Tank Quarry and Ivy Scar Rock – the second in a series about top Earth Heritage sites in Herefordshire and Worcestershire.
Brian Hughes writes: Want to journey to the magma chamber under an ancient volcano? Learn about igneous rocks and see how one igneous rock often intrudes (cuts across) another? See a major geological fault and evidence that rocks have moved relative to each other on the fault? Imagine yourself as a Victorian tourist picnicking under Ivy Scar Rock, the highest natural rock on the Malvern Hills? Have superb views of much of Worcestershire whose underlying geology covers over 500 million years?
First make your way to the North Malvern Road, off the A449 in Malvern, where parking is available on the road or for £2 in the North Quarry car park.
Our first stop is Tank Quarry. Since a major rock fall in the quarry in 2018, faces remain unstable and must not be approached. For this reason, the Tank Quarry car park has been closed, but visitors on foot can keep a safe distance from the rock faces.
On approach, on your right, examine the rocks on the grass in a couple of levels (site 1). Some have interpretation boards. These rocks are from the hills. They are from the Malverns Complex which formed at depth below a volcano in the Precambrian (about 677 million years ago). These are igneous rocks made by the cooling of a red-hot magma. These plutonic rocks (so called as they cooled at depth) are the oldest in southern Britain, apart from the Stanner-Hanter complex which is just across the border in Wales, west of Kington. One must travel to Scotland to find older rocks in the UK. The slow cooling of a magma at depth results in the formation of large crystals. This type of rock, with some crystals > 3 mm, is described as being coarse-grained. Diorite is the most common rock type on North Hill. It contains more magnesium and iron than granite, the other common igneous rock on the hills. Most Malverns Complex diorite has in fact been changed (metamorphosed) by earth forces, shearing, infiltration of hot fluids (metasomatism) and weathering. Malvern granites can often be identified by their pinkish colour.
Many of the plutonic rocks are crossed by “veins”. These are called intrusions and represent later magmatism (hot magma) penetrating weakness in the original rock.
Now ascend a short section of path on grass up a bank from close to the NW corner of the car park. At site 2, there is a display panel with a panoramic view across the Severn Valley.
This shows a wide range of rocks that underline the north and central parts of east Worcestershire. The Clent Hills have Permian breccia (about 299 to 273 million years old), and the Lickey Hills Ordovician quartzites (around 486 to 444 million years old). The Severn Valley is underlain with Permian and Triassic sandstones and mudstones (about 265 to 230 million years old). The Severn Valley is famous for Quaternary river terraces formed during the last 450,000 years. The slopes flanking the Malvern Hills on which the town is built have gravity flow deposits (head). Rocks from the hills, weathering due to frost-thaw action, were able to slide down very gradual gradients towards the valley bottom in the periglacial conditions (cold here but not under an ice sheet) of the Devensian glacial stage (about 100,000 to 11,000 years ago). Across the Severn Basin (which only began to open in its present form in the late Permian and Triassic) is Bredon Hill and the Cotswolds (the latter mainly in Gloucestershire and Oxfordshire) with Jurassic limestones.
In this area are other boulders, some with an explanation board. One stone, not from our area, is a Carboniferous sandstone with a plant fossil.
From the panorama display board, facing the valley, look left towards the hills. There is an isolated display board at the edge of trees (site 3). This gives details about a huge quartz vein intruding the Malverns Complex which can be examined on a nearby face.
At site 4, on a cliff under trees at a bend in the car park approach road, look for parallel scratch marks on the surface. These are slickensides which indicate that another block of rock moved past the exposed face.
This movement occurred in the Late Carboniferous (about 300 million years ago) when two continents collided and mountain building occurred across NW Europe (the Variscan orogeny).
Site 5 (near North Quarry) is also marked by a display board under trees. This is close to a steeply dipping face of Malverns Complex rocks. The face is part of the major East Malvern Fault system. The East Malvern Fault was originally a Precambrian suture (join) where two small portions of continental crust (microplates) collided and coalesced. This weakness was reactivated (rock movements started again on the fault) during the Variscan orogeny. At this time, the collision between two major continents created compressional forces. One effect in our area was to cause uplift of the root of the ancient volcano whose rocks now constitute the Malverns Complex. After this phase, the earth’s crust began to be pulled apart (extensional plate tectonics). During the late Permian and Triassic periods (about 260 to 201 million years ago) successive phases of rifting produced the Worcester Graben (valley) which was to fill with up to 2.5 km of Mesozoic sandstones and mudstones, with the East Malvern Fault marking its western boundary. Note that we cannot call this the Severn Valley in this context as the Severn did not drain this way until very recently (23 to 13 thousand years ago)!
The fault plane here is of diorite. What makes this site special is the breccia stuck to the surface of the fault plane. A breccia is a deposit containing eroded lumps (clasts) of a rock with jagged edges. This example is a fault breccia. A likely explanation of this exposure is that movement on the fault plane in the Triassic fragmented Malverns Complex rocks which became stuck to the diorite fault plane.
Site 6 is the famous Ivy Scar Rock, which was once featured on GWR posters. The crag is the highest natural exposure on the Malvern Hills and presents an impressive aspect. It is reached by an old carriage way that ascends gradually from the southern end of the Malvern Hills Trust’s North Malvern Car Park. After about 800 m a path comes in sharply from the right and the cliff is seen straight ahead, adjacent to the path at SO 773 464 as the main path bends left around a shallow valley.
The rock is a dyke which was intruded into the plutonic rocks of the Malverns Complex. The rock at Ivy Scar is an example of a hornblende microdiorite. This is a finer grained rock than the diorites and granites on the hills. Like the diorite, the dyke rocks contain much magnesium and iron but little quartz. Unlike the plutonic rocks, the intrusions have not been dated radiometrically, though studies of cross cutting relationships on the hills suggest that a Precambrian age of around 600 million years is likely.
An unusual feature of this exposure is the wavy fabric in the middle of the crag that is shown on the next image. Most geologists interpret this as an example of flow banding, in which the intrusions are built up in layers. Each successive episode of magmatism released a small amount of magma into the plutonic rock.
If you have a hand lens, examine the microdiorite in the centre of the face and at its edges. Samples from the middle of the exposure have crystals which are clearly visible through a lens but are much smaller than the crystals in a diorite. Such a rock is said to be of medium grain size. At the extremities of the crag, the crystals are so small that they can hardly be seen individually, even with a lens. This is a fine-grained rock. The very small crystals formed because the magma at the edges cooled rapidly in contact with the older, cold plutonic rocks.
If you have enjoyed this introduction to the rocks of the Malvern Hills, then you may wish to explore this website further. There are many gems to be unearthed!
Acknowledgements: Photos by Ian Fairchild, Kay Hughes and John Schroder and other geologists interested in this area.
Click here for directions to the sites. Do wear stout footwear and bring a stick after a period of rain or in winter as the approaches become slippery and there are some steep slopes. Please do not attempt any climbing or damage the rock surfaces – it will be dangerous both for the rock and you!