25 people and a dog gathered in the car park of the Farmer's Boy pub; our starting point for exploring the geology around May Hill, Gloucestershire. From here we drove part of the way up the Hill, some of us getting lost on the intricate network of country lanes in the process. Then we walked the rest of the way to the top.
Here. Dave outlined the geology of the May Hill area, with the help of very informative hand-outs and the splendid views: west, towards the Forest of Dean; and south, across the Severn, towards the Cotswolds.
A key player in the geological history of this area has been the Malvern Fault. This strikes north-south, running on the east side of the Malvern Hills. It has been traced as far south as Bath, producing the famous hot springs in the Mendips.
The fault is believed to date from Precambrian times, with the last movement occurring during the Triassic period. At various times, movement has been in the form of a reverse fault, with the downthrow side on the west and a strike-slip fault Vertical throw to the east has as much as 2km.
There is a clear difference in rock types and their age on either side of the
fault. On the west side are Precambrian and Paleozoic rocks, including the
Silurian strata at May Hilt. To the east are Mesozoic sediments. The Triassic
sediments of the
Severn Vale were deposited as rifting occurred along the Malvern Fault To the
southeast was the escarpment of Jurassic oolitic limestones in the Cotswolds.
The underlying geology shapes the topography of the area, with harder, more
resistant rocks forming the hills and softer rocks forming the flat plain of
the Severn Vale.
Another key factor in shaping of the landscape here has been folding. The pattern of this folding has been influenced by movements along the Malvern Fault. During the Variscan Orogeny, in the late Paleozoic Era, there was a continental collision to the south of what is now England and Wales. This should have produced east-west striking folds; but the folds in the May Hill area strike north-south. This was because the collision produced movement along the Malvern Fault. Dave used the analogy of a vice: the sediments here were squeezed and crumpled between two rigid blocks as the fault moved.
May Hill is both an inlier (older Silurian rocks, surrounded by younger Devonian and Triassic rocks) and an example of one of these folds; a domed anticline. Looking west from the top of the hill, we could see further evidence of folding in 3 prominent ridges, all striking north-south. The ridges ended just before reaching the Severn Vale, marking the line of the Malvern Fault.
Moving west and progressively into the distance, the three ridges were as follows:-
Near distance (just across the A40 from the pub) - Bishopsfrome Limestone, Upper Silurian age, part of the Old Red Sandstone formation. This limestone was terrestrial in origin.
Middle distance - Devonian Brownstones. These form part of a syncline beneath the Forest of Dean.
Far Distance - Devonian Upper Old Red Sandstone and Carboniferous Limestone of the Forest of Dean
Descending the May Hill Anticline to the Pub
The descent to the pub turned out to be a slow one, spent looking for clues to the rock layers on which we were walking and their past environments. As we walked down the hill, we were moving west, in the direction of dip, so the rock layers became progressively younger. Their characteristics pointed to an offshore marine environment, the sea gradually deepening as the land was subsiding.
We began at the top of the hill, where the oldest rock was exposed: the Huntley Hill Beds. Not very much of this was exposed. However, we found some pebbles near the remains of a fire. These were coarse grained sandstones and conglomerates; both physically and chemically immature: the grains were poorly sorted; and, although mainly quartz, there was some pink feldspar. This pointed to a shallow marine environment, with a rapid rate of deposition as the landmass was subsiding; and erosion of igneous rock on land.
The next layer down was the Yarleton Beds; the geological boundary evident from the break of slope (it became less steep) and springs indicating a less permeable layer. Again, this was poorly exposed but the boulders we found revealed a yellow-brown, medium grained sandstone with lamination, and white mica flakes. The grains were well sorted. There were some fossil remains.
Then, we came to the Woolhope Limestone; the change from sandstone to limestone clear from fizzing with acid. Here there were some brachiopod fossils. This still pointed to a shallow marine environment; but it was now warm enough for calcium carbonate to be precipitated from solution, forming limestone. At the time, the land, now forming England and Wales was moving south towards tropical latitudes.
Next, we entered a field of sheep where there were some north-south trending ridges. It was now muddier underfoot, suggesting underlying shale. This was the Wenlock Shale. The ridges stopped on the west side of the field, indicating a fault here.
Beyond the field and in the woods was the Wenlock Limestone. This was very well exposed, having been quarried along the strike. It dipped steeply west and was very fossiliferous. There were abundant corals, including colonial corals, crinoids and brachiopods.
Next in the succession, was the Ludlow formation (mid-upper Silurian). Compared to the older layers further up the hill, these sediments were very thin, a few centimetres thick. We saw the Ludlow exposed along the bank of a sunken lane near the bottom of the hill. It consisted of mudstones and siltstones, including the Lower Elton Beds; and grey sandstones deposited by rivers (Clifford Mesne Beds).
At the bottom of the hill, we were rather distracted from geology by having to wade through mud and silage in a farmyard. All we noted was that the soil was red, so it probably had something to do with Old Red Sandstone. Luckily, the Farmer's Boy provided the additional service of a hose so that we could wash off the worst of the gunge from our boots before going in for lunch.
Exploring the Quarries
Unfortunately, the weather after lunch wasn't very co-operative, i.e. raining; so my notes about the quarries were more patchy.
Huntley Quarry
First we drove east along the A40 to Huntly quarry, which was on the north side of the main road.
Here, we saw exposures of sediments on both sides of the Malvern Fault. Up a steep slope (and near someone's garden) was Triassic Bromsgrove Sandstone, an example of the Mesozoic sediments, on the east side of the fault, in the Severn Vale. This was a dark rusty red colour, very poorly cemented and very permeable.
Of more interest were the Huntley Quarry Beds, in the quarry itself, on the west side of the fault. These form the core of the May Hill anticline. The exposure here consisted of massively bedded layers, 50cm or more thick. These overlay more thinly bedded layers of a few centimetres thick. All the rock here was dark brown-grey, fine grained and very hard. I don't think anyone found any fossils. Following the exposure through the muddy, very damp undergrowth, we could make out an overturned anticline, on the scale of a few metres. Dave told us that this fold formed through movement along a reverse fault (the Malvern Fault) and that we were looking at a "volcanic sediment" - sedimentary rock containing clasts of volcanic rock. These clasts were relatively unweathered, implying a rapid rate of deposition
However, other aspects of the Huntley Quarry Beds were a matter of debate:
Firstly, the rock is of "uncertain age", anything from late Precambrian times (about 550Ma) to Silurian (443Ma or less).
Secondly, do the Huntley Quarry Beds form a conformable succession with the overlying Silurian layers of May Hill: or is there an unconformity?
Lastly, where did the volcanic clasts come from? The nearest igneous rocks form the Malvern Hills. However these are plutonic granites and diorites, which were formed deep in the Earth's crust and not erupted at the surface.
Dave suggested that Huntley Quarry would be IigIIt for further research projects to attempt to answer these questions.
Hobbs Quarry
To get to Hobb's Quarry, we shared about 3 cars and 2 Landrovers; and had to negotiate a steep, narrow, muddy country lane. Sheila had been right to warn that parking was "very limited".
Here, we had a closer look at the Wenlock Limestone which we had seen on the way down from May Hilt. It was exposed as an impressive series of folds, which set people thinking.
The folds were fairly symmetrical folds, on a scale of several metres; with clearly defined bedding planes; each layer being a few centimetres thick. In the cores of the anticlines were ballstones: algal reef deposits, formed in warn, shallow water. There were no bedding planes visible here.
We spent sometime here, carefully examining the structure and looking for fossils.
On closer
inspection, we could see that the bedded layers, next adjoining the ballstones
had not been folded, unlike the ones on top. This gave some clues to how the
folds formed. Unlike the much larger scale folding at May Hill, these folds
were not formed by fault movement. Instead, layers of limestone were deposited
around and on top of pre-existing reefs deposits, which formed an uneven, rough
surface. The layers on top of the reefs were subsequently folded by compression.
This was produced by the weight of younger, overlying layers, which have since
been eroded away.
We ended the trip here, thanking Dave for a fascinating and enjoyable field trip. I now bad more knowledge about the geology of a part of country which 1 have enjoyed visiting since childhood.
Joan Lee