THE ROADSIDE
GEOLOGY OF WALES
Chapter 6
SOUTHERN DYFED--SOUTHWEST WALES
See SW
Wales introduction for a Time
Scale and outline of the geology of SW Wales
THE INLAND ROADS, A40, A477 and A478
Fig. 43. Carmarthen to Fishguard.
THE A40
Carmarthen
(241 220) to Fishguard (196 237). 46
miles (74 km)
The
interior of southwest Wales is a low grass-covered
plateau from
which the Preseli hills rise gently to an elevation of about 500
metres.
Outcrops are rare except for the rocky remnants, or tors, of volcanic
rocks of
the Preseli hills.
The
A40 between Carmarthen and Haverfordwest, or
Hwlffordd, follows
a belt of dark slate that was deposited as mud in the Welsh Basin during Ordovician time.
Exposures are
rare, except for good road cuttings west of Whitland (220 216). They
show slate
with good colour banding.
The
A40 crosses grazing land on soft, Ordovician, slate between
Haverfordwest and
Fishguard, or Abergwaun. The only outcrops are in a gorge at Treffgarne
(196
223).
Glacial
meltwater eroded the gorge during the ice ages and exposed hard
Precambrian
volcanic rocks. They are greenish grey and have a glassy look, but they
contain
no glass. They began their careers as volcanic ash. Thin layers of
Cambrian
sandstone cover the Precambrian volcanic rocks near the north end of
the gorge.
These sandstones were deposited as the rising Cambrian seas flooded the
continent.
Once
out of the gorge, the road again crosses grassland eroded across
Ordovician
slate. The tors on the Preseli Hills to the east are igneous rock -
rhyolite,
basalt and gabbro - erupted through cracks in the Welsh Basin in Ordovician time.
The A477
St. Clears (228 216) to
Pembroke (198 202) to Haverfordwest
(196 215). 33
miles (53 km)
The
A477 begins at St Clears at the intersection with the A40.
Three kilometres west of St.
Clears, the
road passes over Lower Palaeozoic
rocks and the road cuttings expose grey slate. These are the rocks
folded
during the Caledonian collision before the Devonian Old Red Sandstone
draped unconformably
across the eroded surface.
Two
kilometres west of Red Roses (220
212), red mudstone of the Old Red Sandstone overlies the Lower Palaeozoic slates beneath the surface
and colours
the soil red. There are no outcrops in this productive farmland. To the
west,
the change to the Carboniferous Coal Measures is reflected only in a
change in
the colour of the soil to brown.
Excellent outcrops of the
Carboniferous Coal Measures appear in road cuttings along the A477 east
of
Begelly. These outcrops expose mudstone, and some sandy layers. Because
this
area is north of the former Hercynian mountain front, the rocks are
only weakly
folded.
From Begelly to Pembroke the
A477 follows the Cleddau estuary to the north, with a prominent ridge
capped by
and Old Red Sandstone conglomerate, called the Ridgeway Conglomerate.
The red
soil in the surrounding fields is typical of soil derived from the Old
Red
Sandstone.
Pembroke Castle stands on a knob of
Carboniferous
Limestone, but outcrops are rare around Pembroke. You will see no rock
outcrops
along the A477 between Pembroke and Haverfordwest. The road crosses the
Hercynian front at Johnston (193 211), where a fault
thrusts
Precambrian rocks to the surface. Unfortunately, you cannot see the
Precambrian
rocks close to the road. The rocks north of Johnston are the easily weathered
Ordovician
slate. The closest Precambrian rocks are in the Bolton Hill Quarry (192
211)
west of Johnston. The quarried rock is in
Precambrian
diorite, an igneous rock similar to granite.
The A478
Tenby to Cardigan.
35
miles (56 km)
From
Tenby north the A478 first crosses sandstones of the Carboniferous coal
measures, then, after crossing the Hercynian front, the red beds of the
Old Red
Sandstone, though outcrops are non-existant.
North of the intersection with the A40, the
A478
crosses Ordovician rocks over its entire length, but there are
virtually no
road cuts into the rocks. They are soft slates except where the
road skirts
the Preseli Hills. There, a few tors of Ordovician volcanic rocks
dot the
landscape, but none are close to the road. The only road cut of
any size is
just south of Cardigan; it is a typical color banded Ordovician slate.