The South Staffordshire Coalfield/Chapter 3
CHAPTER III.
DESCRIPTION OF THE Rocks—continued.
2. The New Red Sandstone.
Since the publication of the first edition of this Memoir, the New red sandstone of the midland counties has been thoroughly surveyed, and its several parts analysed much more completely than before.
This has been done chiefly by the labours of Messrs. E. Hull and H. H. Howell, under the direction of Professor Ramsay, who now supplies the following abstract of the results which have been arrived at, in the passages included within inverted commas.
"The New red sandstone around the South Staffordshire coalfield consists of the following subdivisions:-—
| Maximum Thickness. Feet. | |||
| Keuper beds. |
Red Marl | 600 | |
| White and Brown Sandstone with beds of Red Marl. Waterstones | 300 | ||
| Bunter Beds |
Upper Red and Mottled Sandstone | 500 | |
| Conglemerate or Pebble beds | 500 | ||
| Lower Red and Mottled Sandstone | 200 | ||
"The division of the Bunter beds into three sub-formations was first made out by Mr. Hull while mapping the New red sandstone near Coalbrook Dale, and I considered it so important that, with the approval of the late Sir Henry De la Beche. I instructed the members of the Survey engaged on the New red sandstone to map all the subdivisions, more especially as in doing so we had a means of detecting a number of the faults that traverse the country.[1] This accordingly has been done from the south end of the Malvern hills to Lancashire and Nottinghamshire, and the sub-formations have been found always to maintain the same order of superposition, although occasionally one or more members may be absent. In general they are easily distinguishable from each other, except when the Upper and Lower Red and Mottled sandstones chance to be brought together by faults. All of them occur in the district described in this Memoir."
Keuper.—The Red Marls—This subdivision is principally composed of marl of a dull red colour, with occasional thin bluish shaly beds, containing sometimes small layers of bluish-white sandstone a few inches thick. In the bottom part of the marls occur local beds of sandstone some feet in thickness, sometimes brown, sometimes yellow or white, containing few or no pebbles, generally rather shaly, and not often marked by oblique lamination. The marls are likewise generally shaly, and are sometimes slightly arenaccous, especially in the lower beds. Like all marls, they, when dry, frequently crumble into small cuboidal fragments.
In Staffordshire neither the gypsum nor the salt of this subformation is very abundant. Beds of gypsum may be seen cropping to the surface in several places about Tutbury and Uttoxeter, and the substance is met with elsewhere in wells and pits. The salt is only known by the occurrence of brine springs, which are met with at two places, one at Silkmoor, 1½ mile south of Stafford, the other at Shirley Wych, near Weston-upon-Trent. At the latter place there is a shaft sunk to the depth of 414 feet, of which Mr. Hull procured the following section:—
| FT. | |
| Drift sand and gravel | 54 |
| Gypsum | 6 |
| Red marl | 354 |
| 414 |
It is probable, therefore, that the total thickness of this subformation cannot be much less than 600 feet.
Waterstones.—"At the base the Red marl passes into or rather alternates with beds of white and brown sandstone, formerly considered the uppermost part of the Bunter beds, but now placed by the survey as the lowest member of the Keuper series, and called the Lower Keuper sandstone or Waterstones. These strata are the equivalents of the sandstones of the Peckforton hills and of Warwick, which contain Labyrinthodon remains, and in the district surrounding the South Statfordshire Coal-field fossil plants were found in them near Bromsgrove, and a small ganoid homocerque fish, described by Sir Philip Egerton under the name of Dipteronotus cyphis.
"Frequently the beds are rippled or current marked, and also traversed by sun cracks, showing that some of them were alternatcly covered by water and exposed to the air.
"This subdivision occurs in great force round Bromegrove and in the country between Ombersley and Stourbridge. It lies in the form of a synclinal curve, widening to the south, where it is overlaid by a broad tract of red mar]. The sandstone series is here probably from 200 to 300 feet thick. It also occurs on Penn Hill and Oreton Hill as a faulted outher, and farther north it ranges from Whitwick to Penkridge, expanding in places to a width of several miles. Its base partly consists of calcareous breccia and conglomerate, and its greatest thickness is probably about 300 feet.
"On the east of the Coal-field equivalent strata of the same general type (occasionally interrupted by faults) run from the Caradoc sandstone of the Lickey Hills, by Birmingham, to the north of Sutton Coldfield, and from the border of the Coal-field at Cannock Chase to the Trent near Lichfield.
"BUNTER—The Upper Red and Mottled Sandstone.—The Upper Red and Mottled sandstone round the South Staffordshire Coal-field invariably succeeds, in descending order, the strata described above. It varies in thickness from 400 to 500 feet, and generally consists of fine bright red sand, sometimes streaked and mottled with yellow. These sands are often false bedded, but they never contain pebbles. On the east side of the Coal-field this sub-formation ranges from the neighbourhood of Harborne by Birmingham to Sutton Coldfield, and again from the southern part of Cannock Chase eastward to Shenstone. On the west the beds strike in a narrow strip from the Birmingham and Gloucester railway to Hagley, and elsewhere, interruptedly and broken by faults, from the neighbourhood of Stone in Worcestershire to Tettenhall near Wolverhampton. A patch about 5 miles in length also occurs west of Cannock, between Cross Green and Manstey Wood. Good sections may be seen in a cutting of the Birmingham and Gloucester railway, south of Blackwell station, in the new cemetery at Birmingham, in the road cuttings about Tettenhall and Compton near Wolverhampton and at other places marked by arrows on the map.
"The Conglomerates or Pebble Beds.—The pebble or conglomerate beds lie below the Upper Red and Mottled sandstone. 'They vary in thickness from 300 to 500 feet, and in our district are probably thickest south of the Lickey Hills and in Cannock Chase, north of Cannock. They range from a fault near Blackwell station south of the Lickey, to Hagley, lying here directly on the Permian strata, without the intervention of the Lower Red and Mottled sandstone. West and north of Hagley the same strata, repeated by faults, run from the neighbourhood of Church Hill and Heathside to Wolverhampton and the country west of Trysull, and beyond Wolverhampton they strike northward to Cannock in a band on an average about a mile wide. North of Cannock they widen out, and form almost all Cannock Chase between Rugeley and Bednall. On the east the chief area lies between Harborne, Sutton Park and the Brown Hills, where the New red sandstone is faulted against the Coal-measures.
"The conglomerate is generally more or less incoherent, but occasionally it is cemented into hardish rock by the presence of carbonate of lime. In general, however, it still so completely retains its original character of gravel, that it may be dug out with the pickaxe and shovel, and is used for gravel pits. Occasionally it consists chiefly of sand, with only a few scattered quartz pebbles, as, for instance, in the small patch between Northfield and Frankly.
"The pebbles found in these conglomerates consist chiefly of brown and liver-coloured quartz rock, well water-worn and rounded. These were for long considered to' have been derived from the waste of the altered Caradoc sandstone of the Lickey Hills; but when we consider that the same conglomeritic formation, only interrupted by faults, extends all the way from the neighbourhood of Stourport to Lancashire and Nottinghamshire (wherever it has been mapped by the Geological Survey', it is evident that the pebbles have been derived from some other source, the locality of which is unknown. Besides these, there are other occasional pebbles of white quartz, coal-measure sandstone, or millstone grit (stigmaria markings being sometimes discernible in them), chert containing casts of crinoidal stems from the mountain limestone, dull red sandstone, traps, agates more or less decomposed, altered slate, and jasper."
The Drift gravel that more or less covers the country is in a great measure derived from the waste of this conglomerate, and without practice it is at first, sometimes, very difficult to distinguish between these ancient gravels of the New red sandstone period and the other gravels that belong to the much more modern period in which "the Drift" was formed.
In examining a gravel pit for this purpose, the first thing to look for is a chalk flint. True chalk flints, with chalk fossils, may be in some places pretty abundantly found in the gravels of Staffordshire, as sometimes also oolitic and liassic fossils[2] Where these occur they are, of course, conclusive evidence against the gravel being of the New red sandstone period. Very often, however, there are large deposits of gravel belonging to the Pleistocene or some of the more recent periods, in which no fragments of rock are found that can be identified with anything newer than the Coal-measures. Still, even these may, after a little practice, be distinguished from the New red (or Triassic) gravels, by their more irregular heaped-up method of deposition; by the sand in which the pebbles lie being of a paler and yellower colour than that of the New red; by the pebbles lying edgeways and confusedly, instead of horizontally and more or less regularly stratified; by the pebbles being often dirty, while those of the New red are generally quite clean (a remark of Mr. Hull's), and by a vague but sufficiently appreciable facies, which is not describable, but can be learnt by a little practice, and by that alone.
Figures 1 and 2 represent two modes of occurrence of the conglomerates in the New red sandstone.
"The Lower Soft Red and Mottled Sandstone.—The Lower soft red and mottled sandstone, when the section is complete, invariably comes between the pebble beds and the Permian or Coal-measure strata of our area. It is in general a bright red soft sandstone, occasionally streaked with yellow and white, and in most points strongly resembles the Upper soft red sandstone. It is, however, generally coarser and exhibits more false bedding. It is quite destitute of pebbles. Between the south end of the Lickey Hill and Hagley it is absent, the Pebble beds resting directly upon the Permian breccias; and the same is the case on the north and east of the Coal-field. West of Stourbridge it underlies the pebble beds in a narrow strip dipping east, and nearly 5 miles long, being faulted on the west against the Upper soft red sandstone. Still further west, repeated by this fault, it runs north from Kinfare by Spittlebrook Mall.
"On the east side of the Coal-field it is entirely absent, the Pebble beds resting directly on the Permian strata. Its thickness is very variable, and probably within the limits of the map described it never exceeds from 100 to 200 feet, but further west, in parts of Shropshire, it is very much thicker."
- ↑ This may some day throw much light on the mode of working the coal below, should it ever be searched for and found.
- ↑ Gryphæa incurva and large fragments of ammonites and other fossils are sometimes found in great abundance in patches of red clay, belonging to the Drift deposits, near Wolverhampton and other places.