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Ashes Hollow, The Long Mynd

The Longmynd is the large hill that forms the west side of the Stretton Valley. Ashes Hollow is one of the many valleys, batches and hollows that dissect the Longmynd plateau.

The valleys cut down into some of the oldest rocks in the county – those of the Precambrian period. This area is now protected as an SSSI – click here to see why.

Ashes Hollow
The Longmynd is made up of sediments that were deposited under the sea. This was at the margins of an ocean where two tectonic plates were coming together.

You can see how the rocks have been squeezed, buckled and folded during a collision between two huge plates, making a huge mountain chain somewhat like the Alps of today.

This part of Shropshire during the late Precambrian was at the margins of an ocean where two tectonic plates were coming together.

The plate to the west (as it it today, towards Wales) was moving into the plate to the east (in the West Midlands). Their zone of contact was in the vicinity of the present-day Stretton Valley, along a gigantic crack through the Earth’s crust that we now know as the<. The westerly plate sank beneath the easterly one and as it did so the sediments became piled up, forming the thick sequence we see today within the Longmynd. This is known as an Accretionary Prism.

These rocks contain few distinct fossils but those which do occur are very important, as you can learn here.

As the westerly plate sank deeper into the mantle so the heat caused partial melting and volcanoes developed above, within the easterly plate. Their lavas and intrusions formed the rugged crags of Care Caradoc and its associated hills. However, the volcanoes released a lot of ash and this accumulated as beds within the sedimentary sequence, known as Tuffs, notably within the Synalds Formation.

Some ash would fall through the air and then settle into water, becoming quite well sorted and forming thinly bedded sediment.

However, if the ash fell onto dry land then it would not get sorted and instead form thickly bedded sediment, as shown here for a somewhat younger (Ordovician) tuff from the Stapeley Volcanic Formation.

Other ash was erupted with clouds of dense gas that prevented it from going up into the atmosphere but instead flow like an avalanche over the ground surface, even over the sea bed, to eventually lose momentum and be deposited as pyroclastic flow deposits (often referred to as “ignimbrites”).

© photograph by Ian Stimpson, Hypocentre

Tuff from the Stapeley Volcanic Formation

Structure of an Ignimbrite

Within Ashes Hollow each ignimbrite deposit can be seen to comprise a number of zones, as follows (utilising specimens collected and prepared by local geology enthusiast Martin Allbutt).

ASH FALL LAYER (at the top of the ignimbrite)

As the bulk of the pyroclastic flow passes it leaves behind a suspended cloud of fine ash. This settles out as planar ash beds.

WELDED LAYER

More than half the total thickness of an ignimbrite is typically welded, a strong compact state created because the fragments of rock within the avalanche were still sufficiently hot when it was deposited to become fused together. This layer comprises poorly sorted ash with pumice and crystals (now largely altered to epidote by subsequent metamorphism) ejected from the volcano. A common feature is the presence of flame-shaped pumice (“fiamme”) which were squashed flat by the weight of overlying ash whislt still very hot and flexible.

Samples from welded layer

BASAL LAYER

Relatively large blocks of pumice showing some degree of reverse grading (i.e. coarsening upwards) within a bed about 10 cm thick. This represents the “basal surge” as the hot avalanche squaeezed air out in front of the advancing flow.

Samples from welded layer

TRACTION LAYER

A 2 cm thick layer at the very bottom of the ignimbrite, firmly welded onto the pre-existing ground or sea-bed surface (usually the purple shales of the Synalds Formation, as in this photograph, expanded below), often reflecting turbulent (i.e. violent!) deposition.

Click all images to enlarge

Samples from welded layer

Longmynd – GEOMORPHOLOGY

The valleys that dissect the Longmynd have quite a distinctive shape and character because they were all created at the same time by the same process. 

During the last ice-age (25,000 years ago) the ice stretched down the Stretton Valley between the Longmynd and 
Caradoc. This eroded the valley a lot lower (50 metres) than previously which forced the small tributaries to deepen.  They are continuing to readjust to the new level of the main valley.  Each valley has a waterfall which is an indication of the extent to which they have cut down into the hillside.

Explanation of numbers on the diagram.

1. Springs break from base of head below the summit and down valley sides.

2. Undulating Summit plateau above 450 metres.

3. Open valley with moderate gradient.

4. ‘Nick point’: steep gradient and waterfalls mark upper limit of post-glacial down-cutting of lower valley.

5. Narrow gorge deepened by post-glacial down-cutting (erosion).

6. Lower valley with gentle gradient and meandering stream.

7. Small terraces and disused channels created as the stream cuts down through the level, stone-filled, valley floor.

Valley features

Diagramatic view of a typical
 Longmynd valley.

Here are two of the waterfalls found at ‘nick points’ as described above.

Above. Callow waterfall (a much less dramatic affair) up Callow Hollow.

The Lightspout, part way up Cardingmill Valley.

The Ice Age is responsible for the thick stony soil that covers the whole of the top of the Longmynd. This obscures the underlying rock except where it has been re-exposed by the creation of the batches and hollows.

Top of Long Mynd - northern end

This uniform layer has resulted in the moorland vegetation that we see today and appears as the reddish colour in the photo of the north end of the Longmynd above.

The Ice Age has left a few other clues that it affected this part of the county including this erratic at Womerton (on the north end of The Longmynd).

Erratic boulder at Womerton northern end of Long Mynd

Longmynd – FOSSILS

At the time that the rocks of the Longmynd were deposited there were no animals with hard skeletons on the land or in the sea, and no plants on land. Therefore the only fossils of living matter that can be found in these rocks are of algal mats (evidence of where these once lived, or rather died, can be found in the Stretton Shale and Lightspout Formations) and a few possible worm burrows. An overview of these fossils is published here in our Proceedings.

These look like tiny craters a few mm across. Recent research by carefully slicing through a number of these features shows that many of them were in fact formed by escape of gas from rotting algae, buried by mud on the sea floor. Yet others may be fossilised seaweed!

However ‘fossils’ of raindrops and ripples can be found, most commonly in the Synalds Formation.

As rain falls onto mud that has been exposed by a low tide and dried out slightly the impact will result in a small crater in the mud.

When the rain stops and normal gentle sedimentation continues on the incoming tide, the rain pits may be in-filled with fine sediment. In some conditions these pits may then be preserved in the rock record.  In the rocks of the Longmynd these can be found as marks about 2-3 mm across on the bedding planes. Some may even have evidence of the raised rim that was created as rain fell and some are elliptical suggesting the rain hit the mud at an angle.

Fossilised ripples can also be found on some of the bedding planes, such as those exposed near the ‘Lightspout’ waterfall.

Bedding planes near ‘Lightspout’ waterfall

Longmynd – INDUSTRY

The shaley rocks of the Longmynd have been little used.  It is a relatively poor building stone but has been quarried at the entrances to most of the valleys for local rubble-stone building construction and for road sub-base.  The largest of these quarries is Buxton Quarry at the entrance to The Batch, in All Stretton, which as late as 1902 provided stone for All Stretton Church.

The nature of the valleys means that the water of the stream was sometimes put to work in providing power to water mills, the best known being the Carding Mill.  It has also been collected for drinking purposes, though the reservoirs in Town Brook Hollow and New Pool Hollow have now both been superseded by supplies from the River Severn. Nevertheless there is still a spa in Church Stretton and spring water can be collected by the roadside north of the town centre. 

Very few minerals dissolve out from the ancient rocks and the resulting softness and purity of the water was the basis of Church Stretton’s efforts to establish itself as a spa town after the railway first opened. Water was first bottled here, at the mouth of Cwm Dale, in 1883.

The Midland Gliding Club takes advantage of the high flat land for their airstrip and thousands of visitors come each year to sample the delights of this large area of common land.

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