Faulted Glacial Sediments

A couple of years back I started doing some geophysics surveying over some local glacial sediments. It turned out that these morainic sediments were quite strongly deformed, first by thrust compression by the ice front and then with normal faulting as the ice retreated and the moraine ridge collapsed gravitationally. Glacial sediments have never been particularly that interesting to me, coming as I do from a more structural, ‘hard-rock’ background they have been little more than ‘soil’, but the structures did intrigue me.

Last weekend I was invited to attend the 11th Glacial Landsystems Working Group field meeting on the island of Anglesey in North Wales. As I said, a weekend of looking at ‘dirt’ wouldn’t normally appeal that much but it has been a very hard semester teaching so far so any excuse to get out in the field was good enough for me. We spent much of the first day looking at localities where the sea had cut sections through drumlins. There was some good discussion about whether the material in (or is it under?) the drumlin was actually glacial till. I think we decided it wasn’t.

However, it was on the second day where we visited a section with some unusual glaciotectonic features. This is the section at Lleiniog, on the shore of the Menai Straits through fluvioglacial sands and gravels.

The main gravel channel location would appear to be controlled by faulting on either margin. The faults are unusual in that they are near vertical. They are not strike-slip as the pebbles and cobbles can be seen to rotate into them on the left channel margin.

On the right-hand margin of the channel the faults have apparently rotated to that they now appear to have a reverse offset.

One theory for the formation of the structure is that the fluvioglacial sediments were deposited over a large block of ice which then melted out, causing the overlying sediments to collapse downward and channel concentrating in the still downfaulting region.

I’d like to thank the Glacial Landsystems Working Group for their hospitality over the weekend and I’ll finish the post with a couple of Anglesey coastal landscapes.

Accretionary Wedge 28 : Deskcrop & Rock365 #300

October’s Accretionary Wedge is being hosted by Matt Kuchta at Research at a Snail’s Pace on the topic of deskcrops. The deadline for the wedge is fortuitous as I can get it to coincide with my three hundredth deskcrop this year!

Back at last New Year’s Eve I was sitting with a group of fellow travellers around a camp fire at a Bedouin encampment on the edge of the Sahara Desert in Morocco exchanging new year’s resolutions. I rashly suggested that I would take a photograph of a rock every day in 2010 and Project Rock365 was born. How long ago that seems. It has been a long slog since but I have now made it to day / rock 300.

I have saved today’s rock for day 300 and the accretionary wedge as it is one of my favourite samples which has pride of place in my home collection. It is a pegmatite sample from the Narestø Feldspar Quarry, Flosta Island, Arendal, Aust-Agder, Norway. The rock contains some really large feldspar and biotite crystals. Unfortunately, I can’t remember but else about the rock and google is not providing me with much help.

The pegmatite was collected on the Keele Geology foreign fieldcourse to Norway in 1991 (in fact my field guide tells me it was on Friday July 12). The Norway fieldcourse was a long tradition at Keele, now sadly superseded. To keep costs down, the geology department (as it was in those days) had its own tents, folding tables and chairs, cooking equipment and gas stoves. We took the ferry to Bergen and traversed Norway twice, out to Oslo and then back to Stavanger, staying at camp sites along the way. I actually did this fieldcourse twice, once in 1989 and again in 1991. We even took enough tinned food to last a fortnight to keep the cost low as Norway can be expensive, but the same logic didn’t quite work the following year when we went across the Alps and actually took tinned Italian tomatoes back into Italy!

The fieldcourse mainly covered high-grade metamorphic and igneous rocks so I was not that much use of the teaching side apart from the structural mapping at Slemmestad and some of the Caledonian nappe structures at Röldal, my role was much more that of van driver. I did however learn a huge amount from our former mineralogist, George Rowbotham.

Matt asks if we could include a scary dimension to the post. I can’t really think of anything scary except I’ve a sneaking feeling that the sample might be a bit ‘hot’.

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.

View Rock365 in a larger map

Columnar Jointing Meme : Iceland

Coat of Arms of Iceland - source Wikipedia
Since there is a current meme in the geoblogosphere on columnar jointing I’m going to have to join in and add an image from a country that actually has a piece of columnar jointing incorporated in its coat of arms – Iceland. In case you are wondering, it is the plate that the shield rests on.

This columnar jointing is from the Jökulsárgljúfur National Park, part of the larger Vatnajökull National Park in the north of Iceland. It shows that columns do not have to be parallel, just perpendicular to the cooling surfaces, in this case lava tubes.

Accretionary Wedge #27 : Important Geological Experiences

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.

Pembrokeshire Fieldwork

I have been back in Pembrokeshire doing fieldwork for my MIS:TIQUE project. I love the county. It is fieldwork like this that makes my job one of the best in the world. I’d like to thank the Keele Learning and Teaching Innovation Grant scheme for funding the fieldwork.

I’ll post more about the geology and some of the gigapans that I have shot in future posts when I have done the image processing but, in the mean time, here are a few scenic shots. Note that the weather was not always this good!

Rainbow at Whitesand Bay
Rainbow at Whitesand Bay
St David's Head from Abereiddi Bay
St David's Head from Abereiddi Bay
Ramsey Island from St David's Head
Ramsey Island from St David's Head
Marloes Sands
Marloes Sands
Strumble Head from St David's Head
Strumble Head from St David's Head
Skomer Island from Marloes
Skomer Island from Marloes

Higher resolution and other Pembrokeshire photos are in my Flickr Set.