Chapter 5
GLAMORGAN AND GWENT-- SOUTHEAST WALES.Ross-on-Wye (361 225) -- Carmarthen (241 220). 107 miles (171 km)
Fig. 25. Geology between Ross-on-Wye and Brecon.
The M50 is a convenient route between England and South Wales. It becomes the A40 just east of Ross-on-Wye and then continues around the northern end of the Coalfield to Carmarthen.
Between Exit 3 and Exit 4, the road crosses the northern end of the Forest of Dean. A few road cuttings provide rather poor exposures of sandstone and mudstone of the Old Red Sandstone.
Large road cuttings at
Ross (Fig. 26) show coarse sandstone and conglomerate of the Old Red
Sandstone.
The sand settled from fast running rivers flowing out onto semi-arid
coastal
plains. The closely-spaced layers at an angle to the main bedding are
known as
cross-beds, which formed where streams emptied into deeper water as
miniature
deltas. The
scarp of Old Red Sandstone that rims the Forest of Dean
rises prominently to the south.
Fig. 26. Ross-on-Wye. Road cutting in Devonian Old Red Sandstone.
About eight kilometres east of Monmouth (351 213), the A40 passes the cliffs of Symonds Yat. Layers of Carboniferous Limestone tilting gently down to the west make the prominent cliffs.
The red sandstone in the church and many of the walls in Monmouth was quarried nearby from the Old Red Sandstone that lies beneath the Carboniferous Limestone.
The A40 between Monmouth
and Abergavenny crosses the Usk Valley where reddish soils betray
the Old Red
Sandstone, which makes no outcrops. Many of the road cuttings appear to
be
daisy strewn, because
of pebbles
of white vein quartz that weather out of
gravels,
now conglomerate, of the Old Red Sandstone. Raglan Castle is built of local blocks of
the Old Red Sandstone.
Fig, 27. Abergavenny. Sugarloaf Mountain (327 219). The mountain is capped by limestone overlying a hard sandstone at the top of the Old Red Sandstone.
The road crosses flat ground west of Raglan Castle (342 208). To the south is the scarp of Pennnant Sandstone that encircles the Welsh Coalfield. Soil covers most of the lower slopes that are underlain by the Lower and Middle Coal Measures and older rocks.
Fig. 28. Abergavenny. Blorenge Mountain.
West of Abergavenny (330 214) the road follows the valley between the plateau of Pennant Sandstone to the south and the Black Mountains to the north (Fig.s 25 & 28). The rounded hills of the Black Mountains consist of Old Red Sandstone that rarely appears in outcrops.
The A40 continues to follow the River Usk between the Black Mountains on the east and the Brecons to the west. Twenty thousand years ago, this valley was the main outlet of the meltwater from the glaciers to the west. The rocks in both mountains are the Old Red Sandstone, which appears as horizontal ledges of harder sandstone alternating with softer sandstone and mudstone. Hard bands of purple and grey sandstone cap the Brecon Beacons. The clearly visible sandstone ribs create steps in the slopes.
Fig. 29. Geology between Brecon and Carmarthen.
Only a few road cuttings expose mudstone and sandstone of the Old Red Sandstone west of Brecon (306 227). The reddish soils are typical of those that develop on the Old Red Sandstone. A small quarry about ten kilometres east of Llandovery works brown sandstone that dips about 60 degrees down to the south. East of Llandovery, the soil becomes lighter brown as the road passes onto folded grey Silurian siltstone, well covered with grass. The road descends steeply into the Tywi Valley. Llandovery (276 254) stands on an old river terrace above the present flood plain of the River Tywi.
Between Llandovery and Llandeilo (264 222) the A40 follows the broad Tywi Valley, deepened by glaciers that eroded soft Silurian siltstone and slate. The ridge to the south is of Carboniferous Limestone, and the Old Red Sandstone of Black Mountain lies beyond. This is not the same as the Black Mountains north of Abergavenny.
The
road between Llandeilo and Carmarthen (241 220) follows the terrace of the
River Tywi.
The river marks the southern edge of the folds created in the
Caledonian
collision. All the rocks farther south are younger and were deformed
only
during the Hercynian collision. Soft Ordovician slate lies beneath the
terrace.
The only place to see those Ordovician rocks is to follow the B4300,
parallel
to the A40, on the southern bank of the River Tywi. A few cuttings
expose
layers of hard sandstone in the slate.