This lower division of the
Old Red System, though of much smaller dimensions than the overlying formations,
has very marked characters both in structure and fossil contents, and is
very clearly defined by occupying a position in which it passes upwards
into the cornstone and marls, and downwards into the Silurian rocks. In
this relation it has been already alluded to at Pont-ar-lleche (bridge
on the tiles 32), near Llangadock in Caermarthenshire, from whence it is
seen to run in a nearly rectilinear course, from the Tri-chrug on the south-west,
to near Builth (SO 044507) on the north-east, occupying the loftiest part
of the escarpments of the wild tracts of Mynidd bwlch-y-groes and Mynidd
Epynt, at heights of fifteen hundred and sixteen hundred feet. In this
range, the tilestones are extensively quarried, and the strata, which are
inclined at seventy and eighty degrees near Pont-ar-lleche, diminish to
forty and forty-five degrees at the north-eastern end of the Mynidd Epynt,
the dip being invariably to the south-east. After a great flexure on the
Wye, to the east of Builth, the tilestones are again found in similar relations
overlapping the Silurian rocks in the Begwm and Clyro Hills, Radnorshire,
and ex-tending thence to Kington in Herefordshire; in which part of their
range they are much less inclined. Throughout their course from Caermarthenshire
to Kington, the distinguishing beds are finely laminated, hard, reddish
or green, micaceous, quartzose sandstones, which split into tiles. Although
the greenish colours prevail, these beds are usually associated with reddish
shale, and the decomposition of the mass uniformly produces a red soil,
by which character alone the outline of the division is easily defined;
being always clearly sepa-rable from the upper beds of the Silurian System,
which decompose into a grey surface. In Shropshire and the contiguous parts
of Herefordshire, this lower member of the Old Red System rarely occupies
high ground, (except in the instance of the outlier of Clun Forest, hereafter
to be described,) and being for the most part recumbent on the talus of
the upper Silurian rocks, where the latter sink down into valleys, it is
generally much obscured by alluvial detritus. In the gorge of the Teme,
however, between Ludlow and Downton Castle, it is well laid open, particularly
at a spot called the Tin Mill. Flaglike, micaceous, dark red sandstone
‘Bur Stones’ rise there at an angle of about fifteen degrees from beneath
the red argillaceous marls of Oakley Park, and pass down into a lightish-coloured
grey, yellowish, and greenish grey freestone, of which Downton Castle is
built, which will presently be described as constituting the upper stratum
of the Silurian System. Similar relations are visible at Ludlow, and at
Richard’s Castle to the south of Ludlow. |
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In this district, however,
these lower red and yellowish beds, or ‘bur stones’, are seldom so fissile
as the ‘tile stones’ described in South Wales … |
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Tilestone Group, east side
of Herefordshire. As the Old Red Sandstone lies in a vast trough bounded
by the Silurian System both on its eastern and western flanks, we ought
to find its lower member, or tilestones, forming the western fringe of
the Malvern Hills. Owing, however, to high inclination, the accumulation
of detritus, and other results of disturbance, these beds are rarely well
displayed for any distance along the eastern frontier of the Herefordshire
basin. They are, however, clearly laid bare in a natural transverse section
at Brockhill Knell between Mathon and Ledbury, where thin bands of yellowish
green, micaceous flagstone, one and two inches thick, are subordinate to
red, green and purple marls, the whole dipping away to the west and overlying
the grey Ludlow rocks at an angle of forty-five degrees. Hard and thin
flaggy rocks belonging to this group are also seen at the north-eastern
suburb of Ledbury, dipping fifty-five degrees west-north-west, but the
flanks of the ledges of older rocks near that town are encumbered with
so much stiff red clay and detritus that the exact junction beds can rarely
be distinguished. The same causes of obscuration, apply to the line of
junction between the Old Red Sandstone and the Sulurian rocks of the May
Hill range. In some valleys of elevation, however, the upper surfaces of
the grey-coloured Silurian Rocks, which are thrown up in their interior,
ex-hibit on their external faces clear examples of passage into the bottom
beds of the Old Red Sandstone. This is well seen on the eastern slopes
of the Clytha Hills, two or three miles east of Ragland, and will be further
alluded to in the sequel. |
Murchison R
I, 1839 The Silurian System. 181-3, London, John Murray
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