Malvern Hills with Dave Green
It was about as perfect a day for a field trip as one could wish for; clear, dry and sunny. At the hotel car park, Dave gave us a brief overview as to the events that
shaped the Malvern hills that we see today. The north-south aligned hills are ancient rock, dating to 680Ma ago in the Precambrian. The Variscan Orogeny, during the Carboniferous, caused the rocks to be thrust over the existing Silurian strata to the west forming a monocline. During the Permian, faulting produced a graben structure to the cast, of which the Malvcrns formed the western component. The graben extended to the Cotswolds and is now occupied by the Severn Vale. The Severn Vale has exposures of Triassic origin in contrast to the Silurian to the west of the Malverns. It is worth bearing in mind that the same rocks that constitute the Malverns are buried about 2.5 km deep within the graben. The north-south fault system running through this area extends south to Bristol and north to Lancashire. There is evidence that it dates back to the Precambrian, possibly almost as old as the rocks themselves. Palaeomagnetic data reveals that this part of the continental crust was formed at about 60 degrees south, it was probably an island arc setting associated with South America and West Africa. Back-arc spreading and collision with what would become Greenland was followed by the rifling associated with the opening of the Atlantic. Hopefully, we would be able to see some evidence for these early events.
In order to appreciate the landforms and the effect of geology on the landscape, we walked up the Herefordshire Beacon to British Camp, an Iron Age hill fort. The axis of the Malverns was plain to see, although the Herefordshire Beacon itself has been thrust further west and is noticeably off-axis. East-west trending faults cutting across the axis allowed colls to form that have split the ridge into separate hills. To the east, the Cotswold escarpment was visible across the flat basin fill of the Severn Vale. Westwards, there were clear views of the Black Mountains, the Sugarloaf at Abergavenny and to the south, May Hill, which we visited last year with Dave, stood out distinctively, topped with a group of trees. Immediately below us was an instant geological map expressed in the vegetation. The weathering of the alternating sequence of Silurian shales and limestones had produced a series of ridges and little valleys, with the limestone ridges accentuated by a covering of trees. The Wenlock Limestone was a particularly graceful curve.
As we came down the hill, the underlying geology changed to a younger, but still Precambrian, volcanic formation known as the Warren House Formation that has been dated to 566Ma ago (Uriconian). Clutter's Cave gave us an opportunity to examine the outcrop closely. It is man-made, possibly a shepherd's shelter, but must have been quite a feat as the rock is so hard. These fine-grained, mafic, igneous rocks were basalt and pillow lava structures could he identified. There were no vesicles indicating that they were extruded in deep water and analysis has shown them to he more mafic than ocean ridge pillow lavas, so they have been attributed to a secondary spreading centre in a back-arc basin. Not far away, a small outcrop of a fine-grained pinkish rock was identified as rhyolite, resulting from differentiation of a cooling lava. Mineral deposits are often associated with volcanics, Chris found what appeared to be a small galena vein in a hand specimen.
Having recharged everybody's batteries at the hotel pub, we went to Dingle quarry to have a closer look at the Malvernian rocks. As would be expected with rock of this age they have been altered, despite this they retain the label of diorite. The events of their history arc well illustrated by the granite and dolerite intrusions, two stages of metamorphism, injections of pegmatites, cross-cutting features and shearing in different directions. The minerals in the rock had responded to the stresses and many samples showed foliation that resembled gneissic features. By the way, apparently we should now refer to dolerite as 'microdiorite'!
To round off the day, Dave suggested a spot of fossil hunting, so off we went to Whitmans Hill quarry in the north of the area. Until recently, the Wenlock Limestone quarry was worked for hardcore and aggregate. The Wenlock Shale immediately underlying the limestone was just visible in the quarry bottom; this has recently acquired the new name of the Coalhrookdale formation. We avoided the crumbling faces and the coloured pools further down and concentrated on the large variety of weathered out fossils and rubbly limestone fragments on the floor near the entrance. The fossils reflected the environment at that time - bivalves, brachiopods, corals (solitary and chain), trilobites, crinoids, slromatoporoids and Heather's find. oncolites (algal growth). This was a little bit of mid-Silurian, Caribbean-style paradise (but with no coral reefis in this area), that existed when the Iapetus Ocean was in its final stage of closure - how the world has changed! As the sun went down, the failing light and the sudden chill in the air prompted us to return to the hotel, where an enjoyable evening was had by all.
Debbie Tabner
Malvern hills, Second Day, Sunday 5th March 2000
Leader Dave Green
The second day of the meeting included, like the first, visits to quarries in the Precambrian of the Malvern hills and in the adjacent Silurian limestone, but this time at the southern end of the Malvern ridge. A short visit to see Permian exposures was appended.
The major quarry at Hollybush is the eastern quarry this provided much of the aggregate for the construct ion of the nearby M5 motorway but is now closed. Our morning was spent in the nearby Hollybush middle quarry. This is long unused and is much smaller. Our leader Dave walked to an apparently uniform rock face and declared that, after his instruction in Dingle quarry on the previous day, we should now be able to analyse its Structure and build a hypothesis of its history without his direct help. His only clue was to tell us that there were five or six distinct rock types present. After about an hour's sketching, limited hammering and much mutual discussion the party was indeed able, under Dave's guidance, to assemble a historical sequence.
Pegmatites were intruded into the diorites of the Precambrian basement. Faulting displaced the pegmatites and at the same time introduced a small block of a foreign rock, possibly metaquartzite. Much later, a dolerite dyke was intruded, without metamorphosis of the country rock, and this was followed by uplift and erosion. In the final phase, a basal Cambrian quartzite containing conglomerate layers was unconformably deposited.
The party retired to the Farmers Arms public house at Birtsmorton to regain strength after these mental exertions. Some took advantage of the fine early March weather to eat lunch in the garden. Everyone joined in expressing thanks to both our leader, Dave Green, and Sheila Alderman, who organised the trip.
In the afternoon the scene moved to Eastnor quarry. This lies in the upper part of the Silurian Wenlock limestone and has been renowned for trilobite fossils. Recent use of a part of the lower quarry
In the afternoon the scene moved to Eastnor quarry. This lies in the upper part of the Silurian Wenlock limestone and has been renowned for trilobite fossils. Recent use of a part of the lower quarry had exposed some fresh rock. Consequent hopes of some worthwhile fossil finds were confirmed when a stromatoporoid colony was observed embedded in the quarry face and was extracted. Many other fossils were found, mostly from debris on the ground. They included the remains of various types of brachiopod, gastropod, coral, criniod and trilobite. The 'star' discovery, by Debbie, was of a crinoid showing calyx plates.
The afternoon ended with a short drive to Bromsberrow to see a roadside exposure of the Permian Bridgnorth sandstone. The exposure shows soft red sandstone with rounded grains covered in hermatite. Dune bedding is preserved produce by a wind from the direction of the present north. A little further on, the party saw an example of the Haffield breccia. This rests at the base of the local Permian rocks. It was water-lain in a basin resulting from activity on the East Malvern fault, which is a major feature of the geology of this part of the Midlands. Fragments of granitic diorite typical of the Malverns, and Silurian limestone, wer extracted from the siltstone matrix.
Still enjoying the good weather, goodbyes were said at four o'clock and the party dispersed.
John Payne, Malvern