Accretionary Wedge #42 : Countertop Geology

On a brief parole from grading gaol …

Ian Saginor over at Volcanoclast is hosting this month’s accretionary wedge on the topic of countertop geology. As with many wedges, the geoblogosphere has largely decided to ignore, well at least modify, the brief (probably because we mostly have laminate counter tops). Fortunately, Ian has kindly expanded the topic to include any rocks ‘as long as they’re decorative and completely detached from their origin’.

Unusually for me, my example is a palaeontological offering. This is the centrepiece in the foyer of the wonderfully named Hotel Kasbah Meteorites, in Southern Morocco.

The hotel / kasbah is close to the Berber town of Alnif, famous for it’s trilobite fossils. The hills around Alnif Djbel Issimour are called the trilobites mountains and trilobite sellers line the roads like fruit sellers do in other countries. The hotel owners decided to go with the geology motif with highly fossiliferous bathroom surrounds (edit: see below) and this centrepiece of large ammonites and orthocones.

I have a traditional Berber scarf from the rack in the background that I sometimes wear, much to the befuddlement / amusement of my students in the field.


The ammonites and orthocones also appeared as a drinks table in a hotel in Ouarzazate that we stayed in on the last night.

Returns to grading gaol …

Accretionary Wedge #36 Stuff Left Behind

I am already late for this month’s accretionary wedge which has already appeared at geosciblog. Anyway, in the spirit of better late than never, here is my contribution to the topic of “things left behind”.

Garnet Hornblende Garbenschiefer from Lukmanier, Switzerland.

Almost thirty years ago I was fortunate enough to undertake my undergraduate geological mapping work for my degree dissertation in the Passo del Lucomagno / Lukmanier Pass region of Ticino, Switzerland. I have already talked about some of this in Accretionary Wedge #27 “Important Geological Experiences” and Accretionary Wedge #11 “Field camp”.

Google Earth view of my undergraduate mapping area at Lukmanierpass.

The topography, quite naturally for the Alps, was fairly severe, the campsite where I was staying at Acquacalda was at 1800m, the top of the ridge to the south was 2600m, and I had to descend 100m from the campsite before starting to climb up the other side. When working on the southern ridge it would take a couple of hours to walk up to the base of the ridge. I would then eat my lunch early, leave my rucksack and, travelling light, start to work my way up the side of the ridge carrying just my mapcase, notebook and compass-clinometer. I would spend the rest of the day working upwards until about four in the afternoon and then rapidly descend back to the campsite, collecting my rucksack along the way.

The south ridge of my mapping area from the campsite at Acquacalda

After about three weeks, about halfway through my stay there, I was working up to the south ridge. I had already ditched the rucksack but collected a fair sized sample of high-grade gneiss. I was crossing a boulder field and noticed an interesting looking exposure up a steep face to my right. I put my notebook down on a rock, placed my map case on top of it, and my gneiss sample on that to stop it blowing away. I then headed up to the steep outcrop with my compass-clinometer thinking I could easily remember a couple of readings and rock details and return to record the information in my notebook. At the exposure I took a couple of structural readings and then noticed that the rock structure looked even more interesting up and to my left. I traversed across and took another three readings, committing all five to memory.

I then turn round to return to my notebook. To my horror all I could see was a large boulder field. Although I had a luminous yellow field notebook, I had placed a grey map case on that, and then a large rock on top of that. They were perfectly camouflaged. I descended to where I thought I had left them and still couldn’t see them. I started to hunt for them. After an hour a mild panic started to set in. Had I just lost three weeks work down to my own stupidity? Since I knew that they had to be in the boulder field somewhere, I decided that the only solution was to criss-cross the boulder field in a grid search.

After another two hours without success, time was beginning to run out. As I was mapping alone, I had an agreement with the two others I was camping with and who were mapping adjacent areas, to meet up at 5pm. In the case of one of us not being there at that time, the other two would go out to look for them in case something had happened to them. I was going to have to give up soon to make the rendezvous deadline. My map and notebook could be ruined by weather if I left them out, even if I returned to find them the following day.

I decided to do just another couple of passes. And there they were, right in front of me, I practically walked into them. I was so relieved. I realised I was only 10m from where I thought I left them in the first place. I went back to that point and yes, I could see them from that point once I knew where they were, but the camouflage had been excellent. It was time to go back. But not before I had recorded the details of the outcrop including the five structural readings that had been burnt into my memory over the past three hours.

So, I haven’t ultimately left stuff behind on this occasion, but there is a follow up to the story. After my degree, I went to Cardiff to do a Ph.D. and I told the metamorphic petrology lecturer about the wonderful metamorphic rocks at Lukmanierpass, including hornblende garbenschiefer and kyanite schists that were so shiny you couldn’t look at them in the sun without sunglasses. I showed him the box of my rock samples that I had collected there. He asked if he could hang on to them for a while and I agreed. With the passing of my Ph.D. I completely forgot that I had lent him the rock samples. I moved on to Keele, and he moved on from Cardiff. I don’t know what happened to those rocks, but I do wish I hadn’t left them behind.

Accretionary Wedge #35: Crozzle Redux

Once again I find myself returning to a previous blog post with reference to the latest Accretionary Wedge, this time hosted by Evelyn Mervine of Georneys. The theme this month is favourite geology words. This is so difficult. I love the english language and the origin of words. In my first year structural geology course I try and include the origin of technical terms partly because I think it might help people remember them (e.g. graben = ditch in German, imbricate = roof tiled in Latin) and partly because I love the words themselves (e.g. parautochthon = nearly native [alongside the Earth itself] in Greek – I’ll leave Chris & Anne ‘allochthonous’).

So, what do I go for? As I have been working on Staffordshire geology for several years and the county has coal fields in both the north and south of the region, I have come across several old mining terms in the literature and many are, I think, beautiful words that need recording before they are lost forever. For example, Baum-pot is hole left by the removal of a tree stump from extracting a coal seam below (or a calcareous nodule in the Halifax Hard Coal); Bibbley Rock is a conglomerate; Clod-tops are clays above a coal; Creeshy Bleas are nodules of bituminous shale which fall out when the coal is worked away from beneath them; Horse Beans or Shaggy Metal is a stratum of a granular structure immediately overlying rock salt beds; Quoiceneck is greyish black clay with shining surfaces; Rattle-Jack is a carbonaceous shale. There are several other lovely geological mining terms given in a previous blog post.

A little more common, but largely unknown outside of coalfields is Cleat or otherwise known locally within Britain as back and cracks, board-way’s course, hugger, nannies, reed, smooths, thread or veises.

Cleat: Image from Coal Seam Gas Exploration and Production Services

The origin of cleat is somewhat debated. It appears to be extensional in origin and therefore, in least in part, due to contraction in coal formation as the carbonaceous material is devolatilised. However, if this was the sole cause then it would be expected that cleat would form a hexagonal pattern, similar to columnar jointing. Cleat, however, typically forms a rectangular pattern, with two types of cleat at right angles to each other and the bedding planes. One set of cleat, face cleat, is dominant and through-going. The other set, butt cleat, terminates at the face cleat planes so must be later than the face cleat. The orientation of face cleat can be consistent over large areas (in England it is typically north-south) so must tectonically controlled (in this case by a regional east-west extensional stress field). It is thought that the butt cleat forms as a result of later uplift and exhumation.

Although cleat has been a largely ignored and somewhat obscure structural feature, its properties are gaining a resurgence with the advent of coal bed methane extraction. Dropping the pore fluid pressure in a coal seam by draining the water allows the cleat to open up and methane released from the coal can travel to an extraction well via the permeability provided by the cleat. The degree and orientation of cleat development can therefore strongly influence the productivity of CBM. As the cleat is well-developed in North Staffordshire we have high hopes for our own CBM project.

So, I nearly went with cleat as my favourite geology word, but I really have to go with something more local and Staffordshire in origin. The Staffordshire Coal Measures have, in places, been quite strongly folded during the Variscan Orogeny. Where a thin shale is over- and underlain by sandstones, the less competent shales can get quite strongly deformed by thrusting or flexural-slip folding. The black, carbonaceous shales get contorted, burnished, and highly slickensided and were locally known by the miners as Hussle. Where the shales were not carbonaceous the miners of the Staffordshire/Derbyshire moorlands called the rock Crozzle. I think that this has a wonderful local, rural ring to it, so that is my favourite geology word – Crozzle.

Crozzle and Hussle are first described by the founding professor of geology at Keele, Prof. F. Wolverson Cope in:
Cope, F.W., 1949. Crozzle and Hussle. Geological Magazine, 86, 36-42 doi:10.1017/S0016756800074094

For a more detailed discussion of cleat see:
Laubach, S.E., Marrett, R.A., Olson, J.E. & Scott, A.R., 1998. Characteristics and origins of coal cleat: A review. International Journal of Coal Geology 35, 175–207

I extracted the geology relevant mining terms in a previous blog post “Do you know your crozzle from your hussle? from the following:
Gresley, W.S., 1883. A glossary of terms used in coal mining, E & F.N. Spon, London.

Accretionary Wedge 34 : Weird

Dana Hunter at En Tequila Es Verdad is hosting this month’s Accretionary Wedge with the topic of Weird.

First, here is an exposure from last weekend’s Llangollen mapping course.

Folded cross beds and weird tafoni

Weirder still is the fact that the tafoni are not salt weathering related.

Second up, is something I put up for accretionary wedge 6 (things that make you go hmm – back in 2008) but I like it a lot so I’ll put it up again …

What happens to the orange sandstone and the faults?

You can see a wider view in the gigapan below.

Accretionary Wedge #33 : Rock Garden

Rock Garden

Rock garden

This month’s accretionary wedge is being hosted by John Van Hoesen over at Geological Musings in the Taconic Mountains, who muses how geologists have incorporated geology into their homes, offices and gardens. My rock collection has been banished from the house and incorporated into an alpine rock garden at the front of the house, where a pocket-sized area on a steepish slope doesn’t lend itself to much else.

Rock Garden

The beauty of having a rock collection like this is that it brings back so many memories of places visited and geology seen.

Rock Garden

In this corner are the folds, including a flow folded rhyolite from Pembrokeshire, Wales and refolded folds from Loch Leven in Scotland. Behind are pegmatitic gabbro from Pembrokeshire, a gabbroic dreikanter from the Atacama desert from Chile, a water-lain tuff from the Lake District of England and a Welsh Old Red Sandstone conglomerate.

Rock Garden

Here are a siderite nodule from Pembrokeshire, blue john from Derbyshire, England, larvikite from Norway and rose quartz.

Rock Garden

Around the back, the acer has just come into leaf on the ‘pebble beach’. Most of the pebbles amongst the ferns and strawberry plants are actually glacial erratics collected from the local soil as we live on the edge of a glacial meltwater channel. They are mostly Triassic quartzite pebbles, but also include Carboniferous limestones and Lake District granites.