Handbook of Birmingham / Prepared for the Members of the British Association, 1886

by British Association for the Advancement of Science

The Local Rock Formations.

As will be apparent upon a study of the accompanying geological map, the geological formations exposed within thirty miles of Birmingham, include the entire stratigraphical succession between the Cambrian and the middle portion of the Jurassic, with two notable exceptions. The two formations locally wanting are the Ordovician and the Old Red Sandstone. The nearest known Ordovician rocks occur in central Shropshire, to the east of Church Stretton; the nearest Old Red Sandstone beds are met with to the south of the Forest of Wyre. The natural sequence of formations, and the localities where the several formations are most conspicuously displayed, are given in the following table:—

Table of the Geological Formations of the Birmingham District.

MESOZOIC, OR SECONDARY ROCKS.
Liassic.	Middle Lias (Marlstone)—Edge Hill, Fenny Compton.
	Lower Lias Clays—Harbury, Rugby.
Rhœtic.	Marls and White Lias—Harbury, Knowle, Wooten Wawen.
Triassic.	Upper Trias or Keuper:
	(a) New Red Marl—Moseley, King’s Norton, &c., &c.
	(b) Lower Keuper or Waterstones—Birmingham, Warwick.
	Lower Trias or Bunter:
	(a) Upper Mottled Sandstone—Harborne, Edgbaston, Hockley.
	(b) Pebble Beds and Conglomerate—Sutton Park, Smethwick.
	(c) Lower Mottled Sandstone—Clent Hills, Stourbridge.
PALÆOZOIC, OR PRIMARY ROCKS.
Permian, or Dyassic.	(a) Permian Breccia—Clent Hills, Northfield.
	(b) Red Sandstones and Marls—Halesowen, Enville, Rubery.
Carboniferous.	(a) Upper Coal Measures with Spirarks Limestone, &c.—Sandwell, Arley.
	(b) Lower Coal Measures—Oldbury, Bilston, Hawkesbury.
	(c) Millstone Grit—Absent near Birmingham, present near Colebrookdale.
	(d) Carboniferous Limestone—Absent.
Old Red Sandstone or Devonian	Absent near Birmingham, present in Forest of Wyre.
Silurian.	(a) Ludlow Shales and Limestones—Sedgley Hill.
	(b) Wenlock Shales and Limestone—Dudley Hill, Wren’s Nest, Walsall.
	(c) Woolhope Beds—Barr and Rubery.
	(d) Llandovery or Mayhill—Rubery.
Ordovician.	Absent.
Cambrian.	Upper Cambrian:
	(a) Tremadoc Beds and Lingula Flags.—Shineton, Nuneaton, Lower Lickey, and Malverns.
FUNDAMENTAL, CRYSTALLINE AND IGNEOUS ROCKS.
(a) Charnwood Volcanic Rocks—Charnwood Forest, Caldecote Hill, The Wrekin.
(b) Malvern Crystalline Rocks—North Hill, Hereford Beacon, &c.

The Fundamental Crystalline and Volcanic Rocks of the Malverns, the Wrekin, And Charnwood Forest.

The rocks which undoubtedly occupy the lowest place in the geological formations of the Birmingham district are those crystalline and partly schistose masses which form the core of the Malvern Hills; and certain well-marked volcanic rocks which occur at the Wrekin and Nuneaton, and which appear to have their equivalents in the great igneous series of Charnwood Forest. That all these more or less crystalline rocks are of higher antiquity than the Upper Cambrian of Wales is demonstrated by the fact that fossiliferous rocks containing Cambrian fossils of this age overlie them, while the lowest recognisable zones of these overlying fossiliferous rocks (the Hollybush sandstone of the Malverns, the quartzite of the Wrekin, and the Hartshill quartzite of the neighbourhood of Nuneaton) are in part composed of their fragments. Whether, however, they belong in part to the Middle or Lower Cambrian, or wholly appertain to the earlier formations of the Archean, must as yet remain an open question.

(a.) Malvern Hills.—The core of the Malvern Hills is composed of a coarse syenitic, and more or less gneissose rock, pierced by veins of quartzo-felspathic rock of igneous origin (Hereford Beacon, &c). The main mass which is coarsely crystalline, becomes occasionally distinctly gneissose and even schistose, and its mineral bands strike from north-west to south-east, i.e., transverse to the general trend of the Malvern Ridge. The basement beds of the Hollybush sandstones (Cambrian) and the Llandovery rest unconformably upon this rock, which has consequently been claimed by some geologists as distinctly of Archean age, representing in part the Laurentian of Logan.

The best localities for studying the essential characters of the Malvern rocks are the quarries of the North Hill and the Wych, and the eastern slopes of the hills between Malvern Wells and Herefordshire Beacon.

(b.) The Wrekin.—The core of the beautiful hill of the Wrekin is formed by a magnificent series of highly acidic volcanic rocks—rhyolitic lavas and ashes. As first pointed out by Dr. Callaway, they rise out unconformably from below fossiliferous rocks of Upper Cambrian age. (Hollybush Sandstone and Shineton Shales) and are believed by him to be of Archean age. The finest exhibitions of the volcanic ashes of the group are met with in quarries on the flanks of the Wrekin itself, while the rhyolitic lavas occur in scattered localities along the hill. A broad mound of the same igneous series rises out from below the Triassic to the south of Walcot Station, and a most beautiful and instructive section of banded and spherulitic rhyolites is shewn in the quarry at the locality known as the Lee Rock.

The truly volcanic nature of these remarkable rocks was first pointed out by Mr. S. Allport, F.G.S., and their original characteristics, and the changes they have undergone since their formation, have been described by him in a well-known series of memoirs. Their geological position, and their relation to the associated fossiliferous strata, and to the similar rhyolitic rocks of Caer Caradoc and Pontesbury have been fully discussed by Dr. C. Callaway, F.G.S.