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The word “ Tilestones,' originally a local term in Caermarthenshire and Breconshire, was first used by Murchison to designate the beds between the Upper Ludlow Rocks and Old Red Sandstone.
The Ludlow Bone-bed is capped by light-coloured, thinbedded, and slightly micaceous sandstones, which have been quarried near Downton Castle, and have been called the Downton Sandstones : this term is now used for the lower portion of the Tilestones, the Ledbury Shales constituting the upper portion.
These rocks consist of red, grey, and yellow flags, and micaceous sandstones, attaining a thickness of 100 feet : in many of their features they are connected with the Old Red Sandstone.
Murchison first observed this apparent passage at Hay, in Brecon. Junctions may be observed in Caermarthenshire, Pembrokeshire, Herefordsbire, Gloucestersbire, Worcestersbire, and Shropshire.
The town of Ledbury stands partly on these junction beds. They are quarried at Dymock.
At Malvern, the Downton sandstones are about 100 feet in thickness: they consist of sandstones and marls of different tints-red, grey, and yellow.
The beds contain Pterygotus, Hemiaspis, Beyrichia, some Phyllopods, Annelids, Lingula cornea, and other Mollusca, Onchus and Pteraspis, also traces of land plants
1 The word “Tilestones' (happily abandoned by Sir R. I. Murchison) is altogether inappropriate. There is not a stone capable of being formed into a tile, from the Downton Sandstone to the Cornstones of Wall Hills; but there are thin muddy marls over the Downton beds, which would have been tilestones had they sufficiently hardened, and which are doubtless equivalents of the true tilestones. (Rev. W. S. Symonds.)
(probably Lycopodiaceous seeds), the oldest yet known in England and Wales.
Murchison has stated that the tilestones are visible all along the eastern frontier of the Silurian rocks, and scarcely exceed 40 or 50 feet in thickness.
This group, which rests conformably upon the Downton Sandstones, constitutes the passage-beds between the uppermost Silurian rocks and the Old Red Sandstone.
It comprises red, grey, and purple marls, shales, and sandstones, having at Malvern a thickness of 300 feet.
The section exposed near Ledbury was described by the Rev. W. S. Symonds—the beds contain Pterygotus and Eurypterus among the Crustacea ; also Onchus :Old Red Sand- Red marls with grey and reddish sandstone, Pteraspis stone.
and Cephalaspis. Grey marl passing into red and grey marl and bluish
grey rock (Auchenaspis-grits), with Auchenaspis,
Cephalaspis, Onchus, Pterygotus, Lingula, &c. 20 feet.
Purple shales and thin sandstones, 34 feet. Ledbury Shales
Grey shales and grit, with Cephalaspis and Pterygotus,
8 feet. Red and mottled marls, and thin sandstones with i Lingula, Pteraspis, 210 feet. Downton sandstone, 9 feet.
OLD RED SANDSTONE AND DEVONIAN.
OLD RED SANDSTONE. The Old Red Sandstone consists of red and grey micaceous and mottled sandstones, sometimes false-bedded, quartzose conglomerates, slaty micaceous marls, and shales. Its name, however, bespeaks its most prominent character. In some localities bands of nodular or concretionary limestone called “cornstones' occur ; and ripple-marks are met with on the surfaces of some of the beds of sandstone. The maximum thickness of the series may be taken at about 10,000 feet, but in many places it would appear that an estimate of 4,000 or 5,000 feet is sufficient.
It has been observed to pass insensibly in places into the Silurian rocks below, the passage beds called Downton Sandstones and Ledbury shales belonging perhaps as much to one system as to the other.
Three divisions are generally made in the Old Red Sandstone, as follows:
Red and variegated sandstones, and quartzose conUpper glomerates.
Marly sandstones and flagstones, red shale, and thin Middle cornstones. (Brownstone series.) Lower
Pale coloured sandstones, red and variegated marly Cornstone
beds with cornstones in lower part. (Cornstone Group Lower series.)
Pterygotus, Pteraspis, Cephalaspis, &c. About 2,500 The Old Red Sandstone extends from near Bridgenorth in Shropshire, southwards, through a considerable portion of Herefordshire, Monmouthshire, and Brecknockshire, into Glamorganshire, Caermarthen, and Pembroke.
Fig. 7.— Section of Old Red Sandstone on the north-west escarpment of the Black
Mountains, on the borders of Herefordshire.'
(Sir R. I. Murchison.)
(The beds consist of red sandstone and quartzose conglomerate.]
Near Malvern and Abberley, the Old Red Sandstone rests on the Ledbury Shales and Ludlow Rocks. Quartz conglomerates are conspicuous at Kymin Hill, Monmouth. In the Forest of Dean country, the nodular beds of cornstone give the rock a conglomeratic appearance. The Vans (Fans) of
) Brecon are formed of the Old Red Sandstone, the upper beds consisting of white quartz-conglomerate.
Slaty cleavage in nearly vertical lines traverses the beds around Milford Haven. Upper Old Red Sandstone, consisting of conglomerate, red and grey sandstone, and cornstone, 600 feet, has been observed in Anglesea, resting unconformably upon the older rocks, and overlapped by the Carboniferous Limestone. Some traces of red conglomerate and sandstone, belonging to the upper division, have been mentioned as occurring in the Lake district : they rest unconformably upon the Silurian rocks, and indeed contain pebbles with Cambrian and Silurian fossils. The beds are overlaid conformably by Carboniferous Limestone, and, according to
1 This woodcut is borrowed from Mackintosh's Scenery of England and Wales, p. 142.
Sedgwick, beds of red sandstone, of similar type to that of the Old Red Sandstone, alternate in thick masses with the limestone. The Old Red Sandstone, or rather conglomerate, is stated by Dr. Nicholson to attain a thickness of from 2,000 to 3,000 feet in the Ulleswater area, and of about 150 feet beneath the Pennine Chain. Professor Harkness considers its thickness in the Vale of Birbeck as 270 feet. The upper beds of the Old Red Sandstone occur in Denbighshire,
The Old Red Sandstone may be traced near Berkeley in Gloucestershire, where it passes downwards into the Silurian rocks. It appears in places near Bristol, and on the summit of the Mendip Hills. (See fig. 8.) Its occurrence in West Somerset, North Devon, and South Devon, is not disputed : at present, the beds of the same mineral type as the Old Red Sandstone are grouped with the Devonian rocks, of which they form the base.
The organic remains of the Old Red Sandstone are few. Some plant-remains (Sphenopteris, Knorria, &c.) have been observed, but the most conspicuous fossils are those belonging to the classes Crustacea and Pisces. Of Crustacea the genera include Pterygotus and Stylonurus; of Fish the genera are many, and include Pterichthys, Holoptychius, Cephalaspis, Pteraspis, Coccosteus, &c.
The physical and palæontological evidence goes to prove that the Old Red Sandstone was essentially a freshwater deposit, formed in large lacustrine areas.
Old Red Sandstone yields a strong loamy soil, which is generally fertile : many orchards are situated upon it, and a few hop-yards. In some places, however, the soil is wet and boggy, and consequently unproductive. The Cornstones form the richest land in Herefordshire.
Some beds known as “firestones' have been employed for making hearths. Many sandstone beds are used for building