the west and Charnian Terrane to the east, that forms the solid basement of England. These Precambrian igneous rocks are unconformably overlain by Middle Cambrian Malvern Quartzite, and then Upper Silurian (Pridoli) Raglan Mudstone, and Upper Carboniferous (Moscovian) Halesowen Fm. This sequence was folded and thrust during the Variscan Orogeny at the end of the Carboniferous into the north-south Malvern Axis. Extension during the Triassic produced normal faulting along the Malvern Line and deposition of Middle Triassic (Anisian) Bromsgrove Sandstone to the east in the Worcester Graben. All this is being exposed in just one 100m trench, albeit somewhat tectonically shortened.

Standing on the axis, this is the view to the east. In the trench, the light coloured material in the foreground is Precambrian Malvern Complex, succeeded by grey/green and grey Carboniferous, red Silurian muds and Triassic sands towards the car.

To the west, the white is Cambrian followed by Carboniferous and Silurian on the other side of the axis. Note that many of the lithological identifications are still tentative.

The section is still in the process of being created and is on private land, but should be stunning when finished.

I’m late for this one and most of my geology photo archive isn’t on this laptop, so I’m going to go with something that I have to hand. This image is one of my gigapans (in this case a matrix of 16 x 6 photographs stitched together). It shows the foreshore at West Angle Bay, Pembrokeshire, Wales. The view looks westwards towards Milford Haven and shows the Lower Carboniferous Limestone contorted by a series of Variscan thrust related folds. One of the thrust planes is seen in the left of the image, over-steepened by the folding. To the centre of the image are a pair of whaleback periclinal anticlines. The beds then steepen again to vertical on the right via a tight syncline.

But the beauty of a gigapan image is that one can dive in and view the detail like the slickenside lineations on the thrust plane or the writing on the buoy.

All three hundred deskcrop images can be found on my companion posterous blog and at my flickr site.

A google map with links to the geotagged images is embedded below.

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Lockwood DeWitt at Outside the Interzone is hosting this month’s accretionary wedge where he asks “What is the most important geological experience you’ve had?”. The stress here is on the word important.

Picking the most important is incredibly difficult for me. I have been fortunate in my earlier career to have all sorts of important geological experiences, from climbing the summit of Mt Fuji in Japan to exploring the deepest wastes of the Atacama desert, from standing at the top of Monte Perdido in the Pyrenees to the bottom of the Black Canyon of the Gunnison in Colorado.

However, the most important for me has to be my undergraduate mapping in Lukmanierpass, Switzerland because it was important to me on so many different levels. I’ve already covered this way back in Accretionary Wedge 11, Field Camp, so I’ll try not to repeat myself too much.

The first part of the importance is because it was a turning point in my life, the point where I grew up. Up to that point I had had something of a sheltered upbringing. I had not travelled abroad except for a “De la Beche Club” (the student geology society at the Royal School of Mines) cycling geology field weekend in Northern France, and I certainly hadn’t been abroad alone. There were three of us sharing a large frame tent in Switzerland, but the car could only take two plus the tent, so I had to make my way there by train. The Swiss railway system is incredible and runs to the second. It was the first time that I saw proper mountains. The metre gauge train from Göschenen at the northern end of the Gotthard Tunnel climbing up to Andermatt is an experience in itself. Travelling alone across Europe gave me the confidence to go to so many other places since then.

(Note: I’ve converted these to black and white because the older colour photos have faded badly)

It was also the first time that I had done proper independent mapping. Prior to this our mapping training was done as buddy pairs but here I was on my own. We wouldn’t be allowed by health and safety regulations to do this today, which is a real shame because it was a wonderful experience. It was just me against the rock. I had to sort things out for myself. It took me about four weeks to work out why in one part of the area the bedding/cleavage relationship was telling me that the beds were upside-down where as I knew from the stratigraphy that they were the right-way up. It was a struggle, but I cracked it – myself.

The geology was incredible. I’ve never really seen anything like it before of since. The sediments trapped between the internal and external basement zones of the Alps exhibit one of the highest metamorphic gradients in the world with one unit going from amphibolite grade (shown above) to sub-phyllite in just a couple of kilometres. Some of the faces with white kyanite acted a mirrors in the bright sunlight. You couldn’t examine the mineral texture without sunglasses.

My last day in the field was my 20th birthday. After almost six weeks mapping, I had just one last valley to map. And, halfway up the valley I found a rock that I was not anticipating to find. This has taught me never to assume anything where mapping is involved and always check everything out. I had to work very hard to sort out that valley’s geology because I had a train booked home the following morning. I returned to camp absolutely exhausted but ultimately triumphant.

Google Earth view of my undergraduate mapping area, from the lake to the top of the ridge in the middle distance. The foreground ridge by the lake is Precambrian external zone basement gneiss and the middle distance ridge is Precambrian internal zone basement gneiss. Between them is a sliver of highly deformed and metamorphosed Mesozoic sedimentary cover rocks.

The first impression of the site is its relatively small size, about the size of a football pitch and how quiet it is. From 100 metres away the road noise from the M6 motorway is louder.

The drill rig is also smaller than I expected it to be and, as can be seen in the image, is actually mounted on the back of a truck. The drilling plan is shown schematically below [not to scale].

The ‘mother’ borehole (in blue in the diagram) is drilled to about 1000m depth using directional drilling in an ‘S’ shape, shallowing through the target coal seam, and then steeping downwards to form a sump. This is the current stage of the drilling.

The drill bit will next be withdrawn back up the hole to the level of the target seam (in this case the Great Row Coal) and a spur drilled along the the coal seam itself, slightly dipping upwards (in red on the diagram). Water then drains from the coal seam to the sump where it is pumped out. The draining of the water decreases pressure in the seam and stimulates the release of methane from the coal.

If this proves successful, and the mother bore is already showing promising signs of methane, then the next stage will be to drill a number of other bores through a variety of seams, extract the methane which will then be piped to the University’s boiler house.

Keele is blessed with several unmined coal seams below campus. The land on which the university was built was once owned by the Sneyd family who built their family seat, Keele Hall, here. Whilst making, and losing, a fortune from coal mining in North Staffordshire they had the common sense not to undermine their own house!

The other thing that we are interested in are the bottom hole temperatures. North Staffordshire coal mines were amongst the warmest in the country, and if, as we expect, the downhole temperatures are around 50-60°C then we also have the potential for geothermal energy as well.

Disclaimer: As always, this blogpost represents the views of myself only, and are not necessarily those of my employer.

Now that teaching and exams have finished for another year, thoughts turn to the field. Whilst the likes of Geotripper and Dr Jerque get to visit some spectaclur and unspoilt places, my field work takes me to somewhere interesting but not quite as pretty. These are Beech Caves in Staffordshire.

The caves aren’t natural but the result of mining the Triassic Bromsgrove Sandstone (formerly the Keuper Sandstone) for building stone. The pillar and stall workings were begun possibly in 1633 for the construction of the nearby Trentham Hall. The Trentham records for August 31st 1633 note a Roger Low being paid 22 pence per score for carrying 130 foot of stone from Beech.

Trentham Hall 1686 : Image source – Lost Heritage

This was the first of several halls at Trentham, being rebuilt in 1690 and again in the 1830s, ultimately becoming one of the finest buildings in England. Unfortunately, pollution from the growing Potteries conurbation filled the lakes with sewage and the magnificent hall was abandoned and demolished in 1912. The gardens did remain and now the lakes have been cleaned and the gardens refurbished (well worth a visit) there are plans to rebuild the hall as a five star hotel.

Beech Caves still show the evidence of the hand-pick marks by the miners as they followed a layer of pale-coloured sandstone dipping gently down into the hillside. The thick overburden made mining rather than quarrying a more attractive proposition.

In more recent times the caves were probably used as a munition store in the second world war but lately they have been used for raves and other undesirable activities. The caves are now litter strewn and graffiti covered. The land owner and the local council now want to block off the entrances to stop the ne’er-do-wells from getting in. However, in doing so, they will bury an important piece of Staffordshire’s geological history. It would be a great shame if these historic pillar and stall workings were lost. Whilst understanding the landowner’s concerns for the site, it is hoped that some limited, secure access can be maintained for historians and geologists alike.

Reference: Middleton T, 1986. A survey of Beech Cave, Staffordshire. Bulletin of the Peak District Mines Historical Society, 9, 401-403

Also some nice crinoid fragments.

I’ve not been putting the images here on this blog (other than in the sidebar widget down on the right) for two reasons. The first is that I thought that they might clutter up this blog and get in the way of the other posts. The second is that by using posterous I can post images via a simple e-mail which might be easier for me when I’m away in the field.

If you haven’t been subscribing to my posterous blog (either directly or via Chris’ allgeo feed) then this is what you have been missing …

The images can also be found on flickr or on google maps.

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Only 265 days to go …

Two visits this weekend to Wenlock Edge, Shropshire with strongly contrasting weather (cold and wet on Saturday, warm and sunny on Sunday). This is Knowle Quarry at Presthope, the original GSSP of the Wenlock, and studied by Roderick Impey Murchison in his establishment of the Silurian System. Knowle Quarry is owned by the National Trust and open to the public but being a SSSI no collecting of material is allowed.

There are two distinct facies present, massive reef knolls and interbedded limestones and marls. The red colour in the photograph is below is iron staining on a strike-slip fault plane that cuts the reefs, the near horizontal slickenside lineations can be seen on the surface.

After studying the features here we moved on to the neighbouring Lea Quarry, which was active until recently. Note that the quarry is on private land and permission from the landowner must be given before entering. Here is the margin between the two main facies.

Here the recently quarried faces and the low sun (on the Sunday at least) led to some very successful fossil hunting. Note that I’m a geophysicist who was along on the trips as a van driver so I apologise in advance for any mis-identification.

Crinoid and brachiopod fragments

Ketophyllum

Favosites

Heliosites

Stromatoporoid