Great Oolite Group, Middle Jurassic, Bath, Bath—Cotswolds Province
|Green, G W. 1992. British regional geology: Bristol and Gloucester region (Third edition). (London: HMSO for the British Geological Survey.)|
The northern limit of the Dorset–Somerset province is transitional, with tongues of clay extending northwards for considerable distances into the Bath–Cotswold province. It is, however, convenient to take the Mendip ‘Axis’ in the Frome area as the dividing line between the provinces because important thickness changes affect the Fuller’s Earth here, even though the first major change to the shelf facies, the incoming of the Great Oolite, takes place a short distance to the north (P948985).
The influence, if any, of the Mendip ‘Axis’ on sedimentation in Bathonian times, together with the lateral equivalence of the Great Oolite to the strata farther south, has long been discussed by geologists. Following the primary six-inch survey of the area by the Geological Survey a series of cored boreholes were put down specifically to throw light on these problems. Additionally, a site investigation borehole drilled at Horsecombe Vale on the south side of Bath, in which the Fuller’s Earth succession has been divided into 24 distinct sedimentary units, has been proposed as the type section for the Fuller’s Earth (Penn et al, 1979).
The Lower Fuller’s Earth and the Fuller’s Earth Rock continue across the Mendip ‘Axis’ into the Bath area virtually unchanged. There is, however, good evidence for strong attenuation over the ‘Axis’ of the strata between the Fuller’s Earth Rock and the horizon of the Wattonensis Beds, though the correlation of the strata on either side of the ‘Axis’ is disputed, as explained below. The higher Bathonian strata appear to be unaffected by the structure. The current Survey view of correlation across the ‘Axis’ is given in P948998 and P948985.
Lower Fuller’s Earth and Fuller’s Earth Rock
The Lower Fuller’s Earth comprises 9 units, one of which (Unit 6 of Penn et al., 1979) has been defined as the Acuminata Bed, about 4 m below the base of the Fuller’s Earth Rock. This distinctive marker bed, although commonly less than 1.5 m in thickness, persists northwards from some distance south of the Mendips to near Cirencester. The Fuller’s Earth Rock comprises two units; the limestones of the lower one form the Milborne Beds, whilst the upper unit consists of the Ornithella Beds overlain by the Rugitela Beds. The type area of the Rugitela Beds is in the eastern Mendips (Sylvester Bradley and Hodson, 1957), where a recently described section gave the complete sequence (Torrens in Cope et al., 1980, pp.27–28). Here, the Ornithella Beds of the Fuller’s Earth Rock are succeeded by nearly 3 m of mainly rubbly marly limestone, then about 1 m of clay, and finally 0.6 m (seen) of clay interbedded with nodular limestone. The brachiopod and bivalve faunas in both the limestone and the clay with nodules horizon are identical to those of the Wattonensis Beds, and similarly lack ornithellids. The nodular limestone beds also contain an abundant, diverse ammonite fauna of hodsoni Zone age. Torrens thus referred all the exposed strata above the Ornithella Beds to the Rugitela Beds and regarded them as equivalent to the Wattonensis Beds of the basinal area to the south and to the Rugitela Beds of the Bath–Tormarton area to the north. Therefore, mudstones assigned by the British Geological Survey to the Frome Clay are regarded by Torrens as being equivalent to the Upper Fuller’s Earth. Penn and Wyatt (1979), however, suggest that the lower rubbly limestones be assigned to the Rugitela Beds and the upper nodular limestones to the Wattonensis Beds; the intervening thin clay then corresponds to the 0.5 m of clay, recorded by them as Upper Fuller’s Earth in the nearby Frome Borehole (P948985). They consider the similar faunas of the two limestones to be facies repetitions, not necessarily of the same age. This alternative interpretation has been substantiated by the results of the Winterborne Kingston Borehole to the south-east of the present region (Rhys et al., 1982).
Upper Fuller’s Earth
The Upper Fuller’s Earth, which consists mainly of calcareous mudstone with thin limestones in the upper part, ranges from 28 m in thickness at Bath to as little as 0.5 m at Frome to the south. It is divided into thirteen units; the lowest eleven of these units can be resolved into five sedimentary cycles, each of which represents an upward transition from shallow- to deeper-water sedimentation. The shallow-water phase is generally represented by pale, very calcareous silty mudstones or marls dominated by bottom-living forms such as rhynchonellids. These pass up gradually into dark grey to black smooth mudstones representing the deeper-water phase, in which a free-swimming fauna including pectinids becomes important. In boreholes between Bath and Frome the individual units and cycles can be shown to attenuate and overlap each other to rest on the underlying Fuller’s Earth Rock as the Mendip ‘Axis’ is approached.
The commercial Fuller’s Earth Bed occurs between Wellow and Bathampton Down. Its vertical distance below the top of the Upper Fuller’s Earth varies from 3 to 9 m due mainly to the effects of pre-Great Oolite erosion. It has yielded an ammonite fauna assigned to the upper part of the hodsoni Zone. The bed, about 2.5 m in thickness, has long been exploited on the south side of Bath in the vicinity of Combe Hay and Midford, mainly from underground levels driven into the hillside beneath the Great Oolite plateau. Working ceased in 1980. Over 80 per cent of the bed consists of a clay mineral belonging to the smectite group which swells up by absorbing excess water. It also has the property of adsorbing oil; hence the traditional use of fuller’s earth in the west country and elsewhere for cleaning the grease from wool and woollen cloth (fulling). Nowadays it has many other industrial uses. The discovery of glass shards and other pyroclastic material in the bed established its volcanic origin, but the source of the ash remains uncertain (Jeans et al., 1977).
The Frome Clay, as south of the Mendips, consists mainly of calcareous mudstones with a few limestone beds. The sequence passes northwards by lateral passage into the Great Oolite of the shelf facies (P948985). Three units have been recognised in borehole cores, each corresponding to a member of the Great Oolite. Persistent shelly limestone beds (‘smithi’ limestones) occur at the base of the lower two units, of which the one at the base of the formation is considered to be equivalent to the Wattonensis Beds at the base of the Frome Clay south of the Mendips. In a borehole at Frome this bed is separated from the underlying Rugitela Beds by only half a metre of mudstone. South of the ‘Axis’ this interval expands in conformity with the Fuller’s Earth basinal development (P948985).
Bath is the type area for the Great Oolite where it gives rise to the bold scarps and wide plateaux that form such an attractive feature of the district. It is exposed in many quarries (P209650) though few remain in work at the present day. The succession here, fully described by Green and Donovan (1969), is 20 to 30 m thick; in summary it is:
|[Forest Marble, Upper Rags]|
Cream coloured, cross-bedded oolite freestone.
Top surface planed and bored. Base locally sharp.
|5 to 8.5|
Buff, compact, fine-grained, shell-detrital limestones with
occasional ironshot pisoliths (Winsley facies), resting on
fossiliferous cream-coloured, pisolithic, rubbly marly
limestones (Freshford facies), with similar but strongly
ironshot beds at the base (Twinhoe Ironshot).
|2.5 to 8|
|Coombe Down Oolite
Cream-coloured, cross-bedded oolite freestone passing
down into shell detrital marly oolitic limestones. Top surface
strongly planed and bored, base often sharp and erosional.
|10 to 17.5|
The dominant facies of the Great Oolite comprises the massive oolites that have provided the celebrated Bath Stone. This was used by the Romans in the construction of the hot baths; it was also employed in many mediaeval buildings and is still used today. The attractive appearance of the City of Bath owes much to the consistent use of Bath Stone through the ages, more particularly in the 18th century, when the magnificent terraces and crescents were built.
There are three distinct freestone horizons in the Bath area. The lowest occurs in the top half of the Combe Down Oolite. This includes the stone quarried from and formerly mined on Combe Down and Odd Down around Bath itself, and farther east on Box Hill. It is said to be the best weathering variety and can be distinguished by scattered fine-grained shell debris and common calcite stringers (watermarks). The next above is in the Bath Oolite which has been extensively mined in the Westwood–Limpley Stoke–Monkton Farleigh area. It is very pure oolite and is softer than the lower freestone. Lastly, the Upper Rags (Forest Marble) have provided the freestone which has been used in Bradford-upon-Avon from Saxon times onwards. This contains numerous seams of shelly debris that show the cross-bedding to striking effect. During the 19th century the main focus of the Bath stone industry moved eastwards to the Box–Corsham area, following the discovery of thick freestones during the construction of the Box Railway Tunnel. The stone, mainly Bath Oolite, was extensively mined, access being gained by inclines driven through the Forest Marble. Though it hardens on exposure to the air, when newly dug it is readily cut by saws and is ideal for mouldings. The only working mines are at Westwood, Hayes Wood and Monk’s Park.
The Twinhoe Beds provide a distinctive datum within the otherwise dominantly oolitic sequence. They are level-bedded and the lower beds are much bioturbated and highly fossiliferous. They appear to have been deposited in deeper, quieter waters than the oolites. The Twinhoe Ironshot includes an important ammonite fauna ascribed to the aspidoides Zone. Between Box and Corsham the Twinhoe Beds pass laterally into the Bath Oolite, which is about 18 m thick in the Corsham area.
From its full development on the Twinhoe ridge the Great Oolite has completely disappeared within little more than one kilometre to the south. Mapping and borehole evidence show that the Upper Rags pass into argillaceous Forest Marble facies, while the beds below pass into the Frome Clay (Penn and Wyatt, 1979).
- Penn,I E, and Wyatt, R J. 1979. The stratigraphy and correlation of the Bathonian strata in the Bath-Frome area. Report of the Institute of Geological Sciences, No. 78/22, 23–88.
- Sylvester-Bradley, P C, and Hodson, F. 1957. The Fuller’s Earth of Whatley, Somerset. Geological Magazine, Vol. 94, 312–322.
- Cope, J C W, Duff, K L, Parsons, C F, Torrens, H S, Wimbledon, W A, and Wright, J K. 1980. A correlation of Jurassic rocks in the British Isles. Part 2: Middle and Upper Jurassic. Special Report of the Geological Society of London, No. 15.
- Rhys, G H, Lott, G K, and Calver, M A (editors). 1982. The Winterborne Kingston borehole, Dorset, England. Report of the Institute of Geological Sciences, No. 81/3.
- Jeans, C V, Merriman, R J, and Mitchell, J G. 1977. Origin of Middle Jurassic and Lower Cretaceous fuller’s earth in England. Clay Mineralogy, Vol. 12, 11–44.
- Green G W, and Donovan, D T. 1969. The Great Oolite of the Bath area. Bulletin of the Geological Survey of Great Britain, No. 30, 1–63.