We should have been looking at mammoth tusks in a Cotswold gravel pit, but no one, least of all inoffensive geologists, can fight against the might of the RSPB. Apparently there were sand martins nesting and rampaging hosts of geological hooligans might scare them off.

So the rampaging hosts were consigned to the Upper Soudley Valley where it was anticipated that we could cause little harm. Our leader was still Nick Chidlaw and our aim was to study the geology of the valley and its effect on the industrial development of the area.

We met at the entrance to the Old Railway cutting on a fine, sunny day. The route we were to follow is given here. The geology is shown here. The map of the various workings of the mines is here. The Geological Succession is shown here.

After the ritual of handing out the handout we set off along the old railway to the first set of outcrops and spotted the junction between the (Early Devonian) Brownstones and the (Late Devonian) Quartz Conglomerate. The junction was easy to find but characterising it as an unconformity was more difficult. Indeed, with the evidence available at the cutting, one cannot tell whether there was any break in sedimentation. One has to take it on faith that evidence elsewhere indicates a disconformity (an unconformity without any difference in bed angle between the two components).

Then on to pick the junction between the Quartz Conglomerate and the Tintern Sandstone. Unlike on geological maps, rocks in railway cuttings don't come ready labelled. It is very much just a matter of choosing a horizon. No doubt an intimate knowledge of Devonian palaeontology and a lot of time for fossil collecting could pin down the junction, but we just passed on.

 

Nick Chidlaw gets us started	Nick points to the Brownstone (on left) and Quartz Conglomerate disconformity	Are we in the Tintern Sandstone yet?	The search for the Carboniferous contact

Then we looked for the Carboniferous contact. There is no significant break in sedimentation at the start of the Carboniferous so we are looking for a fairly subtle change in rock type. This search was hindered by the fecundity of nature. The Forest of Dean was covering everything in vegetative matter, so we had a hot sweaty tramp through the undergrowth looking for enlightenment. Eventually the contact was seen and we had a good look at the Limestone Shale.

As we walked along the railway line we looked at the valley side to our left, across the Soudley Brook and saw a tree covered hillside. An unspoiled landscape? It certainly looks nice and so must be unspoiled. But it is certainly not an unchanged landscape. If you look at Nicks handout you will see that there are three different mines in the hillside - limestone, iron ore and coal. And extraction has continued for more than two thousand years, from the Ancient Britons at Cudleigh Holes, through the Romans at Chimney Scowles, almost to the present day. Indeed extraction continues with reworking of colliery waste at Eastern United Colliery Tip. (And the Forestry Commission work the woodland at the present day.) So the Soudley Valley is very much a working landscape - and a very beautiful one too.

 

Hot work in the undergrowth	Walking through an industrial landscape	Climbing into Blue Rock Quarry	Excavations for sand in the Quarry

We left the old railway to climb into Blue Rock Quarry where we saw some strange excavations where there had been small scale digging for unconsolidated sand which is found in one particular horizon. This was probably done for local domestic use. The quarry itself was probably a source of agricultural lime. We passed a ponor - rather disappointing in the hot dry weather, possibly on the contact between the Tintern Sandstone and the Limestone Shale, and came to Shakemantle Quarry.

 

The main face of Shakemantle Quarry	The top of the Lower Limestone Shale	Lunch in the Quarry	Examining the Whitehead Limestone

This quarry closed a few decades ago and the Forest is steadily taking it over. The stone - the Lower Dolomite - was largely used for roads. Seeing this quarry with its vast faces (some of the faces are classified as climbs) one wonders why there is a limestone mine nearby. It seems to be in the Lower Limestone Shale. But no one knows why they went to the trouble of mining limestone when they could have picked it up from the hillside nearby. Perhaps it had something to do with land ownership. Or perhaps they were happier mining than quarrying.

After lunch we had a look at the site of the Shakemantle iron ore mine. There is not a lot to see. All the shafts have been filled in and the site cleared. If you have the equipment, experience and the inclination you can get into some of the adit mines. The casual visitor can visit Clearwell Caves.

Why is there iron ore in the Forest of Dean? The iron is usually found in what were once limestone caves. It is in a form known as brush iron ore. This form indicates that the ore came from above - the "bristles" of the brushes hang downward. The supposition is that during the Triassic the area was a desert with the folded limestone full of caves and open fissures. The land surface, not very different from what it is today, was a desert covered with red, iron rich deposits. Despite being a desert, there were occasional flash floods which washed iron into the caves and fissures. There the limestone gave a less acidic environment and the iron came out of solution, forming little stalactites of iron ore on the surfaces of the limestone. This iron helped make the Forest an early industrial area.

We continued through the succession until we came to the Pennant Sandstone. At the base of the sandstone is the Coleford High Delf Coal Seam, one of the most important coal seams in the Forest. As we walked south through the woods we could see the tips at the entrances to the various adits which exploit this seam.

 

Nick expounds	In the Drybrook Sandstone Quarry	On our way to the High Delf	The Chimney Scowle

And we could see the scowles where the Romans (and others) had extracted iron ore. One of the Scowles has a chimney which was part of the ventilation system for an underground mine within it. And just as we turned downhill we passed the strange holes - the Cudleigh Holes - which may be pre-Roman iron mines.

And then we were back in the village. It had been a great day. The geology had been interesting and so had the industrial archaeology. We all got sun tans and had worked up a thirst which the local pub was very willing to help us with.

So despite this walk being a last minute replacement it was well worth doing. In fact it has become one of our favourite family walks. It is a great introduction to the Forest of Dean - both geologically and historically.