Dingle Quarry, the ninth in a series about top Earth Heritage sites in Herefordshire and Worcestershire.

Whether novice or expert, Brian Hughes encourages you to visit this fascinating quarry in the Malverns.

A red-letter day in my (limited) field experience occurred when we were shown around Dingle Quarry by local expert, John Payne. John and his team, working in conjunction with the Malvern Hills Trust, had done a grand job in clearing the vegetation from crucial features of interest on the cliffs. Additionally, a few very small, discrete patches had been cleaned using an angle grinder and exposed rock in pristine condition. The sun shone, the light was great, and the fine condition of the exposures made identification and interpretation so much easier. Make sure that you too delve into the Dingle!

General view, showing the conspicuous ledge dividing upper and lower tiers.  Photo by Moira Jenkins.

Before visiting a crag, I try to read any available literature on field identification, but more often than not I am left disappointed. Either I cannot find what is referred to, or I am left unconvinced by the interpretation offered. Why have reasonable alternatives been rejected? None of this applied at the Dingle, where we were able to find all the structures and rock types explained with an annotated diagram in John’s excellent book, Herefordshire’s Rocks and Scenery: A Geology of the County (pp.68-9).  Great stuff!

The rocks at Dingle are a part of what geologists call the Malverns Complex. If you are keen to get to grips quickly with what Dingle has to offer, then skip the next 3 paragraphs which explain some of the background to the Malverns Complex rocks.

Malverns Complex Rocks

These Precambrian igneous and metamorphosed rocks were intruded into a ‘country’ (pre-existing, probably sedimentary) rock at a subduction zone located at around 60°S, close to what is now Amazonia in S America, about 677 million years ago (Ma). Some evidence of this sedimentary country rock has been found at the Gullet Quarry, a few miles to the S of the Dingle along the ridge.

The Malverns Complex is appropriately named as the rocks are indeed very complex. In places the nature of the igneous/metamorphosed rocks can change rapidly within the space of a few metres. Originally igneous (formed from a cooling magma) the rocks have been subjected to at least four mountain building events (orogenies) and enormous earth forces. Rocks have been dynamically metamorphosed by shearing, altered by hydrothermal activity, (the intrusion under pressure of hot water containing minerals different to those originally present) and modified by even later chemical changes due to weathering. The uplift during the mountain building events (the last around 300 Ma in the Carboniferous Period, Variscan orogeny) has caused much shearing, faulting and fracturing of the rocks. As a consequence, there are relatively few unaltered igneous rocks on the hills, and many quarries expose a bewildering mix of rock types.

Naming Malverns Complex Rocks

The naming of rocks from the Malvern Hills in the field is also problematic. Igneous and metamorphic rocks are now tightly defined and classified by e.g. British Geological Survey based upon work by researchers such as Le Maitre. This causes difficulties for the field geologist. I next illustrate the nature of these problems. For example, the early Victorian geologist, Rodney Murchison (famous for The Silurian System) called the Malvern rocks “syenite” though it is probable that few of the rocks on the hills would now fall into the syenite classification. Yet, some contemporary authors still use this term. Further, intrusions correctly termed microdiorites by today’s standards, are often called dolerite (which has a very different chemical composition) in older and not so old literature. North Hill is noted for its diorites, but an anecdote, which seems reliable, has it that a respected local geologist failed to find any pristine diorite in The Northern Quarries where it was said to abound. All the “diorites” had suffered some degree of shearing, metamorphism or alteration. Correct identification of the rocks in 2021 requires chemical analyses and the use of thin sections to identify constituent minerals and their proportions. All this is beyond the scope of the amateur field geologist. Fortunately, Dingle Quarry has a fine selection of structures and rock types that can be distinguished with some confidence in the field.

Dingle Quarry- Structures and rocks

Dingle Quarry (SO 7654 4567) is a classic site on the Malvern Hills where many features typical of the Malverns Complex can be seen. The abandoned quarry is easily accessible from the West Malvern Road to the west of the hills and north west of Worcestershire Beacon. From the approach track, the upper and lower tiers, separated by a conspicuous ledge or shelf, can be glimpsed through the trees.

As always when visiting a quarry exercise great caution and beware of drops and the possibility of rock falls.

From J. Payne’s “Herefordshire’s Rocks & Scenery: A Geology of the County”, Fig 4.11, p.68.  Image by John Stocks, annotation from a sketch by P. Olver. By kind permission of John Payne and John Stocks.

Just above the lower tier is the first interesting feature of the crag, the afore-mentioned near-horizontal ledge. Quarry men use the term “bench “for a feature like this left untouched in a quarry.

The Lower Tier

Well below this ledge I was delighted to find several examples of slickensides, standing out clearly in the bright light. A slickenside is a set of polished scratches or mineral fibres which indicates that two rocks have moved relative to each other along their length. Slickensides provide good evidence of faulting.

Slickensides were found in a detached block (not in situ) and elsewhere.

Slickensides, evidence of faulting.  Photo by Kay Hughes.

Slickensides – plunging almost vertically.  Photo by Kay Hughes.

There are several pegmatites to the right of the lower tier. A pegmatite forms when an intrusion cools very slowing, producing a rock with large crystals. Their pinkish colour and lack of many dark (mafic) crystals indicates that these pegmatites are of granitic composition. Because the intrusions appear to be near-vertical, they are called dykes. But were they emplaced vertically originally?

Granitic pegmatite in the lower tier. The pinkish colour stands out from the surrounding grey diorites. Photo by Kay Hughes.

Very large crystals typical of a pegmatite from the exposure in the image above.  Photo by Kay Hughes.

The centre of the lower tier has a speckled rock with what is often called a “salt and pepper” appearance. This is typical of a type of relatively unaltered diorite commonly found on the hills.

Probable diorite from the scree below the lower tier. The salt and pepper appearance is common in diorites on the Malvern Hills. Each gradation on the scale is 1mm.  Photo by Kay Hughes.

The darker patches are formed from minerals rich in magnesium and iron (mafic) such as hornblende (an amphibole mineral which contains water, consistent with formation at a subduction zone). The paler specks are largely composed of felsic minerals, quartz, plagioclase feldspar (sodium, calcium, aluminium, silica) and orthoclase feldspar (the sodium and calcium replaced by potassium). This medium to coarse grained rock is essentially an unaltered diorite. In the lower quarry are pinkish rocks of granitic composition (coarse grained with quartz and both feldspars). As orthoclase is usually pink and unusually the plagioclase feldspar on the hills is also pinkish, granitic type rocks which contain white quartz are fairly easy to identify in the field both by their pinkish colour and their paucity of black minerals. Often granites occur as later intrusions into already solidified diorites.

In the lower quarry some geologists have described areas without a clear contact between the two types, with the granite merging diffusely into the diorite. This has led to the proposal that the source of these rocks was a magma chamber containing a mix of different mafic and felsic molten materials.

Before leaving the lower tier identify an overhanging, almost horizontal, surface well below the ledge. The image below shows the author, a rock climber in his youth, pointing to the overhang.

This feature has been identified as the underside of a fault plane. The under surface here was smooth perhaps reflecting the relative motion of two rock blocks. Because the fault is currently at a low angle to the horizontal it is usually called a thrust fault (or just a thrust) in the literature. To be certain that this is a thrust there would need to be evidence that the rocks were in this or a similar position when faulting occurred.

The smooth underside of the almost horizontal fault plane.  Photo by Kay Hughes.

The Upper Tier

The safest way to get to the upper tier is to return to the access track, walk a short distance uphill along this and then take a rough, indistinct path onto the ledge with a steep drop to the left.

The next feature of interest is just above the thrust plane. This is the long, nearly horizontal ledge which separates the lower and upper tiers and dips eastwards into the hill. This is an intrusion of magma which forced its way through pre-existing older igneous rocks. Professional geologists tell us that the chemical composition of intrusions like this is different from that of the surrounding rocks. Fortunately. the two rocks have slightly different colours and can be separated in the field. The intrusion is darker grey and has slightly finer grains than the pinker rock into which it was intruded.

The contact between a grey microdiorite intrusion on the left and the pinker igneous rock (probably a pegmatite) into which it was intruded on the right. The small white vein at the contact is calcite, which would be a later addition.  Photo by Moira Jenkins.

Another view of the intrusion showing a clear contact above with pegmatite. The camera case is 12cm long.  Photo by Kay Hughes.

The pinkish igneous rock seems to have been identified as a granitic pegmatite on the annotated sketch in The Herefordshire Geology. However. we could see few large crystals perhaps because of shearing and strong alteration.

Magnified view of the rock above the microdiorite intrusion, possibly showing a metamorphic fabric?  Photo by Kay Hughes.

In places the contact of the intrusion with rocks of the upper tier is visible. I was excited to be able to confirm with a hand lens that the crystals in the intrusion are much smaller at the contact than they are in the middle. This is called a “chilled margin”. Because the magma cools fast in contact with the cold, solidified rock very small crystals are formed at the edges. In contrast, in the middle cooling is slower and larger crystals result. This is a classic method of demonstrating the existence of an intrusion.

Field examination shows clearly that this medium-grained (crystals visible to the naked eye) to fine-grained (crystals visible with a hand lens) grey rock is very different from the greyish diorites in the lower and upper tiers.  Laboratory analysis confirms this type of intrusive rock as a microdiorite, though some of the literature wrongly refers to it as a dolerite, which is a rock of very different chemical composition. This near-horizontal intrusion is often called a sill. However, it has never been clear to me, given all the forces to which the Malverns have been subjected in the last 680 Ma or so, how geologists can be certain that the original emplacement occurred horizontally?  Perhaps the feature would better be called a sub-horizontal intrusion?

The upper tier has more diorite and pegmatites. The literature tells us that on examination under a microscope both these rocks, unlike the diorites in the lower tier, show an alignment of crystals which is typical generally of regional (large scale) metamorphosed rocks. However, on the hills, it is thought that this alignment (foliation) is due to localised shearing.  Strictly, the term diorite refers to an unaltered igneous rock, so perhaps they should be called “metadiorite”? However, this term has not found favour with geologists.

There are a couple of further interesting observations that have been made about the sill and the thrust fault.  By examining in the field the cross-cutting relationships between the sill and rocks in the lower and upper tiers, the deduction has been reached that the sill was one of the later intrusions of the several evident at Dingle Quarry. Further, these relationships suggest that thrusting occurred even later than this, probably exploiting the weakness along the plane of the intrusion. The thrusting most probably occurred during one of the orogenies, with the latest, the Variscan orogeny perhaps being the most likely. I am unaware of any research that throws light on the timing of the this thrust.

Further reading following my visit reveals that geologists have also identified shear zones in the quarry. Pyrites has been recorded associated with a pegmatite intruding a biotite- rich diorite.

A rock from the a recently fallen block from the upper tier of Dingle Quarry. A possible altered biotite diorite?  Photo by Kay Hughes.

A quick re-visit on 19th November 2021 showed that the quarry was still in very good condition and that the features mentioned above can still be found. I hope that you enjoy your visit to Dingle Quarry as much as I did.

Brian Hughes, 30.11.2021

More information about Dingle Quarry is available at:

https://earthheritagetrust.org/sites-to-visit/

and in the Malverns Champions booklet where Dingle Quarry is on the map and page 30 of part 2:

https://ehtchampions.org.uk/ch/wp-content/uploads/pdfs/Malvern_booklet_part1.pdf

https://ehtchampions.org.uk/ch/wp-content/uploads/pdfs/Malvern_booklet_part2.pdf

https://ehtchampions.org.uk/ch/wp-content/uploads/pdfs/Malvern_booklet_Champions_Trail_map.jpg