Barnt Green Road Quarry – the fourth in a series about top Earth Heritage sites in Herefordshire and Worcestershire.
Visit the Barnt Green Road Quarry to see rocks that have been folded so intensely that some of them have been turned upside down. (OS grid reference SP 001753.)
The Lickey Hills Country Park is home to rocks that reveal a fascinating fragment of Earth history. Erosion and quarrying have exposed the bedrock in several places, but nowhere is the evidence of the area’s geological past more dramatic than in Barnt Green Road Quarry.
To the south of Birmingham, just inside the county boundary of Worcestershire, lies the north-south ridge of the Lickey Hills. It is easily accessed from the A38 or Junction 4 of the M5, and there is free parking at the Lickey Hills Country Park Visitors’ Centre (B45 8ER).
Location Map for Barnt Green Quarry – follow the Champions’ Trail, shown in blue, to find the locations marked in the text. Adapted from the Lickey Hills Barnt Green Road Quarry booklet. Copyright © Herefordshire and Worcestershire Earth Heritage Trust.
Arriving by road, turn off Rose Hill to drive down Warren Lane. Passing the Visitors’ Centre on your right, continue to the main car parks beyond, park the vehicle, and proceed on foot. Where the tarmac driveway turns left from the lower car park to the upper terrace, walk straight ahead, leaving the tarmac and crossing a smaller parking area, to find a descending footpath. Proceed downhill for about 200m until you reach a junction with a steeper downhill path on your left. Descend this path carefully until you almost reach Barnt Green Road; at this point you will see the path continuing on your left, parallel to the road. Walk approximately 100m northwards along the path until you reach a wooden gate, which gives access to the public viewing area for the quarry.
The access gate for the public viewing area.
The public viewing area, with its interpretation boards.
The rocks you see in the quarry are layers of sandstone, separated by thin layers of mudstone, and belong to the Lickey Quartzite formation. If you pick up a fragment from the quarry floor and examine it with a handlens, you will be able to make out individual sand grains of clear grey quartz and cream-coloured feldspar. In some thin finer-grained layers there is an abundance of the silvery-looking mineral muscovite. Despite the age of these rocks, the clay-rich mudstone layers have remained soft: although green when first exposed, they rapidly turn red as they oxidise in the atmosphere.
The mineral content, rock texture and sedimentary structures have all led to the conclusion that these sediments were deposited in an off-shore marine environment. It is possible that some of the green clays are so-called ‘bentonites’ derived from volcanic ash, but this has yet to be proven. As yet, it has not been possible to date the Lickey Quartzite directly, as it contains no dateable fossils, and no minerals of igneous origin (which could be dated radiometrically) have been identified. However, we know the age of the older igneous rocks that lie beneath the quartzite, and fossils provide a date for the younger rocks above. This has allowed geologists to conclude that the rocks in the quarries of the Lickey Hills were deposited between 485 and 444 million years ago (source: BGS Geology of Britain Viewer) in the Ordovician period.
A thin slice of rock from the nearby Warren Lane Quarry (not open to the public) provides evidence of the next episode in these rocks’ history. As you can see in the photograph below, the grains of sand have been squeezed together so tightly that they fit together like jig-saw pieces, with no gaps between. This ‘pressure welding’ phenomenon is consistent with burial to a depth of at least a kilometre, but the fact that the clays are still soft means that the rocks did not suffer any significant metamorphism.
A thin section of the Lickey Quartzite from Warren Lane Quarry in cross-polarised light, showing the tightly sutured boundaries between the sand grains.
Field of view approximately 3mm.
From the middle of the viewing area, look across to the main face of the quarry. Although vegetation and accumulations of weathered rock debris partially obscure the exposure, it is still possible to see dramatic changes in the orientation of the strata. In 2010, the face was thoroughly cleaned revealing the extent of the contortions suffered by these rocks.
The main face of the quarry in 2010. Near the top of the face, the layers of quartzite sandstone can be seen to bend over to the right. The yellow dotted lines trace the shape of the fold in two places.
This fold is significant – imagine the force needed to distort solid rock to this extent. The point at which the rocks bend around is called the fold hinge, and it separates the two limbs of the fold. The upper limb of this fold has been turned upside down: it can be described as a recumbent fold as it appears to be lying down on its side. These layers of rock were originally flat and horizontal, so the area they now occupy is a lot smaller than before they were squashed up. The convulsions that deformed these rocks are likely to have been associated with the closing of an ancient ocean, called Iapetus, during the Silurian period.
Now turn your attention to the southeast corner of the quarry, over to your left. At first sight, there appears to be another upward-curving fold, but this is an optical illusion. In fact, the rocks are dipping towards you, and as the quarry penetrates deeper into the layers, it exposes progressively higher points within them, creating the illusion of a fold. However, to the left of this illusory deformation sits a real structure.
In the southeast corner of the quarry, a fault cuts through the hinge of the recumbent fold. The movement of the rocks on the left has dragged the broken edges of the layers on the right, revealing the fact that they moved down, relative to their neighbours.
Close inspection of the rocks here reveals the fact that some layers have been reduced to collections of broken fragments called breccia. This is commonly the result of stresses on the rocks causing them to break and move against one another along faults. The rocks on the right of this fault have been bent by the downward movement of the rocks on the left. Hold your hands at eye level and lay the left one over the right, slanting them in the same orientation as the fault in the quarry face (see photo below). Now slide the left hand downwards – are your elbows getting further apart, or closer together? You should see them getting further apart.
Hold your hands like this to see how the rocks moved along the fault.
A similar movement must have caused the faulting of these rocks; in other words, they must have been pulled apart – they were being stretched rather than being squashed. So, this movement must have happened during a different episode in the rocks’ history to the squashing that produced the folds.
The interpretation panels give lots more information, so take the time to read them before leaving.
Two of the interpretation panels in the Barnt Green Road Quarry (shown above).
Retrace your steps to the carparks, and turn right to follow the path to the right of the upper car park. After about 100m, pause to read the information panel that offers an excellent overview of the geology of the Lickey Hills.
The interpretation panel on the summit ridge of Bilberry Hill.
Having read the panel and taken in the panoramic views, continue along the ridge until you reach a small exposure of rocks close by the right hand side of the path. Close examination reveals that this rock consists of angular fragments of white quartzite that have been cemented together by natural processes. A rock comprising angular fragments is a breccia, but unlike the breccia in the Barnt Green Road Quarry, this one is not the result of faulting.
On either side of the cleft in the middle of the exposure there is a junction between the Lickey Quartzite, and the overlying breccia.
Walk to the cleft in the middle of the exposure and examine the rocks on either side. You will find an irregular junction between the Lickey Quartzite sandstone below, and the breccia made of broken fragments of quartzite above. This has been interpreted as an ancient erosion surface, or ‘unconformity’. Millions of years after the layers of sediment had been hardened into Lickey Quartzite sandstone and folded, the rocks were uplifted, and forces of erosion bit ever deeper into the newly-elevated strata. At the very point at which you are now standing this prolonged period of destruction came to a halt as the sea gradually encroached across the land. As waves pounded the coast, the first sediments to accumulate were the broken fragments of the exposed quartzite.
On the east side of the cleft in the exposure, an irregular surface separates the Lickey Quartzite below, from the breccia above.
The same boundary between the two rock units is also clearly visible on the east side of the cleft.
This unconformity can be seen more clearly at the junction of Leach Green Lane with the A38 at Rubery. Fossils found here show that the sediments lying above the unconformity were deposited in the Silurian period. Like any unconformity, this one represents a gap in the geological record, as natural forces have destroyed rocks that once lay above the deeply-buried Lickey Quartzite.
If you would like to read more about the oldest rocks in the Lickey Hills, you can download a pdf copy of The Lower Palaeozoic Geology of the Lickey Hills here: https://ehtchampions.org.uk/ch/wp-content/uploads/pdfs/Lickey_Geology_Review_2019_v4.pdf.
Alan Richardson
28.09.2020
