All round the Wrekin 2003
Normally on my trips to the UK for committee meetings, I spend the Sunday getting to know Birmingham better, but this time, Sunday 6 April, I was lucky to be able to join the West Midlands Branch for a lecture and field trip, which took me to the Welsh Borders to an area I had never visited before – the Wrekin.
At 8.00 am, Chris Gleeson, area BO, and myself were on our way to a community centre near the trip venue. By 10.30 am the room was bustling with people drinking coffee and chatting and checking out maps or looking at rocks. Our leader for the day, Liz Etheridge, an expert on the Wrekin, gave a very informative lecture on its history.
The Wrekin is home to the oldest rocks in Shropshire, 667 Ma old schists and gneisses. There are layers of volcanic ash and lavas, mostly rhyolite from explosive eruptions. Since the initial igneous activity, the area has been extensively folded and faulted. The main igneous rocks to be found are: rhyolite – crystalline and either pink or deep purple; tuff – fine-grained ash deposits with flinty fractures and dark in colour; dolerite in dykes, sometimes rust coloured from weathering.
At the time of formation, south and central England was 60°S of the equator and moving northwards. The environment changed from subduction to continental plate volcanics, and ca 540 Ma ago, micro-continental collision caused folding and faulting and the emplacement of igneous intrusions.
In the Cambrian, eustatic sea-level rise resulted in erosion and deposits of beach conglomerates, and ripple-marked sandstones can be found. As the sea deepened, finer material was deposited, forming dark green sandstone containing glauconite. Low energy deposition in the Ordovician resulted in shales rich in graptolite fossils. The end of the Ordovician saw the start of the closure of Iapetus and tectonic activity during which comptonite sills were emplaced. The Silurian and the closure of Iapetus found Shropshire in a tropical zone. At the foot of the Wrekin a classic marine cross-section can be seen. The shallow marine environment left beds full of Pentamerus oblongus, a small brachiopod, the fossils of which look like arrow heads.
The Devonian is absent, but from the Carboniferous an unconformity can be found, resulting from erosion, and sandstone, limestone and coal measures, with some basaltic lava flows. During the Permian the area was at a latitude similar to the Sahara today and red sandstone was deposited. Dune bedding is present near Bridgenorth. Ice Ages left their mark with glaciers forming rounded hills above flat plains and a thick layer of rock flour. The presence of glaciers resulted in the course of the River Severn being diverted.
After the lecture it was time for lunch, and not having been able to make my own sandwiches for the trip, I was left with no alternative than to seek out a pub for nourishment. This diversion into the local hostelries resulted in two of us visiting three pubs in the space of less than an hour, before we found somewhere serving snacks. I am told the local beer is worth a try!
After lunch we drove to the Wrekin. My first impression was that of a green version of Ayer’s Rock, rising sharply from the plain. It did rise sharply, as knees and ankles will testify, and we slowly wound our way towards the top, stopping at interesting outcrops, eg. a dolerite dyke and a deposit of what is locally termed ‘quartzite’ but which is not metamorphic and can probably be best described as an arkose sandstone. On the way up it was pleasant to chat to the participants and exchange stories. I stopped a little below the actual summit and sat on a grassy mound admiring the view, together with Alan Bates, local newsletter editor. It was cloudy, but the sun broke through, revealing the patchwork of fields below and the hills in the distance. After the march back to the cars it was time for me to head for Birmingham airport and home, but my flight was late and I missed my connection, arriving in Bern, the last part of my journey in a chauffeur-driven Mercedes, courtesy of SWISS, around midnight. I may have been tired, but it was a thoroughly enjoyable trip with some great people and interesting geology. Thanks, West Midlands, for giving me a day to remember!