Fig. 31. The Welsh borderland.
The rocks of the Welsh Borderland include a wide variety of ages: they vary from Precambrian in age near Church Stretton, Ordovician in the north, Silurian to the west and Triassic mudstone north of Shrewsbury.
Shrewsbury (350 312) to Welshpool (323 307). 18 miles (29 km)
Fig. 32.
Geology of the
Shrewsbury/Welshpool area.
The A458 west of Shrewsbury (350 312) follows the River Severn for a few kilometres across Triassic mudstone, then climbs onto soil-covered rolling hills of upper Carboniferous sandstone. Near the Welsh border, the road crosses onto folded Ordovician and Silurian slate. The large hill north of Middletown (331 312) is eroded in volcanic rocks that erupted during Ordovician time.
An old quarry above the
village contains thick beds of Ordovician volcanic ash in layers that
dip
steeply down to the north. Some thick layers contain volcanic bombs as
much as
15 centimetres across, but most are fine volcanic ash (Fig. 33). These
volcanic
rocks are more resistant than the surrounding sedimentary rocks, so
they stand
up to make the hill. The volcanic rocks do not appear near the road. A
small
road cutting near the village exposed a sill of black dolerite.
The hills to the south of Middletown are Long Mountain. They are eroded into slate of Silurian age that does not make prominent outcrops.
Fig. 33. Thick Ordovician volcanic ash beds in the Middletown quarry (331 312). The largest is three metres thick.
Welshpool (323 307) to Church Stretton (346 294) (Shropshire). 27 miles (43 km)
Fig. 34. Geology of the Welshpool/Church Stretton area.
The A490 south of Welshpool crosses Silurian slate as it skirts the southern end of a block of Precambrian to Ordovician rocks. The Ordovician rocks were already gently folded in the early stages of the Caledonian collision before the younger rocks were deposited on them during Silurian time. That relationship is not displayed in any individual outcrop, but it does become clear through study of a geologic map on which it is clear that Silurian rocks overlap the folded Ordovician rocks. Both the Ordovician and Silurian rocks were then folded during the climax of the Caledonian collision at the end of Silurian time.
The A489 meets the A490 at Church Stoke (328 294). Corndon Hill, the rugged black hill 2km to the northeast of Church Stoke, is a knob of dolerite, a black igneous rock similar to basalt except that its crystal grains are larger. This is one of many igneous rocks intruded during Ordovician time.
The A489 southeast of Church Stoke follows a broad valley to Craven Arms, with the Long Mynd to the north.
Church Stretton (346 294) and the Long Mynd (342 295)
This area in adjacent Shropshire is included to show good examples of rocks at the eastern edge of the Welsh Basin. For more details, see Geological Association Guide #27 The Geology of South Shropshire.
The A488 from Shrewsbury to Bishop's Castle (333 288) crosses early Paleozoic rocks. Several quarries work Ordovician sandstone, volcanic rock, and dolerite intrusions. The Ordovician sedimentary rocks contain ore bodies of lead and zinc. They account for the many small mine tips that dot the landscape around Black Marsh (333 299).
From Black Marsh, an unclassified minor road crosses the moors via Shelve (334 298) to Church Stretton. At Bog (336 298) are more old mines that once produced lead and zinc ores. Just east of Bog, the road to Ratlinghope (341 297) crosses a sharp divide with a white cap of pure quartz sandstone. A minor road to the north leads to a large National Trust parking area near this sandstone.
The Stiperstone ridge
(336 300) (Fig. 35) above the parking area is pure white sandstone,
about ten
to 15 metres thick, which dips about 60 degrees down to the west. Most
Ordovician rocks in the region are siltstone and shale that appear to
have been
laid down in deep water, but this sandstone was deposited near shore
when sea
level rose during Ordovician time.
Fig. 35. The Stiperstones.
Stripes of sandstone blocks run down the hill from the main outcrop (Fig. 36). Geologists think they are the products of frost heaving during the Pleistocene ice age. Similar stone stripes are common in modern tundra landscapes.
Fig. 36 The stone stripes below the Stiperstone Ridge.
Outcrops are rare east of the Stiperstone, except for a few poor exposures of mudstone and dolerite. The road climbs onto the Long Mynd (342 295), a mountain exposing Precambrian rocks--though there are almost no outcrops. On the moor, about five kilometres from Church Stretton, is a low outcrop of reddish gravel, now hardened to conglomerate, with pebbles as large as five centimetres across.
Most of the Precambrian rocks are rather uniform siltstone and sandstone. The best place to see them is at the car park about a kilometre and a half west of Church Stretton (346 294). Rocks near the car park are purple and grey mudstone. Those on the hill to the east, called the Devil's Chair, are coarser siltstone and sandstone. The nearly vertical layers make vertical ribs in the valley sides.
The view from the Devil's Chair is across the Church Stretton valley, which follows the trace of the Church Stretton fault, once the edge of the Welsh Basin. The hills across the valley are the Caer Caradoc range, made of a sliver of Precambrian volcanic rocks. Ordovician rocks lie to the east.
Wenlock Edge is a low scarp of Silurian limestone southeast of Church Stretton. Trees cover the scarp, but you can see the Silurian limestone in a cliff at the National Trust viewpoint (357 296) on the B4371. The limestone is the same age as the Silurian siltstone of the Welsh Basin to the north, but was deposited in shallow water at the southeast edge of the basin, whose edge was at the Church Stretton fault. The limestone is rich in fossils that show it was an ancient barrier reef. The coastline and low hills then lay to the southeast.