_jl I I_ CHAPTER 11 REGIONAL GEOLOGY by C. Roger Bristow and Bernard C. Wo rssam The geology ofsouth-west England is indicated in oudine STONE TYPES USED FOR TIIE SCULPTURES in Fig. 9 below;the succession ofstrata isshown in Table 1. In the south-west of the district is the high ground The various types of stone used for sculptures are fo rmed by theCarboniferous Dartmoor Granite intruded described in stratigraphical order. Geological fo rmations into older Carboniferous deposits. may contain other beds than those of building (or The Jurassic limestones, which provide most of the sculptural) stone quality: for example,not allthe Chalfield stones used in Anglo-Saxonsculptures, fo rm prominent, Oolite Formation is made up ofBath stone. mostly west-facing, escarpments.The Inferior Oolite is Bothsandstones and limestones were used, and most in a persistent unit that extends in an approximate north­ each category were arenites, i.e. having sand-sized south line across the whole district. Inthe northern part particles.Arenites lie within the total size-range 0.06- of the district are the Cotswold Hills underlain by the 2 mm diameter. Fine-grained arenites have particles up limestones of the Great Oolite Group. These limestones to 0.2 mm diameter, medium-grained 0.2--0.6 mm, and only extend to just south of Bath, and are replaced coarse-grained from 0.6-2 mm. For identification, the southwards by clays. The outcrops of the Corallian and stone of each sculpture was examined by hand lens wit h Porcland groups are intermittent as they are overlain, in a built-in measuring grid, allowing the diameter of the part, by the unconformable CretaceousGault and Upper grains above 0.1 mm to be measured withconfidence. Greensand fo rmations. The main type of stone used was Bath stone from the The Upper Greensand gives rise to a prominent Chalfield Oolite Formation,which waswidely exploited escarpment in the southernpart ofthe districtand forms in the Bath area. In addition, some ten types of stone the high ground of the Black.down and Haldon Hills, from localities in, and one (Taynton stone) from just respectively north-east and south-west of Exeter. The outside, south-west England were used. A fe w carvings Chalk, which hasa wide outcrop inthe east of the district, are ofstone of uncertain provenance. See Fig. 8. gives rise to tracts ofhigh ground with a generally open aspect. DARTMOOR GRANITB by R. C. Scrivener The Palaeogene sands and clays occur in a downfold in the Cretaceous strata in the south-east of the districtand Dartmoor is the largest of the granite bosses that trend occupy the western part of the Hampshire Basin. west-south-west from Exeter to Land's End and covers In the centre of the district are the alluvial and peaty an area of some 250 square kilometres. Characteristically, deposits fo rming the low-lying Somerset Levels. the granite gives rise to rugged tors overlookingareas of Table 1 shows the relation between chrono­ moorland or rough pasture. Ingeneral, the granite consists stratigraphical divisions based on geological age, and of quartz, perthitic orthoclase crystals up to 17 cm long, lithostratigraphical divisions (as in Fig. 9), represented in plagiodase and dark brown mica (biotite). Common south-west England. accessoryminerals are tourmaline, zircon and apatite. 13 11 11- _j l IL 14 CHAPTER I! 0 50km I p Pg ss su sx sz Alluvium and peat Fuller's Earth to Oxford Clay Palaeogene strata Bradford stone and Great Oolite limestones (including Bath stone) Chalk Fuller's Earth (north of Bath) Upper Greensand and Gault Inferior Oolite (including Dundry and Doulting stone) Portland, Purbeck and We alden groups Lias Group (including Ham Hill stone) Kimmeridge Clay Palaeozoic strata Corallian limestones (including Osmington Oolite and To dber Freestone) FIGURE 9 Geology of south-west England 11 11 _jl I I_ REGIONAL GEOLOGY 15 Chalk Group Boyne Hollow Chert Upper Greensand 0� SbiftaburySancutone tl Formation � Cann Sand � Gault u Lower Greensand We alden Group Pllltleck Made Purbeck Group Stoncl l'ordlad,_ I"' - a Wocldey Member 1, PortlandChert Portland Group -TiiliiiiY.MmaJo.. _ Ji':] Portland Sand Kimmeridge Clay OamiDpm Oolite Corallian Group u 1bclber fleeltxlae ."'.... "' l Oxford Clay & Kella�Fonnation Cornbrash Forest Marble <""' IIIDDe•AIIdilFOalile Great Cbalfield Bath Oolite Oolite Oolite Frome Formation Tw inhoe Beds Group Clay (Bathstone) Combe Down Oolite Tavnt ou Limestone Fm= Lowerer's Full Earth DoultiDg/_.,. stone Inferior Oolite Ham stone Hill - Lias Group BlueLW TABLE 1 Gc:nerilised vertical section (not toscale) ofthe Mesozoic stnta insouth-west England. Stone used for Anglo-Su:on sculptures highlighted 11 11- _jl I I_ 16 CHAPTERII In general, the granite can be divided into an upper (outer) one of'giant' granite which fo rms most of the tors and is characterised by large rectangular fe ldspar crystals, and a lower (inner) one of'blue' that contains fe wer and smaller fe ldspar megacrysts. Muscovite (white mica) occurs as a secondary mineral resulting from post­ crystallisation pneumatolytic or hydrothermal activity, and its presence commonly renders the granite more amenable to being carved or shaped (cf. the Copplestone 1 and Exeter 1 crosses). The typical granite of most of northern Dartmoor, the nearest part of the outcrop fo r the fo ur granite carvings in the present area, isa coarse biotite granite with variable abundance ofK-feldspar megacrysts and does not appear to be the source of the stone for these carvings. The closest match for Copplestone 1 would be the granite of the Merrivale area near Princetown, where coarse­ grained biotite-muscovite granite was worked at Merrivale Quarry until recendy. The stone ofExeter 1 could be a 'contaminated' granite of the type fo rmerly StrauYQUDger thm worked at Swcltor in southern Dartmoor. The Plymstock Ham HillStoue 1 cross is typical of granite on the south side of the Ham HillStoue § Str.ata older thau batholith. The very fresh and hard biotite granite with HamHill Stone sparse K-feldspar megacrysts ofthe Chulmleigh 2 stone (Appendix B) is verysimilar tothat quarried in the Haytor ,) quarry 'blue' granite quarries in the nineteenth century, and extensively used fo r building and facing stone, also by monumental masons. Because of its relative intractabilty FIGURE 10 to the techniques used by the early masons, Chulmleigh Outc:rop ofthe Ham Hill Stone 2 may be a late fa ke. LlAS GROUP,BLUE LlAS FORMATION LlAS GROUP, BRIDPORT SANDFORMATION, HAM HILL STONE MEMBER. The Blue Lias Formation consists dominandy of an alternating sequence of hard, bluish grey, fine-grained, The Ham Hill stone, named from Hamdon Hill, south sparsely shelly limestones up to 0.25 m thick, alternating of Stoke-sub-Hamdon, Somerset, is an orange-brown, with grey mudstones of similar thickness. The fo rmation, shelly, strongly cross-bedded (not obvious in small blocks) which is thick (up to 144m) and widespread in Somerset, limestone developed at the top of the Bridport Sand thins over the Mendips and is only about 5 m thick Formation over a limited area in southern Somerset north of the Mendips in Gloucestershire. (Fig. 1 0). Because of superficial lithological similarity, the In Somerset, the Blue Lias has been widely, and still is Ham Hill stone has sometimes been included as part of locally, quarried as a building and walling stone, but, the Inferior Oolite. However, in lithological detail and because of its hardness, hasrarely been used fo r sculptures. age (as determined by ammonites) it can be distinguished It has also been widely used for gravestones, but is not fro m the Inferior Oolite. The stone on Chiselborough particularly long-lasting as weathering causes it to split Hill about 2 km south of Hamdon Hill has also been and flake. called North Perrott stone (Green 1998). Although the One carved stone (Muchelney 4) from Muchelney Ham Hill stone is up to 27 m thick, only about 15 m Abbey (much of which isbuilt ofBlue Lias limestone) is of it is suitable fo r working as ahigh-<:lass freestone. The a pale grey, sparsely shelly micrite and resembles the stone has been worked from Roman times (Wilson et al. limestones of the Blue Lias, except that it is fa irly soft 1958, 63) and can be fo und in buildings over a wide area (it easily marks with a soft wire brush) and presumably is (Thomas 1990, 297; Robinson 1995, 163-4);Montacute fa irly easy to carve. House and Sherborne Abbey are good examples. 11 11- _jl I I_ REGIONAL GEOLOGY 17 It continues to be worked as a building stone and stone INFERIOR OOLITB GROUP, UPPER INFERIOR OOLITB fo r renovation at both the northern and southern end of 'FORMATION', DOULTING STONE MEMBER Hamdon Hill. The Inferior Oolite Group is a widespread and persistent Sculptured Ham Hill stone has only been ide tified at ": limestone unit across the present district. North of the two localities (Muchelney, Somerset and Stmsford, Mendips, the Inferior Oolite is up to 100 m thick and Dorset) . At Muchelney,two (Muchelney 2 and 3) of the can be divided into mappable fo rmations. From the fo ur sculptured stones are of Ham Hill stone. One Mendips southwards to the Dorset coast, the Inferior (Muchelney 2) is a very pale orange (10YR 8/2), very Oolite is generally a thinner (1-25 m), but nevertheless shelly limestone. Most shells {bivalves) are elongate and persistent, unit. Within this latter tract, however, �e vary from 0.5 to 6.0 mm across and vary from sub­ Inferior Oolite cannot be subdivided into mappable un1ts angular to sub-rounded and are clast supported. A crude and is regarded as a fo rmation (see Bristow et al.