« PreviousContinue »
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 conglomerates.
Marly sandstones and flagstones, red shale, and thin cornstones. (Brownstone series.)
Pale coloured sandstones, red and variegated marly beds with cornstones in lower part. (Cornstone 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 (See fig. 8.) (See fig. 8.)
of the Mendip Hills.
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
purposes, and these, as well as the cornstones, are often used for mending roads. The conglomerates are sometimes used as cyder-millstones.
Old Red Sandstone, of a greyish-brown colour, quarried near Chepstow, was used in the construction of Tintern Abbey. The sandstone is quarried near Monmouth, Ledbury, &c.
The Three Elms Stone, near Hereford, is also much quarried for building-purposes.
The term Devonian was proposed by Sedgwick and Murchison, because from a study of the fossils Mr. Lonsdale considered that the strata constituted a natural history group intermediate between the Silurian rocks and the Carboniferous Limestone.
The Devonian rocks of West Somerset, Devon, and Cornwall consist of a series of slaty rocks, grits, sandstones, and limestones. Much has been written upon the exact relations of the different divisions that have been made out, and considerable difference of opinion has been expressed. By many authors they are considered as the marine equivalents of the Old Red Sandstone, but this classification is by others admitted to be doubtful, and Prof. Jukes expressed his opinion that the greater part of the Devonian strata were of Lower Carboniferous age. Under such circumstances we cannot do better than consider the beds separately as Devonian, while at the same time pointing out some of the ascertained facts The and some of the conclusions which they foreshadow. beds are best displayed in the coast sections between Barnstaple Bay and Lynton, and the following series of beds has been made out in North Devon:—
LYNTON SANDSTONE (FORELAND GROUP).
This series consists of hard red and grey sandstones with some slaty beds, and is developed at the northern end of the Quantock Hills, at Dunster, Porlock, and the North Foreland.
The beds are unfossiliferous, and are admitted to be identical in character with beds of Old Red Sandstone.
This division comprises a series of gritty slates, shales, and sandstones, about 1,500 feet in thickness, containing, according to Mr. Etheridge, Spirifera, Orthis, Fenestella, Favosites, Bellerophon globatus, Orthoceratites, Crinoids, etc.
HANGMAN GRITS (MARTINHOE BEDS).
This group consists of red and grey grits, shales, and sandstones, well seen at Woodabay, and they resemble, according
1 The Marwood Beds were taken as the uppermost Devonian by Sedgwick and Murchison.