The tectonic structure of the Malvern Hills

The Tectonic Structure of the Malvern Hills by N. E. BUTCHER Received 16 January 1961; read 2 June 1961

CONTENTS 1. 2.

INTRODUCTION LOWER PALAEOZOIC UNCONFORMITIES

page 103 ...

(a) The Upper Llandovery Unconformity (b) The Cambrian Unconformity 3.

THE MALVERN STRUCTURE

(a) (b) (c) (d) (e)

The Malvern Fold ... Strike Faults Oblique Faults The Age of the Malvern Structure The Relation of the Trias to the Pre-Cambrian (f) Late- and Post-Trias Movements 4.

THE REGIONAL STRUCTURE ACKNOWLEDGMENTS EXPLANATION OF PLATES ... REFERENCES DISCUSSION

105 105 107 107 107 110 112 112 114 115 116 118 118

118 120

ABSTRACT: The structure of the Malvern Hills is suggested to be essentially that of a single, large north-south monoclinal fold involving Pre-Cambrian and unconformable Palaeozoic strata. The steep limb connecting the upper and lower flat-lying limbs may have been about 1000 feet in length. The fold is one of a regional set of roughly northsouth folds. The Malvern fold appears to be broken by flat and steep dip-slip strike faults which have presumably arisen parallel to the fold limbs. It is further cut by oblique faults with evidence of a strike-slip component of displacement. This faulted fold structure, produced by orogenic movements at the end of the Carboniferous period, presents a picture of maximum shortening in the direction perpendicular to the easterly dipping axial surfaces of the fold. It is suggested that posthumous movements on one or more of the steep dip-slip strike faults during the ensuing post-orogenic period produced the western margin of the area of New Red Sandstone deposition. The Malvern Hills may be a spectacular example of the way in which an ancient Pre-Cambrian crystalline basement can bend and form the core of a much younger anticline.

1. INTRODUCTION

a natural boundary separating the counties of Worcestershire and Herefordshire, must surely be one of the most striking topographical features to be seen in Britain. Geologically, they are something of an enigma. The sharp, north-south Pre-Cambrian (Malvernian) igneous and metamorphic ridge, seven and a half miles in length but nearly

THE MALVERN HILLS,

103

104

N.E.BUTCHER

everywhere less than half a mile in width, is flanked to the west by Cambrian, Silurian and Old Red Sandstone strata. Eastwards the ridge descends abruptly to the Triassic Worcestershire plain. Structural discussion has been principally concerned with the nature of the relations between the Pre-Cambrian and the bordering steep and often inverted Palaeozoic strata on one side, and the flat-lying Trias on the other. Reviewing the principal literature on the Malvern structure with regard to the western boundary of the Pre-Cambrian, it is noteworthy that the pioneer work of Phillips (1848) appears to have come closer to the truth than has any subsequent interpretation. Phillips's interpretation seems to have been overlooked or set aside by most of the later workers. His views are perhaps best expressed in the six-inch sections (Sheet 13, see especially No.7) which he drew across the Old Series one-inch maps LV SE and XLIII NE in his work for the Geological Survey. It is clear from these that Phillips envisaged the Lower Palaeozoic strata to be everywhere unconformable on the Pre-Cambrian, the surface of unconformity itself being folded. Thus Phillips may be said to have shown that the Pre-Cambrian and Lower Palaeozoic have essentially a fold structure. Holl (1865) added to this interpretation by showing the presence of oblique faults, mostly trending north-west-south-east. He also demonstrated the western fault margin of Herefordshire Beacon (see his map opposite page 72). Callaway (1879, 219) set the pattern of subsequent interpretations in regarding the Malvern Hills as primarily a fault structure. From his detailed investigations Groom (1900, 193) concluded that 'the western margin of the Archaean massif appears everywhere to be defined by a fault, lit some cases about vertical, in others reversed, sometimes with a considerable hade'. He envisaged the Pre-Cambrian having upthrust and locally overthrust relations with the Palaeozoic strata, the whole structure being one of complex folds and faults. Groom's demonstration of the structure of Herefordshire Beacon as a mass of Pre-Cambrian thrust westwards over inverted Silurian strata (1900, 140-2) is generally accepted. Blyth (1952) followed Brammall (in Pocock, Brammall & Croft, 1940, 61) in regarding the Pre-Cambrian ridge as a series of staggered fault blocks. In Blyth's view the folds in the Palaeozoic strata immediately west of the ridge are the result of the upthrust of the Pre-Cambrian basement blocks, being 'related in origin to movement on the system of faults which bound the Pre-Cambrian ridge' (p. 192). In the same year Raw (1952) published his interpretation of the hills as the remnant of a thrust nappe. Impressed by the steep attitude of the supposed fault bounding the western side of the Pre-Cambrian ridge (Herefordshire Beacon and Chase End Hill apart), he regarded the Pre-Cambrian as being relatively downthrown, rather than upthrust, by steep easterly dipping normal faults slicing through the thrust nappe (see his fig. 6).

TECTONIC STRUCTURE OF THE MALVERN HILLS 105 There has been general agreement as to the late Palaeozoic age of the main movements producing the Malvern structure (Wills, 1951, 36). It is difficult to see how the interpretations of Groom, Blyth and Raw would fit in to the regional tectonic picture produced in late Palaeozoic times. It is suggested here that the structure of the Malvern Hills is essentially that of a single, large north-south monoclinal fold involving the Pre-Cambrian and unconformable Palaeozoic strata. The fold appears to be broken by flat and steep dip-slip strike faults, which have presumably arisen parallel to the fold limbs, and also by oblique faults. This interpretation allows the Malvern structure to be part of the regional picture of roughly north-south folds produced at the end of the Carboniferous period. Further, posthumous movements on one or more of the steep north-south faults provides a means by which the western margin of the area of New Red Sandstone deposition could have been produced. 2. LOWER PALAEOZOIC UNCONFORMITIES (a) The Upper Llandovery Unconformity Reference to Holl's (1865) excellent map shows that it is the May Hill Sandstone of Upper Llandovery age, the local base of the Silurian succession, which crops out immediately west of the Pre-Cambrian ridge, except at the southern end ofthe Malvern Hills where the Cambrian intervenes. Holl (1865, 94) explained this as being due to the overstep of the unconformable May Hill Sandstone across the Cambrian strata on to the ancient crystalline rocks. Groom (1899, 168; 1900, 193) could not accept Holl's simple stratigraphical picture. He held (1899, 153) that the Cambrian, as well as the Silurian, formerly passed eastwards over the Pre-Cambrian range. Groom regarded the occurrence of Cambrian strata in the vicinity of Cowleigh Park at the north end of the hills 'as indicating that the May Hill Beds do not rest directly upon the Archaean, but are brought into contact with it by means of faults' (1900, 162). Although, therefore, he apparently accepted an unconformity in the stratigraphical succession at the base of the May Hill Sandstone (1910, 699), Groom was, nevertheless, able to assert that 'nowhere is there clear evidence that the Palaeozoic beds west of the hills ever rested on these particular crystalline rocks' (1910, 716). Yet Phillips had in 1842 described such a locality. Phillips relates (1842, 290-2) how his sister first discovered, in that year, loose pieces of a fossiliferous conglomerate at the northern end of the Malvern Hills. Subsequently, the conglomerate was found exposed west of the Worcestershire Beacon (Plate 4). With De la Beche, Phillips observed the surface of the Pre-Cambrian to be 'nearly vertical, but undulating and irregular, and its strike nearly north and south' (p. 291). Phillips figures and describes the conglomerate 'accumulated in an irregular bed above a foot

106

N.E.BUTCHER

or only a few inches in thickness against the trap' (pp. 291-2). The pebbles 'are of the same nature as the rocks immediately adjacent and composing the neighbouring hills' and Phillips concludes that the igneous rocks of the hills and the unconformable Silurian were together 'raised in a solid state' (p. 292). In 1844 Miss Phillips traced the conglomerate 'some distance farther northward' (Phillips, 1848, 69). Symonds (1884, 45-6) lists the localities where 'Miss Phillips's Conglomerate', as it became known, could be seen. In addition to those at the northern end of the hills, he records its occurrence on the western side of the Wyche cutting and, of especial interest, states that 'it has been found resting against the quarry of Holly Bush sandstone on the north side of the Ragged Stone, and also against. the syenitic axis of the Midsummer Hill' (p. 46) in the southern Malverns. This distinctive fossiliferous conglomerate is now displayed in the Gullet Quarry (32(80)762381) near the south end of the hills (Plate 5). Westerly development of this well-known quarry in the Ma1vernian has revealed the overlying May Hill Sandstone. In September 1959 the sediments were observed by the writer only at the extreme southern end of the western quarry face. Later, they evidently became exposed along this face. They were seen to be well exposed in July 1960 at the top of the face in the northwest corner of the quarry. Reading (1960) and Reading & Poole (1961), visiting the quarry since the present writer commenced work there, have already commented on the exposure. Reliable determinations of the attitudes of the rocks in question can only be obtained in the north-west quarry corner (Plate 4, A). Here, the Malvernian shows a well-developed foliation with a dominant dip of 55° due south. The generally even surface of the Malvernian, to which the bedding surfaces in the overlying sediments are apparently parallel, dips at 60° due west. It is not proposed to describe here in detail the palaeontological and sedimentological features of the May Hill Sandstone. Some twenty feet of sediments are incompletely exposed. The sequence comprises shales and thin fine-grained sandstones and siltstones, which average three inches in thickness, with a green coloration; and there are occasional thin, mostly decalcified limestones with a mainly coral-brachiopod fauna. This sedimentary sequence has been proved to be the right way up by examination of the internal structure of ripple marks found on the upper surface of a sandstone bed (Reading University Geology Department Museum No. 14278). The conglomerate is confined to the base of the succession (Plate 4, B) and is one foot or less in thickness. May Hill sediment and fossils, the latter in their usual decalcified form, occur between boulders and pebbles of Malvernian rock, forming a 'shelly mass' similar to that which Phillips (1842, 291) figured and described from the northern end of the hills. Indeed, the whole occurrence is evidently remarkably similar to

PROC. GEOL. ASSOC., VOL. 73 (1962)

PLATE 4

A. The steep Silurian-Pre-Cambrian unconformity

B. The basal fossiliferous conglomerate [To face p, 106

~ ';:;>

--

-

-- - -

- -

-

-

"...

'<" ; .::::!

I

I

'"C :;tl

0 (j

----

o tr1

0 t""'

>

tr:

[/)

0

o <

0

r-J

w

-

,-., 'D 0\ N

'-'

'"C

r-

> ..., Photograph by courtesy Aerofilms Ltd.

An aerial view northwards along the Malvern Hills

tr1 VI

TECTONIC STRUCTURE OF THE MALVERN HILLS 107

that described by Phillips. There can be no doubt as to the unconformable nature of the junction between the Silurian and the Pre-Cambrian in the Gullet Quarry. It therefore seems probable that the interpretation of Phillips (1842; 1848) and Holl (1865) is the correct one, namely that the boundary between the Pre-Cambrian and the May Hill Sandstone is a line of unconformity for the whole of its five and a half mile length, and not, as later writers have supposed, a fault. Thus the Malvern Hills are comparable to Shropshire where Whittard (1952,165-7) in particular has demonstrated this important Upper Llandovery or Valentian unconformity. It is interesting to note that Pocock & Whitehead, in the relevant Geological Survey Regional Guide (1948, p. 54), specifically state that 'in the Malvern country the Upper Llandovery deposits, with a basal fossiliferous conglomerate, rest on the Cambrian and pre-Cambrian rocks'. (b) The Cambrian Unconformity It remains to consider briefly the nature of the boundary between the

Pre-Cambrian and the Cambrian in the southern part of the Malvern Hills. Groom (1899) adhered rigidly to a fault interpretation (see his map, pl. XIII). Unfortunately, this junction is apparently now nowhere to be seen but in the last century it was evidently well exposed in the quarry at the southern end of Ragged Stone Hill, close by Whiteleaved Oak. Groom's sketch of the face (1899, fig. 7) shows a fault separating the Pre-Cambrian from Cambrian Hollybush Sandstone, the fault junction, dipping about 35° in a west-south-westerly direction, being parallel to the bedding surfaces in the overlying sediments. However, Holl (1865, fig. 2) had given a drawing (by Salter) of the quarry face as it was over thirty years earlier. The junction between the Pre-Cambrian and the overlying Cambrian sediments is shown as an unconformity, dipping at 50° in a south-westerly direction. Salter's drawing seems the more reasonable of the two interpretations and Holl's description (1865,88-9), supported by Symonds (1872,34), makes it probable that the surface of the Pre-Cambrian is again an unconformity, with a westerly inclination of the order of 45°. The angle between the Silurian and Cambrian unconformities is not known, but the data shown on Groom's map (1899, pl. XIII) suggest that it is small. The presumed relationships between the Pre-Cambrian and Lower Palaeozoic strata are diagrammatically portrayed in Fig. 1 a. 3. THE MALVERN STRUCfURE (a) The Malvern Fold It is of considerable importance that the surface of the Pre-Cambrian has been observed to be steep in all three of the localities (west of Worcester-

N. E.BUTCHER

108

a

b

c

d

e E<- -

TECTONIC STRUCTURE OF THE MALVERN HILLS 109

shire Beacon, the Gullet Quarry and the Whiteleaved Oak quarry) discussed above, especially since these localities are widely separated. Reference to any of the geological maps of the hills shows that the outcrops of the May Hill Sandstone-Pre-Cambrian and Cambrian-Pre-Cambrian junctions everywhere approach the form of straight lines. Hence it is likely that the surface of the Pre-Cambrian is steeply inclined along the whole length of its outcrop. Since the earliest investigations it has been known that the generally flat-lying Palaeozoic strata to the west of the Malvern Hills acquire a steep attitude as the hills themselves are approached. Groom (1900, 193) concluded that 'inversion is, in fact, the rule along the western side of the Malverns'. He recognised that the steep Palaeozoic strata form the western portion of a north-south anticline which he termed the 'Malvern fold' (1910, 720). Acceptance of the steep western bounding surface of the PreCambrian as one of unconformity means that the Malvernian rocks must form an integral part of the fold structure. In completing the fold form as seen in cross-section it seems reasonable to suppose that the steep attitude changes back again, upwards and eastwards above the present level of erosion, to a dominantly flat-lying attitude similar to that to the west of the hills. The complete Malvern fold is shown diagrammatically in Fig. 1 b. The synclinal part of a fold of this general form can be easily fitted to some of Groom's key sections (1899, fig. 13; 1900, figs. 5,6 and 10) and to Robertson's section (1926, fig. 18) along the line of the second Colwall railway tunnel through the hills. The Lower Palaeozoic outcrops are most narrow near the Wyche cutting towards the northern end of the hills (Groom, 1900, pl. VIII). Here, the changes in bedding attitudes recorded by Groom on his map as taking place in extremely short distances across the strike of the strata, suggest a fairly closed angular shape for this lower synclinal hinge. In this interpretation the Pre-Cambrian basement rocks form the core of the upper anticlinal part of the fold (Fig. 1 b). The crystalline rocks at

EXPLANATION OF FIGURE 1 Fig. 1. The Malvern faulted fold, a model interpretation illustrating the sequence of events. (a) Relationships in the unfolded state in late Carboniferous times. Coarse 'Vs' denote Pre-Cambrian Malvernian; fine 'Vs', Pre-Cambrian Warren House volcanics; coarse dots, Cambrian; Silurian and younger Palaeozoic strata unornamented. (b) The monoclinal fold form produced at the end of the Carboniferous period. Development of (c) flat and steep dip-slip strike faults and (d) steep oblique-slip oblique faults. Arrows marked'S' indicate direction of maximum shortening during stages (b), (c) and (d). (e) The production, by posthumous movements on a steep dip-slip strike fault, of the western margin of the site of deposition of Permo-Triassic sediments. Fine dots denote Trias. Minor normal faults cut the Trias. Arrows marked 'E' indicate direction of extension.

110

N. E.BUTCHER

present exposed to view, with the exception of Herefordshire Beacon, are evidently located within the steep limb. It must be admitted that there is as yet no direct evidence available, derived from the Pre-Cambrian rocks themselves, that supports this fold interpretation. However, most of the Malvernian outcrops show the rocks to be extremely shattered and slickensided surfaces are common. These crystalline rocks would be expected to deform during the generation of the fold structure in a rather different, more brittle, manner than would the overlying Palaeozoic sediments. Brammall (in Pocock, Brammall & Croft, 1940, 58-62) emphasises the tectonic features found in these ancient rocks. (b) Strike Faults During the production of the fold form by bending there will be movements on the bedding surfaces within the limbs which will have the sense indicated in Fig. 1 b. Ideally, one may imagine the folding phase of deformation passing into faulting in which dip-slip strike faults arise parallel to the bedding surfaces within the limbs (Fig. 1 c). The important feature is that the two resulting fault sets present the same two-dimensional movement picture as does the fold. There appear to be representatives of both sets of faults cutting the Malvern fold. (i) Flat Dip-slip Strike Faults. It is common knowledge that the summit of Herefordshire Beacon is one-half mile west of the north-south line of Pre-Cambrian hill-tops to the north (Plate 5). Other features of this Herefordshire Beacon-Broad Down mass of Pre-Cambrian rocks are also thought to be important. Firstly, the topographic form of Herefordshire Beacon, viewed from, say, Worcestershire Beacon, is quite distinctive. All the other Pre-Cambrian hills to the north and south have a sharp, symmetrical ridge form (Raw, 1952, pl. 8; Groom, 1899, pIs. XIV and XV) which presumably reflects the steep attitude of the mass of rocks. Herefordshire Beacon, on the other hand, presents a steep western face and a much flatter eastern face, thus suggesting a flat-lying body of rocks. Secondly, the Pre-Cambrian rocks everywhere exposed in the hills are referable to the Malvernian, a plutonic and metamorphic series, with the exception of the outcrops to the east of Herefordshire Beacon (Groom, 1910, pI. XXIII, fig. 1). These Warren House volcanic rocks were held by Illing (in discussion on Platt, 1925,64) to be younger than the Malvernian, the junction between the two series being a stratigraphical boundary of Pre-Cambrian age. This suggests that, compared with the rest of the hills, a different part of the Pre-Cambrian basement is here exposed. Thirdly, 'a very remarkable feature in connexion with the Herefordshire Beacon is the occurrence of fossiliferous debris of May Hill Sandstone at a number of places high up the hill' (Groom, 1900, 143). Groom mentions

TECTONIC STRUCTURE OF THE MALVERN HILLS 111 the possibility that these fragments may be the remnants of a former general covering of May Hill Sandstone, deposited directly upon the PreCambrian, but discards this view in favour of a fault interpretation (see his figs. 2 and 3). Groom specifically states (p. 145), however, that he searched in vain for Cambrian fragments and is clearly perplexed by the fact that only May Hill Sandstone debris could be found. The hypothesis which Groom discarded seems to be the likely answer to this problem. There can be little doubt, as Groom (1900,140-3, figs. 2 and 3) showed, that Herefordshire Beacon comprises a mass of Pre-Cambrian rocks separated by a flat thrust fault from underlying inverted Silurian strata. The above evidence supports the idea that the superimposed Pre-Cambrian mass is actually a remnant of the upper flat-lying limb of the Malvern fold, thrust westwards over the steep limb of the fold (Figs. 1 band 1 c). It may well be, as claimed by Groom (1899, 148-9, fig. 16), Blyth (1952, 189) and Raw (1952,231), that Chase End Hill in the extreme south of the Malvern range is also a thrust mass of Pre-Cambrian. The topographic form of this hill suggests that these Malvernian rocks belong to the steep limb of the Malvern fold, having been thrust westwards over steep Cambrian strata. (ii) Steep Dip-slip Strike Faults. A notable feature of Groom's sections across the hills is the frequent representation of steep faults, many of them being essentially parallel to steep bedding surfaces in the Palaeozoic sediments which they cut (for example, see Groom, 1899, fig. 4). Many of these faults introduced by Groom probably do exist, though where parallel to steep bedding surfaces they would be extremely difficult to detect in mapping and their sense of movement similarly difficult to determine. Although it has been argued that the steep western boundary of the Pre-Cambrian ridge is one of unconformity and not a major fault, it is quite possible therefore that locally the junction with the Lower Palaeozoic sediments is a steep strike fault. This is indeed particularly likely in the northern part of the hills where the surface of the Pre-Cambrian is near vertical. This probably explains the relations seen near West Malvern which perplexed Robertson (1926, 168). Farther south, it is tempting to suspect from the information given by Symonds & Lambert (1861,155 and 157, fig. 5) and Robertson (1926,167, fig. 18) that the two parallel Colwall railway tunnels were in fact driven through the May Hill SandstonePre-Cambrian unconformity. However, it may be, as Robertson assumes (p. 167), that a near vertical strike fault was penetrated. Steep north-south faults down throwing eastwards in accord with the expected picture (Fig. 1 c) appear to be present near Herefordshire Beacon. Blyth (1952, fig. 2) maps two such faults cutting the Wenlock Limestone immediately west of this hill. The eastern boundary of the narrow PreCambrian ridge of Swinyard Hill is probably another member of this fault

112

N. E. BUTCHER

set. Thus the outcrop of May Hill Sandstone to the east of the PreCambrian is interpreted as that of a portion of the upper limb of the Malvern fold let down by a presumed normal fault. Groom (1910, 720), indeed, suggested this possibility. The evidence gathered by Blyth & Blackith (1953) and a probable bedding attitude of 35° towards N. 60° E., measured by the writer at the point B on their sketch-map (p, 443), would agree with this interpretation. (c) Oblique Faults The strike-faulted fold model (Fig. 1 c) implies maximum shortening in the direction perpendicular to the axial surfaces of the fold. Theoretically, it is reasonable to suppose that further shortening in this direction could be accomplished by the development of two sets of steep oblique-slip oblique faults (Fig. 1 d). One set would therefore ideally show a combination of dextral and normal displacement, the other of sinistral and normal movement. In the Malvern Hills themselves there appears to be evidence of the development of the latter set of faults. Holl (1865, map facing p. 72) and Groom (1900, pl. VIII) show the presence of a number of north-westerly trending faults, mostly cutting the Silurian and Devonian strata, in the middle and northern parts of the Malvern range. Holl (1865, 95) recognised that they have 'given rise to considerable lateral displacement of the beds'. It is clear from the brief account given by Brammall (in Pocock, Brammall & Croft, 1940, 58-62) that members of this fault set are abundant within the Pre-Cambrian rocks. Blyth (1952) followed Brammall and stressed the prominence of these north-westerly 'tear faults' (p. 187, fig. 1). The most prominent of these faults is one which, crossing the railwaycutting about 220 feet south-west from the Colwall tunnel entrance, brings the Wenlock Shale against the Lower Old Red Sandstone (see Robertson, 1926, fig. 17). Phillips's diagram (1848, 136) is a graphic illustration of the sinistral sense of movement along this fault.

(d) The Age of the Malvern Structure The Malvern faulted fold is clearly younger than Lower Devonian in age, since the Lower Old Red Sandstone is known to be involved in the steep limb of this structure east of Colwall (Groom, 1900, pl. VIII). On the other hand it is older than the earliest of the flat-lying New Red Sandstone deposits. These, the Haffield Breccia (Groom, 1910, 725; Pocock & Whitehead, 1948, 72-3; Blackith, 1956), unconformably overlie and terminate the outcrop of the folded Palaeozoic strata at the southern end of the Malvern Hills (Phillips, 1848, pl. 1). Although the Pre-Cambrian ridge ends abruptly at North Malvern the

TECTONIC STRUCTURE OF THE MALVERN HILLS

113

adjacent Silurian strata are traced in a narrow outcrop roughly northwards for twelve miles into the Abberley Hills (Phillips, 1848, pl. 1). These hills are situated at the south-east corner of the Forest of Wyre coalfield (Kidston, Cantrill & Dixon, 1917, fig. 1). Coal Measures are found in two critical areas, on the western flank of Woodbury Hill and again half a mile to the west near Hillside Farm (see Groom, 1900, fig. 15a). Their relations to the adjacent and undoubted steeply dipping Silurian and Old Red Sandstone strata have aroused a considerable amount of discussion (Groom, 1900, 165-75, figs. 15a and 27; King, 1924, 12-13; Dinham, 1937, 46; Falcon, 1947; Mykura, 1951; Falcon, 1952; Wills, 1956,60, footnote and 93). A study of this literature shows that there can be little doubt that the Coal Measures near Hillside Farm, taken by Mykura (1951,389-90) to be of Morganian age, really are folded with the Silurian and Old Red Sandstone as Groom supposed. However, Groom (1900, 166-7) regarded the flat-lying Coal Measures on the nearby Woodbury Hill as unconformable upon steeply inclined Silurian strata and therefore concluded (p. 175) that the intense folding took place during Coal Measure times, before the deposition of these Woodbury Hill beds (see Groom, 1910, 724; Pocock & Whitehead, 1948, 72). It may be noted here that Moore & Trueman recognised and accurately dated an important intra-Westphalian unconformity within the South Wales and neighbouring coalfields and correlated this unconformity with the intense movements along the Abberley and Malvern 'axis' (see Moore, 1948, 291). This unconformity was named the 'Malvernian' (Trueman, 1947, xcviii-ix). It seems unlikely, however, that such intense folding and faulting movements could have taken place within Coal Measure times and have been expressed as a simple unconformity in such short distances. An examination of Mykura's section across Woodbury Hill (1951, fig. 2), which agrees with Groom's section (1900, fig. 27) in showing flat-lying Coal Measures resting unconformably upon steep Silurian strata, shows that it would be easy to fit the fold form of Fig. 1 b to the recorded bedding attitudes. The Woodbury Hill Coal Measures would then form a remnant of the upper flat-lying limb of the fold, brought into juxtaposition with the steep limb by means of a flat dip-slip strike fault (Fig. 1 c). In fact, the relations seem remarkably similar to those across Herefordshire Beacon. In support of this interpretation, it may be noted that King (1924, 12-13) evidently regarded the Coal Measures near Hillside Farm and those on Woodbury Hill as of similar age-and does, indeed, mention the presence of 'thrust planes' on Woodbury Hill. Further, Mykura (1951, 389, figs. 1 and 2) notes 'low angle thrusting' affecting the Silurian and Old Red Sandstone to the north of Woodbury Hill. His map shows a prominent steep dip-slip strike fault and a north-westerly fault which appears to show PROC. GEOL. ASSOC., VOL. 73, PART I, 1962

8

114

N. E.BUTCHER

a sinistral displacement. Thus all the main elements of the Malvern structure (Fig. I d) appear to be present in this area. The Trimpley fold occurs eight miles farther to the north (Kidston, Cantrill & Dixon, 1917, fig. 1; King, 1924, 7). King specifically states (p, 14) that 'the overfold at Trimpley was emphasised after the deposition of the Keele beds and of the Calcareous Sandstones of Enville for they partake in the folding'. Since these strata are now generally accepted as being of Stephanian age (Trueman, 1947, lxxxiii) it is clear that this Trimpley fold, and presumably also the Malvern fold, could not have been fully generated until at least the end of the Carboniferous period . This conclusion was reached by Phillips (1848, 135). (e) The Relation of the Trias to the Pre-Cambrian Perhaps the most spectacular feature of the Malvern Hills is the way in which the narrow Pre-Cambrian ridge, viewed from the east, rises like a wall from the Triassic plain. The fact that the eastern boundary of the Pre-Cambrian is parallel to the western boundary suggests that the two are genetically connected. Since the steep western bounding surface of the Pre-Cambrian is interpreted as one of unconformity it seems reasonable to regard the eastern surface as a major strike fault, developed essentially parallel to the steep limb of the Malvern fold . The following sequence of events may then be imagined to have taken place. Orogenic movements at the close of Carboniferous times produced the Malvern faulted fold which presented a picture of maximum shortening in the direction perpendicular to the axial surfaces of the fold (F ig. I). The steep dip-slip strike faults , downthrowing eastwards, which were initiated during these movements were therefore in a favourable attitude for posthumous movements to take place on them during the ensuing postorogenic period of horizontal crustal extension. In this way a major easterly dipping normal fault zone was developed in Permian times, cutting through the Pre-Cambrian core of the Malvern fold. Once developed, it is reasonable to suppose that the eastern block went on sink ing throughout Permo-Triassic times, receiving a vast thickness of sediments in the process. These sediments would therefore be deposited against the western block, the active fault line being the very western margin of the site of deposition (Fig. I e). This interpretation of the sequence of events is essentially the same as that envisaged by Phillips (1848). It is evident from this classic memoir that Phillips regarded the Malvern Hills as a faulted anticlinal structure of postCarboniferous and pre-New Red Sandstone age. This view was most graphically advanced by Strickland (1851, see pl. 1) in a paper which seems to have been completely overlooked by later investigators. Strickland's

TECTONIC STRUCTURE OF THE MALVERN HILLS 115 paper was known, however, to the Malvern geologist Bennett (1942) who followed a mixture of Strickland's and Groom's interpretations. The geological interpretation by Cook & Thirlaway (1955) of their geophysical results agrees with the sequence of events outlined above. This purely structural reasoning supports their conclusion (p. 56) that the Trias is more likely than the underlying Coal Measures to be very thick in the area east of the Malvern Hills. Falcon & Tarrant (1951, see pl. XI) had postulated a very thick Coal Measures sequence. However, because the Malvern structure is of post-Coal Measures age it would seem likely that the Coal Measures, if preserved under the New Red Sandstone, are of the same order of thickness as they were immediately west of the Malvern fold. Cook & Thirlaway (1955, 51) estimate the eastern surface of the PreCambrian to dip to the east at about 45°. (f) Late- and Post-Trias Movements It is to be expected that vertical movements took place on the faults of the Malvern structure at intervals throughout Mesozoic and Tertiary times. The second Colwall railway tunnel and the adjacent eastern cutting provided a continuous section through the Trias into the Pre-Cambrian. Robertson (1926, 162-3, pl. XI) figures and describes many small faults cutting the Keuper Marl which downthrow, and apparently also dip, westwards (Fig. 1 e). These appear to form a set of normal faults which are complementary to the easterly dipping surface of the Pre-Cambrian (see Cloos, 1931). Hence this surface, seen by Robertson (1926, 163) to dip at 45°, might well show post-Triassic fault movements. These minor faults cutting the Keuper Marl are probably present all along the foot of the Malvern Hills (see Hollingworth in discussion on Cook & Thirlaway, 1955,67). Holl's map (1865) shows that the Pre-Cambrian ridge is terminated at its northern end by a prominent north-westerly fault and at its southern end by a north-easterly fault. The outcrop pattern shown on the Old Series one-inch sheets of the Geological Survey, LV SE. and XLIII NE., suggests that these two faults originated as essentially a sinistral and a dextral oblique fault respectively, and that they were therefore presumably related to the Malvern fold. However, both now bring Pre-Cambrian against Trias, indicating considerable post-Trias movement, presumably of a dip-slip nature. It may be noted here that the outcrops of flat easterly dipping Coal Measures near Newent beyond the southern end of the Malvern Hills (Groom, 1910, 722-4) are probably of the upper limb of the Malvern fold. Presumed dip-slip post-Trias movements appear also to have taken place on one of the north-westerly faults bounding the Herefordshire

116

N.E .BUTCHER

Beacon-Broad Down Pre-Cambrian mass (Blyth, 1952, fig. 2). Farther south, the east-west dip fault through the Gullet, possibly initiated as a fracture perpendicular to the Malvern fold axis, is shown by Groom (1899, pl. XIII) to affect the Trias. It may be that the explanation of the patches of Lower Palaeozoic strata found between Midsummer and Hollybush Hills, and on Ragged Stone Hill (Groom, 1899, pl. XIII) is that these are slices let down into the PreCambrian by late north-south normal faults (see Blyth, 1952, 192).

4. THE REGIONAL STRUCTURE

The strongest argument for regarding the structure of the Malvern Hills as basically that of a single, large fold is perhaps that, as a result of this interpretation, the Malvern structure takes its place as just one of a regional set of roughly north-south folds affecting the Carboniferous and older rocks. This fold set can be readily identified, on the published maps of the Geological Survey, as extending at least from the Gloucestershire to the North Staffordshire coalfields and from the Warwickshire to the eastern edge of the South Wales coalfields. Clearly, the Malvern fold, rising majestically though broken above the Trias plain, is the most important member of this set of folds to be seen at the present level of erosion . Those north-south folds which are well developed at the present erosional level show a distinctive form in cross-section which Phillips (1848, 134) recognised . In his words, 'the western slope of these anticlinals is highly inclined, or even vertical, while the eastern slope is extremely moderate' and his section (p. 7) through Woolhope, Ledbury and the Malvern Hills illustrates this well. This monoclinal fold form , which is that suggested here for the Malvern Hills themselves (Fig. 1 b), also has another important, though obvious, property. The steep limbs of the folds are short in comparison with the great length of the flat-lying limbs. In completing the Malvern fold form in section it was thought that the steep limb could be of the order of 1000feet in length. Subsequently, it was found that the steep limb of the Ledbury fold, three miles to the west of the southern end of the Malvern Hills, is about 1000 feet long (see Blyth, 1952, fig. 3). Farther south, the Staple-edge fold in the Forest of Dean coalfield has a steep limb about 600 feet in length (Trotter, 1942, fig. 9). It is evident that these north-south folds have a similar form to those trending roughly east-west in the Mendip Hills and Somersetshire coalfield (compare Moore's fig. VII.3 with fig. VIII.8 in Trueman, 1954). Many geologists have held that these two fold-sets, perpendicular to each other, developed at slightly different times, the north-south folds being produced

TECTONIC STRUCTURE OF THE MALVERN HILLS 117

first (see, for example, Wells & Kirkaldy, 1948, 168; Squirrell & Tucker, 1960, 183-4). Groom (1900,179-82), however, argued that the movements were 'practically simultaneous' (p, 195). The east-west and north-south folds in the critical Bristol district involve Stephanian strata (Moore & Trueman, 1937, 197; 1939, pl. 4) and there appears to be no firm stratigraphical or structural evidence which would preclude both fold-sets being fully generated at essentially the same time, at the end of the Carboniferous period. The writer is convinced that all tectonic structures should be interpreted in terms of movement pictures. The movements of rock masses during deformation may be three-dimensional in nature and exceedingly complex. Fold-sets with differing trends may occur together, yet have been produced at the same time. These north-south folds and the east-west folds of the Mendip Hills and Somersetshire coalfield indicate a not very intense deformation, being generally large in scale and characterised by an apparent lack of cleavage. It is to be expected, therefore, that the one set would not be found with the other. In any case, the orientation of the folds is such as to make it unlikely that they would occur together, one fold set having southerly dipping and the other easterly dipping axial surfaces. The dominant tendency during the orogenic movements at the end of the Carboniferous period, as shown by the outcrops in south-west Britain, was, however, the production of roughly east-west trending folds. The reason for the development of these apparently anomalous north-south folds possibly lies mainly in the intra-Westphalian movements which resulted in a pronounced unconformity, with easterly overstep by later sediments, within the Coal Measures succession (see Moore & Blundell, 1952). Although now seen as monoclinal folds they must have been initiated as very broad flexures of low amplitude and presumably the trend of the fold axes would be determined during the earliest slight deformational movements, which may well have taken place in Westphalian times. This physiographic control, which indeed Moore & Blundell (1952, 471) advocate, is suggested by the general form of the depositional areas pictured by Wills (1951, pl. XI A) in Upper Westphalian and Stephanian times. Owen (1959,49-50) appears to argue along similar lines. In conclusion, it is worth emphasising the significance of the Malvern fold. It has been held (Blyth, 1952, 192; Whitworth, 1952) that the sharp folds in the Palaeozoic strata to the west of the Malvern Hills are due to fault movements in the Pre-Cambrian basement. In Wills's words (1956, 87), 'it is tempting to picture these sharp foldings as the superficial expression of deep-seated block-jostling'. It is evident that the Malvern fold, one of this regional fold-set, provides a spectacular example of the way in which, as the late Dr. G. M. Lees, F.R.S., held (1952), a crystalline basement can bend and form the core of an anticline.

118

N.E.BUTCHER

ACKNOWLEDGMENTS This piece of research arose from the author's teaching duties whilst employed by Reading University. Thanks are due to the management of the Gullet Quarry for permission to survey there. Mr. J. D. D. Smith kindly answered, on behalf of the Geological Survey and Museum, some queries concerning the second Colwall railway-tunnel section. Professor P. Allen, Professor H. L. Hawkins, Professor L. R. Moore, Professor J. Sutton, Miss P. S. Walder and Dr. W. R. Dearman kindly read and commented on the completed paper. Plate 5 is published by permission of Aerofilms and Aero Pictorial Limited and with the aid of a grant from Reading University Research Board. EXPLANATION OF PLATES PLATE 4 The north-west corner of the Gullet Quarry (32 (SO) 762381), July 1960. The Upper Llandovery May Hill Sandstone unconformably overlies Pre-Cambrian (Malvernian) metamorphic rocks. A. The view is along the strike of the Pre-Cambrian surface (approximate position indicated by broken line) which, together with the sediments, dips at 60° due west. B. The basal fossiliferous conglomerate. Compare with the drawing given by Phillips (1842,291) from a locality west of Worcestershire Beacon. PLATE 5 An aerial view northwards along the Malvern Hills. The Pre-Cambrian ridge is flanked to the west by Palaeozoic strata and descends eastwards to the Triassic Worcestershire plain. Worcestershire Beacon is in the distance; Herefordshire Beacon, offset to the west, is in the middle distance; the Gullet Quarry is in the foreground.

REFERENCES BENNETT, A. 1942. The Geology of Malvernia. Malvern. BLAcKrrH, R. E. 1956. The Haffield Breccias. Sci. J. R. Coll. Sci., 26, 77-85. BLYTH, F. G. H. 1952. Malvern Tectonics-a Contribution. Geol. Mag., 89, 185-94. - - - & R. E. BLACKITH. 1953. A Temporary Section in the Malverns. Geol. Mag., 90,442-3. CALLAWAY, C. 1879. On Plagioclinal Mountains. Geol. Mag. (2), 6, 216-21. Cwos, H. 1931. Zur Experimentellen Tektonik. Naturwissenschaften, 19, 242-7. COOK, A. H. & H. I. S. THIRLAWAY. 1955. The Geological Results of Measurements of Gravity in the Welsh Borders. Quart. J. geol. Soc. Lond., 111, 47-70. DINHAM, C. H. 1937. District Reports. (A): England and Wales: 2. West Midlands and Cambridge Districts. Summ. Progr. geol. Surv., Lond., 1936 (I), 45-8. FALCON, N. L. 1947. Major Clues in the Tectonic History of the Malverns. Geol. Mag., 84,229-40. - - - . 1952. The Age of the Malvern Folding. Geol. Mag., 89, 304. - - - & L. H. TARRANT. 1951. The Gravitational and Magnetic Exploration of Parts of the Mesozoic-covered Areas of South-Central England. Quart. J. geol. Soc. Lond., 106, 141-70.

TECTONIC STRUCTURE OF THE MALVERN HILLS 119 GROOM, T. T. 1899. The Geological Structure of the Southern Malverns, and of the Adjacent District to the West. Quart. J. geol. Soc. Lond., 55, 129-69. - - - . 1900. On the Geological Structure of Portions of the Malvern and Abberley Hills. Quart. J. geol. Soc. Lond., 56, 138-97. - - _ . 1910. The Malvern and Abberley Hills, and the Ledbury District. Geology in the Field (London), 698-738. HOLL, H. B. 1865. On the Geological Structure of the Malvern Hills and Adjacent Districts. Quart. J. geol. Soc. Lond., 21, 72-102. KIDSTON, R., CANTRILL, T. C. & E. E. L. DIXON. 1917. The Forest ofWyre and the Titterstone Clee Hill Coal Fields. Trans. roy. Soc. Edinb., 51, 999-1084. KING, W. W. 1924. The Unconformity Below the Trappoid (Permian?) Breccias. Trans. Worcs. Nat. Cl., 8,3-15. LEES, G. M. 1952. Foreland Folding. Quart. J. geol. Soc. Lond., 108, 1-34. MOORE, L. R. 1948. The Sequence and Structure of the Southern Portion of the East Crop of the South Wales Coalfield. Quart. J. geo!. Soc. Lond., 103, 261-300. - - - & C. R. K. BLUNDELL. 1952. Some Effects of the Malvernian Phase of Earthmovements in the South Wales Coalfield, a Comparison with other Coalfields in South Britain. Compte Rendu 3ieme Congres de Strat. et de Geol. du Carbonifere-s-Heerlen 1951, 463-73. - - - - & A. E. TRUEMAN. 1937. The Coal Measures of Bristol and Somerset. Quart. J. geol. Soc. Lond., 93, 195-240. ---- & . 1939. The Structure of the Bristol and Somerset Coalfields. Proc. Geo!. Ass., Lond., 50, 46-67. MYKURA, W. 1951. The Age of the Malvern Folding. Geol. Mag., 88,386-92. OWEN, T. R. 1959. The Armorican Earth-movements. 7th Inter-University Geol. Congress Swansea 1958, 46-50. PHILLIPS, J. 1842. On the Occurrence of Shells and Corals in a Conglomerate Bed, Adherent to the Face of the Trap Rocks of the Malvern Hills, and Full of Rounded and Angular Fragments of those Rocks. Phil. Mag.; 21, 288-93. ---.1848. The Malvern Hills, compared with the Palaeozoic Districts of Abberley, Woolhope, May Hill, Tortworth, and Usk. Mem. geol. Surv. U.K., 2 (1), 1-330. PLATT, J. I. 1925. The Pre-Cambrian Volcanic Rocks of the Malvern Inlier. Proc. geol. Soc. Lond., No. 1133, 63-5. POCOCK, R. W., A. BRAMMALL & W. N. CROFT. 1940. Easter Field Meeting, Hereford. Proc. Geol. Ass., Lond., 51, 52-62. - - - & T. H. WHITEHEAD. 1948. The Welsh Borderland. Second Edition. Brit. reg. Geol. RAW, F. 1952. Structure and Origin of the Malvern Hills. Proc. Geol. Ass., Lond., 63, 227-39. READING, H. G. 1960. Proc. geol. Soc. Lond., No. 1581, 113. - - - & A. B. POOLE. 1961. A Llandovery Shoreline from the Southern Malverns. Geo!. Mag., 98, 295-300. ROBERTSON, T. 1926. The Section of the New Railway Tunnel Through the Malvern Hills at Colwall. Summ. Progr, geol. Surv., Lond., 1925, 162-73. SQUIRRELL, H. C. & E. V. TUCKER. 1960. The Geology of the Woolhope Inlier (Herefordshire). Quart. J. geo/. Soc. Lond., 116, 139-85. STRICKLAND, H. E. 1851. On the Elevatory Forces which Raised the Malvern Hills. Phil. Mag. (4), 2, 358-65. SYMONDS, W. S. 1872. Records 0/ the Rocks. London. ---.1884. Old Stones. New Edition. London. - - - & A. LAMBERT. 1861. On the Sections of the Malvern and Ledbury Tunnels (Worcester and Hereford Railway), and the Intervening Line of Railroad. Quart. J. geol. Soc. Lond., 17, 152-60. TROTTER, F. M. 1942. Geology of the Forest of Dean Coal and Iron-ore Field. Mem. geol. Surv, U.K. TRUEMAN, A. E. 1947. Stratigraphical Problems in the Coalfields of Great Britain. Quart. J. geol. Soc. Lond., 103, lxv-civ, - - - . Editor. 1954. The Coalfields of Great Britain. London.

120

N .E.BUTCHER

WELLS, A. K. & J. F. KIRKALDY. 1948. Outline of Historical Geology . Second Edition. London. WHITTARD, W. F. 1952. A Geology of South Shropshire. Proc. Geol. Ass., Lond., 63, 143-97. WHITWORTH, T. 1952. Malvern Tectonics. Geol. Mag., 89, 384. WILLS, L. J. 1951. A Palaeographical Atlas of the British Isles and Adja cent Parts of Europe. London. - - - . 1956. Concealed Coalfields. London.

N . E. Butcher Geology Department The University Reading

DISCUSSION PROFESSOR MOORE in a written contribution mentions that Mr . Butcher had kindly sent him a copy of the paper and should welcome the opportunity of contributing to th e discussion. The author has reviewed the old er evidence and added new evidence of cons iderable importance which has a bearing upon the structural history of the Mal vern Hills. The writer is particularly interested in the Upper Palaeozoic history of this region and its possible relationships with Carboniferous basins south of the Malvern Hills. The author has drawn attention to the fact that there is a structural connection between the Malvern Hills and the eastern margin of these basins, a fact which is well established. The absence from the Malverns of Lower Carboniferous and of most of the Upper Carboniferous with the exception of presumed Upper Westphalian strata must be expla ined, particularly since the Westphalian strata are in juxtaposition by unconform ity with Silurian and Old Red Sandstone rocks . Similar conditions are paralleled in the basins to the south notably in the Forest of Dean and Bristol coalfields, where Upper Westphalian strata rest unconformably on horizons down to Upper Old Red Sandstone. Here , along the Lower Severn Axis some thousands of feet of sediments were eroded pr ior to Upper Westph alian depo sition. The movements which include the lat er intra-Coal Measures Malvernian phase were fold movements, e.g. Forest of Dean and Lower Severn Axis, and were instrumental in controlling the sub sequent form of the sediment ary basin. The presence of massive quartz conglomerates at the Drybrook, Trenchard and Pennant horizons suggest a local crystalline source and in conjunction with the fold movements appear to mark an important episode in the history of the Malverns , to which the author has made little reference. Concerning later Carboniferous movements of the Malverns themsel ves much appears to hinge on the age of the Coal Measures present and upon structural relat ionships which have been variously interpreted. Work in progress at Sheffield should establish the age of these measures and by so doing it is hoped ma y prov ide more conclusive evidence with regard to existing interpretations of their structural relat ion ships. The author indicates that the youngest Coal Measures in the basins to the south are Stephanian. In referring to these measures care has always been taken by previous authors to state the age as high Westphalian D or basal Stephanian-the difference is significant. Certainly these measures were involved in the folding of N .-S. bas ins. That of the Bristol and Somerset coalfield was formed prior to the onset of the later E.-W. Mendip folding. If stratigraphic sequence does not allow of such a distinction in age, the structural pattern of the Radstock Basin and its confines indicate deformation and modification of earlier structures by later . The total absence of any deposits in the southern basins from high Westphalian D or basal Stephanian to Trias (Keuper) indicates a prolonged period of uplift and erosion, a period which includes Intra Stephanian (Asturian) and Permian (Saalic) movements. I cannot agree with the author's premise that one period of movement was responsible for the ent ire structural complex including the Mendip front. The strong N.-S. trends, and pressure involving sharp folds and thrusting from the east , which from the author's conclusions intensify northward, are in my opinion a separate and earlier phase in the tectonic h istory ...

TECTONIC STRUCTURE OF THE MALVERN HILLS 121 wished to congratulate the author on a paper which brings together much interesting information on the structure of the Malverns. The new evidence which has enabled Mr. Butcher to restate an early hypothesis as to the origin of these structures will be awaited with interest. In the meantime could Mr. Butcher say any more on the way in which the crystalline rocks responded to the deformation he has discussed?

PROFESSOR SUTTON

regretted that he could not be present at the meeting to hear Mr. Butcher read his paper on the structure of the Malverns. In a closely argued review of previous literature Mr. Butcher had clearly demonstrated that, contrary to usual belief, the past is the key to the present. His new structural synthesis has particularly impressed me for three reasons. Firstly, a structural interpretation based on the fold-form control of continued deformation by faulting has a particular appeal as previously, both individually and later jointly, they had used the same mechanism to account for the structures along the north-west margin of Dartmoor. In the second place he had fitted an apparently anomalous structure into a well-known regional pattern. Thirdly, I have been looking at a monoclinal structure at Burnmouth on the Berwickshire coast in which Lower Palaeozoic greywackes and shales are overlain unconformably by Devonian and Carboniferous sediments. The work is still very incomplete, but, although the potentially steep plane of unconformity in the inverted limb of the monocline is not exposed, there is the great advantage here that the basement rocks have a well-defined structural pattern. It is hoped that it might be possible to demonstrate that this pattern has suffered a second deformation at the upper monoclinal hinge which is related to generative folding and not reversed faulting. I know from personal experience many years ago that the structures within the Malvernian are difficult, but I would like to ask Mr. Butcher if he has attempted a comparison, using statistical determinations of existing planar structures, of certain critical areas. These would appear to be, for example, the Herefordshire Beacon where he suggests that the Malvernian is 'flat-lying' and those other regions where the hills have a sharp, symmetrical ridge form possibly reflecting the steep attitude of the mass of rocks. May I, Sir, take this opportunity of congratulating Mr. Butcher on his most interesting paper. DR. DEARMAN

DR. F. G. H. BLYTH suggested that Mr. Butcher's theory did not carry the interpretation of Malvern tectonics very much farther, and may in fact be incompatible with the presence of the many faulted junctions between the Silurian and Pre-Cambrian rocks of the area. Without a detailed study of the internal structures in the Pre-Cambrian masses themselves, the overall structure can hardly be decided. But as far as the internal structures are known, they support the theory of upthrust Pre-Cambrian blocks along a line of dislocation. The writer would like to know what evidence Mr. Butcher has found in the PreCambrian to support his theory that the Malvernian gneisses themselves have suffered folding in late Carboniferous times. PROFESSOR HOLLINGWORTH commented on the significance of the Coal Measures section across the Abberley Hills, Hillside to Woodbury Hill. The area is one which he had mapped for the Geological Survey prior to Mykura's investigation. He differed from the published interpretations in that he would link up the sharply infolded synclinal tongue of Coal Measures directly with those beneath the Haffield Breccia of Woodbury Hill. The simplest structural interpretation is that the isoclinal folding of the Silurian rocks of Woodbury Hill is older than the Coal Measures there. No evidence to support thrusts has been produced. If it is assumed that the Abberley Fold Belt was sharply limited on the west, it could well be that the later post- Morganian folding produced the Hillside syncline to the west of Woodbury Hill in flat-lying Coal Measures and Old Red Sandstone, but was unable to modify the already isoclinically folded Silurian of Woodbury Hill and its cover of Upper Coal Measures.

122

N.E.BUTCHER

PROFESSOR P. ALLEN said the paper was an interesting reminder of the 'fashions' that inundated geological thought from time to time. The folds of Strickland and Phillips were virtually swept away by the faults of Callaway and Groom. Signs of vigour in structural interpretation were happily still with us, though nowadays usually on a more sophisticated level. The paper was also a reminder of how literature could be 'lost'. Strickland's far-seeing interpretation had not, so far as he (the speaker) was aware, been quoted by any subsequent author. Mr. Butcher's paper was undoubtedly valuable for its exhaustive review of the literature, his new ideas and his interpretation of the Malvern structure. Bibliographically and historically, the author had done much 'to put the record straight', particularly in respect of Phillips and Strickland. The new evidence of the steeply dipping unconformity in the Gullet Quarry was highly important, and the interpretation itself accommodated a number of hitherto intransigent facts as well as relating the gross structure to its regional setting. Above all, the paper pin-pointed which steps needed to be taken next: which hypotheses tackled, what ground-mapping and other work would best be embarked on. This would require several specialists, as the discussion had clearly shown. The author was to be congratulated on bringing an original and simplifying view to bear on a popular wicket. The Malverns were a difficult ground, and many a geologist had surely felt like the eloquent Australian cricketer who, on first seeing the beautiful Worcester 'cathedral pitch' nearby, exclaimed: 'But there's too much .....y grass!'

DR. A. J. SMITH complimented the author on the presentation of his paper, but expressed surprise that so much should be assumed about the structure of the Malvern area on the basis of two exposures of Mayhill (Llandoverian) sandstone. While Mr. Butcher has quite rightly stressed the importance of geological sections continued skywards, Dr. Smith suggested that a section continued westwards, say as far as and including the north-south Ledbury fault, would have been more to the point. Dr. Smith was sure that Mr. Butcher would agree that no account of the structure and origin of the Malvern Hills and hypothesis about the depth of the Pre-Cambrian (Malvernian) basement should neglect the structures displayed by the Silurian strata immediately to the east of Ledbury. These structures include quite tight symmetrical and asymmetrical (axial planes inclined to the west) plunging folds. Nor should any account disregard the steep dip of the Bronsil (Tremadocian) shales and the marked unconformity between that formation and the overlying Mayhill sandstones near Eastnor, especially if the author is postulating a near surface basement.

agreed with Professor Moore that he had made little reference to the intraCarboniferous unconformities. However, the paper was not intended to be an account of the entire structural history of the Malvern region. Rather, it was principally concerned with the nature of the orogenic structure which, the author felt, should be determined before a full discussion of the earlier events, to which Professor Moore referred, could be embarked on. With regard to the regional orogenic structure, the author was inclined to think, on the present evidence, that these north-south folds could have been generated at essentially the same time as the east-west folds to the south. He looked forward to more precise dating of the various critical Coal Measure outcrops. Professor Sutton's question could not yet be adequately answered. At the moment the author envisaged, on the evidence of abundant slickensided surfaces within the Pre-Cambrian, a process in which blocks of crystalline rocks moved past one another, the whole mass conforming to the fold form being produced in the overlying layered (sedimentary) rocks. Clearly, detailed work on the Pre-Cambrian rocks was necessary, a point which Dr. Dearman had also made. The author had made a preliminary examination of the attitudes of the dominant foliation in the Malvernian. Unfortunately, the critical Herefordshire Beacon area showed few good exposures. The preUpper Llandovery igneous intrusions found in the Cambrian sediments would also be expected, on the author's interpretation, among the Pre-Cambrian rocks. It was felt that a search for and a structural study of such intrusions would be a worth-while investigation. THE AUTHOR

TECTONIC STRUCTURE OF THE MALVERN HILLS 123 The author could not agree with Dr. Blyth that the fold interpretation may be incompatible with the many faulted junctions between the Silurian and the Pre-Cambrian. The study of orogenic structures showed that faults occurred with folds. It seemed to the author simpler to add faults to a fold than to fold fault blocks as required in Dr. Blyth's interpretation. The steeply dipping Silurian-Pre-Cambrian unconformity had to be explained. There was as yet no evidence derived from the Pre-Cambrian rocks themselves that directly supported the fold interpretation. Professor Hollingworth's interpretation of the critical Hillside Farm-Woodbury Hill section in the Abberley Hills appeared to demand two periods of orogenic movements, whereas the author preferred an interpretation needing only one. Dr. Smith had rightly drawn attention to the Ledbury structure and its relation to the Malvern structure. It was now necessary to try to determine whether or not the Pre-Cambrian basement was involved at depth in the Ledbury fold. The author hoped that detailed geophysical surveys would try to follow the surface of the Pre-Cambrian at depth westwards from its outcrop on the Malvern Hills. The author thanked Professor Allen for his kind remarks and agreed with him about the pretty grass. He was grateful to all who had contributed to the Discussion and for the critical reception given to the paper. The author had first visited the Malvern Hills in 1959, knowing nothing of the region and its literature. After seeing the new exposure in the Gullet Quarry he had arrived at the structural interpretation now presented. Subsequent search of the literature revealed that this interpretation was essentially the same as that advanced by the pioneer geologists Phillips, Strickland and Holl, It was quite true, therefore, as Dr. Smith had said, that the paper was based largely on two exposures-but these were the key exposures. No detailed mapping had been done and the paper was presented for critical examination with a due sense of humility and respect for all the previous work. The author was sure that future work would modify his interpretation and he was prepared, too, for its rejection if this became necessary. The main aim had been to focus attention on what was undoubtedly an important structure in the tectonic framework of Britain.