• Abstract and Figures
• Public Full-text

CHAPTER XXIX. THE MALVERN AND ABBERLEY HILLS, AND THE LEDBURY DISTRICT. By THEODORE GROOM, M.A., D.Se., F.G.S., F.R.G.S. PART I. PHYSICAL FEATURES AND COMPONENT ROCKS. HE geographical centre of England and Wales is the heart T of the English Midlands. This central area may be described as a relatively low-lying, undulating plain, from which, like islands, rise the more elevated tracts of South Staf­ fordshire, Warwickshire, Leicestershire, and Northamptonshire. The greater part of the plain is formed by gently rolling beds of Trias, but towards the south and east by the Lias. Here and there the plain is diversified by minor scarps and ridges formed by the more durable beds of the Lias and Trias. Its western boundary is the more elevated and hilly tract of the Welsh borderland, which comprises some of the higher parts of Shrop­ shire, Herefordshire, and Gloucestershire. The elevation of this tract is due, partly to the durability of the Palseozoic and Archrean rocks of which it is composed, and partly to upheaval. Much of its surface is occupied by the Old Red Sandstone, but along the eastern border, which runs nearly north and south, emerge the harder Lower Palaeozoic and Archeean rocks of May Hill, the Malverns, and the Abberley Hills. The scenery on the two sides of the Malvern-Abberley range differs appreciably. On the east lies the southern extension of the Midland plain, here narrow and bounded on the east by the Cotteswolds. Watered by the Severn, much of it forms pasture or arable land, dotted with villages and towns. The Malverns rise up abruptly from this plain in a manner suggestive of an ancient coast line. On the western side of the range is the hilly tract of the Old Red Sandstone, with its rich hop lands and orchards, and occasional park lands of fine oak. The population is appreciably less dense than to the east, and towns of even fair size are scarce. The strata are thrown into folds, which towards the Mal­ verns are much more marked than those of the Trias on the east. The Silurian cores of some of the more marked anticlines rise up to form the picturesque wooded heights of the Ledbury and Woolhope Hills. Viewing broadly the disposition of the strata between the Mal­ vern and Abberley Hills on the east, and the hills of Radnor­ shire and South Shropshire on the north-west, the area is seen to be formed by a syncline with a flat or gently undulating base THE MALVERN AND ABBERLEY HILLS. and more sharply recurved edges. In the lap of the syncline where denudation has been less effective stand the Clee Hills with their remnants of Carboniferous Limestone, Millstone Grit, and Coal Measures. The Malvern and Abberley Hills constitute a part of the eastern rim of this basin. The Malverns form a chain of heights extending from Chase End Hill on the south to North Hill; the highest summit is the Worcestershire Beacon. The county boundary between Herefordshire and Worcestershire follows the crest of the range. Continuing the line to the north is a chain of lower and less striking heights, to which the general name of Abberley Hills has been given. The formations represented in the two ranges and their im­ mediate neighbourhood are the following: Archsean, Cambrian, Ordovician (Tremadoc), Silurian, Old Red Sandstone, Car­ boniferous, "Permian" (Midland type), Trias, and Pleistocene. The Carboniferous is represented by certain Coal Measures alone, but at no great distance, in the Forest of Dean and in the Clee Hills, occur the Carboniferous Limestone and Millstone Grit. The Lias and Rhretic are also within easy reach. The outcrop of these formations in the Malvern district is indicated on the map (Plate XXIII, Fig. I). The sequence is interrupted by a number of unconformities, as shown in the following table: PLEISTOCENE. Unconformity. JURASSIC Lias. ~~:; ~~ " Probable unconformity. TRIAS. Upper Bunter Sandstone. Probable unconformity. 1 Bunter Pebble Bed. Lower Bunter Sandstone. Probable unconformity. PERMIAN (?) Haffield Breccia. Unconformity. Coal Measures. CARBONIFEROUS. Unconformity . { Coal Measures. Unconformity, OLD RED SANDSTONE. Ledbury Beds. Downtonian.{Downton Sandstone. Upper Ludlow Shales. Aymestry Limestone. r ( Lower Ludlow Shales. SILURIAN. .\ Salopian, -Ii wenlo.Ck Limestone. Wenlock Shale. Woolhope Limestone. j Woolhope Shales. "ValentJan. 1 May Hill Sandstone. Unconformity, 700 GEOLOGISTS' ASSOCIATION JUBILEE VOLUME. ORDOVICIAN. Tremadoc. Bronsil Shales. (Uppe! White-leaved Oak Shales, CAMBRIAN. Middle. { Hollvbush Sandstone. i Lower. Malvern Quartzite. Probable unconformity. ARCH£AN. Malvernian and Warren House Series. ARCHlEAN ROCKS. The Archaean rocks* are seen chiefly in the Malvern Hills, the core and highest parts of which they form. Small patches are also found in Cowleigh Park and near Martley in the Abber­ ley Range. The rocks include two apparently distinct series: 2. Warren House Series. 1. Malvernian Series. No trace of a sedimentary series comparable with the Long­ myndian of Shropshire and May Hill has been detected. The M aluernian comprises a series of schists and gneisses in which hornblende, chlorite, white mica, felspar and quartz are the dominant minerals. Associated with these rocks are red binary granites, diorites of medium or fine grain, felsites, and occasionally other rocks, such as syenite and olivine diabase. The complex thus formed has not hitherto been mapped in detail, though the rocks are admirably exposed in many places, more particularly in the fine quarries near the northern end of the hills, and in the Hollvbush Pass. So extensive, indeed, has been the quarrying that permanent injury to the picturesqueness of the neighbourhood is feared. The Warren House Seriest (Fig. 120) is found only on the easternside of the Herefordshire Beacon, and in the neigh­ bouring part of Castlemorton Common. It is stated to consist of rhyolites, andesites, basalts and tuffs, but the assemblage awaits detailed description. The prevailing dip seems to be easterly, as though the series overlay the adjoining Malvernian. The junction not improbably dips in the same direction, but it is not clear that the two series are not separated by a fault. There can be little doubt that the Malvernian and Warren House Series are of pre-Cambrian Age. The chief considerations bearing on this question are the following: (I) Although the Cambrian can in no place be observed to rest directly on either, its general dip is away from them. (2) The very considerable metamorphism of these rocks, especially of the Malvernian, suggests an age much greater than that of the slightly altered Cambrian. (3) The rocks cannot be readily paralleled lithologically with any post-Archzean rocks in the country. '~See List of Works on page 738. Nos. 3, 8-13, 16, 19,20,23.24. t List of Works, Nos. 9, II, 19,28. THE MALVE RN AND ABBE RLEY HILLS. 701 (4) The Malvern ian has much in common with the gneissic and schistose rocks of Shropshire believed to be pre-Cambrian, and the Warren H ouse rocks compare well with the Uriconian of the same countv. (5) The conglomeratic beds of the May Hill Sandstone and Mal vern Quartzite include rolled pebb les of alt ered rocks, some of which are very similar to the granites of the Malvernian, and others to the lavas of the Warren H ouse Series. This seems to imply great denudation and pa rtia l alteration of the old rocks before the depo sition of the Cambria n, and probably an un­ confo rmity at the base of the latter. (6) T he plagioclin al structure of the Mal vern s suggests great pre-Cambrian folding and denudation. C AM BRI AN AND O RD OVICIAN ROCKS :*. The Cambrian consists of : 3. The White-leaved Oak Shales (" Black Shales "). 2. The H ollybush Sandstone. 1. The Malvern Quartzite. The Malvern Quartzite is commonly a grey quartzite of rather fine grain. It is seen only in shattered strips, chiefly towards the southern end of the Malvern s, but small exposures are visible in Cowleigh P ark, and at Martley in the Abberley Hills. T he total thickness may be several hun dreds of fee t. The best ex­ posures are at the northern end of Midsummer Hill in the Gull et P ass, and in Winter Combe on the Raggedstone. I n the locality first mentioned beds of conglomerate alternate with quartzite. The ground-mass of the conglomerate consists chiefly of grains of quart z, quartz-schist, rhyolite, orthoclase, microcline, plagio­ clase, white or green mica, chlorite, together with epidote, ilmenite, iron pyrites and glau conite. The pebbles are chiefly metamorphic quartzite or quartz-schist, plutonic quartz, rhyo­ lites, granite, orthoclase, microcline, plagioclase, chlorite-schist, and quartzose diorite ; less abundant are mica-schist, diorite, gneissic diorit e, andesite, basalt, sandstones, and clastic quartzite. Of these mat erials, some have evidently been derived fro m a complex resembling the Malvernian, and others from a volcanic series like the Uriconia n ; but a third group, largely repr esented, including such rocks as the quartz schists, are very poorly represented in the Mal vern Hills. The chief fossils foun d in the Quartzite are Kutorgina plzillipsii, Holl., Obolella (?) groomii, Matley, and H yolitllUs prim e-uus, Groom. The H ollybusli Sa nd stone (Fig. II6) is exposed towards the southern end only of the Malverns. In the la rge quarry at White-leaved Oak it ap pears to pass down into the Malvern * List of Works, No . 27. 702 GEOLOGISTS' ASSOCIATION JUBILEE VOLUME. Quartzite, but only one or two feet of the latter are visible. The Sandstone is prevailingly of a dark-green colour, due to the presence of numerous grains of glauconite, chiefly casts of for­ aminifera.

Load more

  42

Copy Link