I have been out the last couple of days undertaking fieldwork for a building stones project, but today I was treated to a glimpse of a gem of a little geological section being prepared. Those of us who work in geoconservation talk a lot about geodiversity but this must be the most geodiverse section I have come across. It is a trench, only about 100m long but displays rocks from the Precambrian, Cambrian, Silurian, Carboniferous and Triassic.
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 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”.
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”.
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.
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.
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.