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Viaduct Sink – the opening campaign

After their success at Thrupe Lane the Thrupe digging team turned their attention to Windsor Hill an area always thought to be of great promise by older members of the B.E.C.  Simon Meade-King (WSG & Wxxxxx) has sent the following report for publication in the Belfry Bulletin.

Some two miles north of Shepton Mallet lies an area of limestone scarred by the complex of the Windsor Hill quarries, now disused, and pierced by the track bed of the old Somerset and Dorset Railway on its solitary path over the Mendips from Shepton Mallet to Radstock.

The quarrying operations have revealed various small caves which have attracted the attentions of cavers over the years.  Though small in themselves these caves hinted at the possibility of a large cave system taking the considerable drainage which sinks in the area.

Just beyond the quarries the railway crossed the deep wooded ravine of the Ham Woods Valley by an impressive viaduct.  Almost in the shadow of this viaduct a sizeable stream sinks in the floor of the valley, to re-emerge three miles to the west at St. Andrews Wells after dropping nearly 300ft via the limestone.  Viaduct Sink, as this swallet is known, together with Windsor Hill Sink a few hundred yards away, form the most easterly of the ten Sat. Andrews feeders.  These feeders stretch from Biddlecombe Swallet, just to the north east of Wells to Windsor Hill in the east, and include Thrupe Lane, the only sink to have yielded a major system to date.

The principle geological feature of the Windsor Hill area is the sandstone ridge of Maesbury, a mile to the north.  This provides a catchment area for the streams running off its southern flank. Below the Maesbury ridge is a superficial layer of head, Thrupemarsh, which gathers the surface water before it crosses the Lower Limestone Shales and sinks at the limestone boundary.

Viaduct sink has been known for many years, the stream disappearing in its bed 45yds up valley from the viaduct, and in wet conditions running on down the valley to sink in an intermediate swampy area in the woods.  However, although geologically the top sink is in a textbook situation, the immediate surface area did not encourage digging with an absence of solid rock against which to construct the shoring of a shaft.

The first trial excavations began in January 1976 and brought up sandstone cobbles, shale and finally Black Rock Limestone, until at a depth of 3ft a boulder ruckle was encountered.  Work continued through the boulders and until a buried cliff gradually emerged.  This discovery was of importance if only because it solved the shoring problem.

By the spring of 1976, when Atlas (the digging team) were invited to join forces with the West London Caving Club on the site, a large if unsupported pit has been opened up with on all sides but one, a jumble of mud and rock.  At its maximum depth the cliff was undercut, and as we excavated the down dip end of the pit, a rift was exposed choked with tons of rocky material.

At this point with things beginning to look interesting a halt was called to the digging to enable a proper shaft to be constructed.  This was built entirely of wood using sleepers from the nearby quarry siding and unlike that at Thrupe was built to rather more economical design and with the ends interlocking to provided more strength.

As material was brought up it was dumped behind the shoring to give extra stability and at 18ft depth we fixed the last rungs in place and digging then recommenced at the mouth of the choked rift.  It was down this rift that the most promising point of attack appeared to be.  The left wall gave a feeling of security although on the right a low chamber needed walling up to support the unstable roof.

We then moved forwards and downwards against the solid wall, uncovering evidence of a major stream sink in the form of a huge sandstone cobble.  Large quantities of stal were brought up.  As we deepened the area beyond the grouted wall more grouting and consolidating became necessary.  We concreted in a sleeper at roof level to give additional support to the roof and prevent boulders dropping on us from the rift above.  The instability of the roof made progress rather slow, and several boulders had to be banged – the fumes clearing slowly in the absence of a noticeable draught.  An interesting discovery was a small round tube going off to the left, probably an abandoned oxbow.  Beyond this passage, our way forward was unexpectedly barred by a solid wall, and we diverted out attack to the right.

It was now the sweltering days of August at the height of the heat wave, and the stream had completely dried up making conditions very pleasant.  However, as we probed forward into a mass of loose material we began to undermine our second grouted wall, and another sleeper was cast across to prevent collapse.  The main priority had to be to leave the shaky conditions of the entrance rift and getting something a good deal more solid and here we were lucky.  A few feet ahead lay a small chamber almost filled with rubble and with a roof if not absolutely solid very nearly so, and all our effort could now return to digging.

The following day, with a bit of clearing back we were able to crawl up and have a careful look at the chamber.  Up dip to the right, were two obvious stream inlets – one at least coming direct from the surface.  To the left, at its lowest point, the wall was undercut along its whole length and probing revealed a promising black hole which we enlarged revealing a definite way on. An awkward jammed rock barred access to what appeared to be a more roomy passage below – it looked as if we could be nearly in.  The roof was banged and with hopes of a breakthrough running high we forced our way through.  But alas, this turned out to be a space hardly enough room to admit a body – merely a tiny breakdown chamber with no obvious continuations.  On the east wall was much evidence of botyoidal stal and the place had a generally dry fossilised look about it similar to some of the small caves in Windsor Hill Quarry.  The south wall was of a semi-circular shape indicating erosion by the swirly motion of a descending stream.  The floor consisted of a massive slab, fallen from the roof.

So, with no immediate prospects in that direction we resorted to clearing out the first chamber we had entered.  The numerous cobbles that emerged were a constant source of encouragement and after a few hours digging we uncovered the mouth of a bedding plane under the right wall, draughting slightly, and floored with stream debris.

During the Bank Holiday weekend that followed, this bedding plane, some three feet across by eighteen inches high, was cleared out and was soon roomy enough for one of us crawl in to the edge of a six foot drop where a cross-rift cut across at right angles. To the left and right this rift petered out, but straight ahead a flat out gravel lined crawl was forced to an awkward constricted right hand bend.  By squeezing flat out a man sized space could be seen to the right with apparently stal covered walls.  Stones thrown through made promisingly resonant noises and a crowbar pushed through indicated 3ft of width.  Furthermore, one of the team reported the sound of a distant roar – the omens looked good.