Home > Start > Contents > A Tour of the Stone Slate Regions > Silurian: Pridoli
  • Introduction: Stone-slate geology
  • Stratigraphy
  • Rocks and climate
  • Map
  • Cretaceous - Wealden
  • Jurassic
  • Permian and Triassic
  • Carboniferous
  • Devonian - Old Red Sandstone
  • Silurian - Pridoli: The Tilestones
  • Ordovician
  • The Silurian rocks cover much of central and north Wales and the Welsh Marches, parts of Ireland and Cumbria, and the Southern Uplands of Scotland. They are well known for slates and shales or siltstones which were used for roofing in Wales and Scotland but in South Wales there are also famous micaceous sandstones which produced stone-slates from Cilmaenllwydd to Builth Wells. Further east roofing has been produced from the same beds in Powys and South Shropshire.
    From Llandeilo in South Wales to Ludlow, the lowest part of the Silurian, the Pridoli Series, consists of micaceous flaggy sandstones, known as the Tilestone Formation. Today it is more correctly called the Long Quarry Beds in the south and the Downton Castle Sandstone Formation in the north. Its use is differentiated by the initial capital in ‘tilestone’, otherwise used to describe any stone slate. 
    The ‘Tilestones’ were originally named by Murchison at Llandeilo in West Glamorgan, where roofing was produced in a series of long, narrow quarries. In The Silurian System (1839) he described the route of these ‘finely laminated, hard, reddish or green, micaceous, quartzose sandstones’ in a north-easterly direction across South Wales and into Herefordshire and Shropshire.
    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. 
    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 …
    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