13 New Precipice Walk

This well-made path high above the Mawddach estuary follows a tramway which served the gold mines on Foel Ispri. Even though it remains mostly within the Maentwrog Formation there is much of geological interest along it. Access to the walk is by a minor road north of the main road (A496) opposite the Penmaenpool toll bridge. Some visitors leave their cars at the first fork (Figure 33) in the road, where the Nant Cesailgwm joins Afon Cwm-Mynach [SH 690 200]; others follow the right fork through the gate and park near Nant Cesailgwm [SH 696 205]. Most drive to Foel Ispri uchaf where the footpath starts. The path more or less follows the 260 m contour. The usual walk ends near the mine workings at the edge of the forest about 1 km from Foel Ispri uchaf, but it is very pleasant to continue on to Llanelltyd, a distance of about 2.5 km from the farm.

Locality 1 [SH 69650 20673] The road here cuts through the southern side of a tree-covered hillock which is composed of sandy cobbly gravel overlain by about 1 m of brown boulder clay with sandy lenses near the bottom. The gravel was deposited by a stream which flowed under the Devensian ice-sheet before the boulder clay was deposited during deglaciation.

Locality 2 [SH 69657 20648] Along the road there are several outcrops of thinly interbedded cleaved, grey silty mudstone and fine quartzose sandstone of the Maentwrog Formation (p. 55). These sedimentary rocks dip roughly south-east and are intruded by thin sills of grey altered microtonalite.

Locality 3 [SH 69733 20844] Black and dark grey mudstone of the underlying Clogau Formation is exposed here. The mudstone is banded, cleaved and contains plentiful pyrite, including some 1 cm cubes. The mudstone is folded in several small anticlines and synclines, and the cleavage is parallel to the axial planes. Intrusions similar to those in the overlying formation are also present.

At the second parking place the road bends sharply, and amongst the oak trees there are many outcrops of moderately to steeply dipping silty mudstone and fine sandstone of the lower Maentwrog Formation intruded by thin sills. At Locality 4 [SH 69869 20819] the characteristic sedimentary structures of the formation can be examined (p. 23).

Beyond the forest the road climbs to the farm. About 200 m below the farm [SH 6966 2013] is a good viewpoint situated just on the east side of the axis of the Dolwen pericline. To the north, the bedding on Diffwys can be seen dipping west while on Y Garn it dips east. There are splendid views down the Mawddach estuary.

Locality 5 [SH 69802 20091] In the car park at the start of New Precipice Walk, a thin sliver of sedimentary rocks and a quartz vein can be seen between two sills. The upper sill is green dolerite which is exposed at several places at the start of the walk.

Locality 6 [SH 69962 19930] From here the track lies approximately along the strike of the Maentwrog Formation which, with the sills within it, is well exposed in craggy outcrops. Parallel- and cross-lamination, lens-like beds and separated ripples may be seen in the sandstone beds.

Locality 7 [SH 70032 19904] Here, and farther along the path, the rocks are folded. Minor crenulations occur between well-developed fractures, several centimetres apart, which lie parallel to the axial planes of the folds.

Locality 8 [SH 70313 19979] The path here crosses the foot of a tip outside an old lead/zinc mine working. Over the next 500 m there are many adits both above and below the path. These workings lie along quartz veins, trending roughly E–W, which contain mainly sphalerite and galena with lesser amounts of other sulphide minerals. Most of the workings were opened after 1888, though the veins are thought to have been worked in ancient times. Up to 1900, when they were consolidated under the name Voel Mines, several companies worked this vein system for lead, zinc and gold. The workings are extensive, but total production was very small and a great deal of investment money was lost.

Locality 9 [SH 70350 20006] Beyond the well near the last locality the path forks. The lower path is the correct one to follow but, a little to the east of the point where the upper path crosses an old incline, there is an interesting small outcrop of baked silty mudstone in contact with an underlying dolerite sill.

Locality 10 [SH 70550 20119] The lower path leads along a wall to a wooden footbridge above which there are several thick quartz veins within the sedimentary rocks. There is a filled-in adit above the veins, and in the associated tip there is plenty of sphalerite.

The path continues from the footbridge to a stile over a wall into the forest [SH 7070 2013] and can be followed down to Llanelltyd.

(Figure 58) Cross bedding.

References