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Geological notes and local details for 1:10 000 sheet: SP 61 SE (Brill). Part of 1:50 000 Sheet 237 (Thame). British Geological Survey Onshore Geology Series Technical Report WA/88/45

By A J M Barron

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Bibliographical reference: Barron, A J M. 1988 Geological notes and local details for 1:10 000 sheet: SP 61 SE (Brill) British Geological Survey Technical Report WA/88/45

British Geological Survey Onshore Geology Series Technical Report WA/88/45

British Geological Survey Natural Environment Research Council

©NERC copyright 1988

Keyworth, Nottingham, British Geological Survey 1988

Authors: A J M Barron British Geological Survey Keyworth Nottingham NG12 5EE

Introduction

This report describes the geology of the 1:10 000 Sheet SP 61 SE covering the district between the villages of Brill and Chilton in north-western Buckinghamshire. The district was included in Old Series Sheet 45 SE, published in 1863 and revised in 1867. Parts of Brill Hill were resurveyed at the 1:10 560 scale by G.W. Lamplugh in 1904 and T.I. Pocock in 1905. The southern part of the hill and the area west of grid line 658 were surveyed by H.B. Woodward in the same year and this area was included in the 1:63 360 Oxford Special Sheet published in 1908. Notes on the railway cuttings were made by G. Barrow and H.B. Woodward.

The present survey was carried out by A.J.M. Barron in 1986. The survey and the map preparation were funded by the Thames Water Authority. B.M. Cox identified fossils collected in the field.

The Brill No.1 Borehole was drilled between the 18th of September and the 4th of November 1986 by Geotechnical Engineering of Gloucester and was logged by K. Ambrose, A. Horton and A.J.M. Barron. Micro-palaeontological determinations of the strata down to the Purbeck Beds from this and other boreholes were made by I.P. Wilkinson and macro-palaeontological determinations of the material from the Purbeck Beds down were made by B.M. Cox.

Shallow boreholes were drilled in April 1986 by A.J. Dixon of the Institute of Hydrology.

Brill lies in the north-west corner of the district, 16 km west of Aylesbury and 15 km north-east of Oxford. The village is on the summit of a hill which rises to 187 m OD. The village of Chilton and the hamlet of Easington in the south-east also stand on an outlying hill which rises to 142 m OD. The small village of Dorton lies on lower ground in the north-east. In the extreme east, the land rises to another hill the bulk of which is outside the district. The ground falls away in the south-west towards the River Thame and in the north-east towards the River Ray. There are many scattered farms and the area is mainly arable with some grazing especially on the steeper slopes, and there are a few woods and copses. The formations to be described are all very poorly exposed.

Reports covering adjacent sheets are:

SP 61 NW Arncott (K. Ambrose)

SP 61 NE Ludgershall (M.G. Sumbler)

SP 71 NW Waddesdon (M.G. Sumbler)

SP 61 SW Oakley (A. Horton)

SP 71 SW Chearsley (A.J.M. Barron)

SP 60 NE Long Crendon (K. Ambrose)

Geological sequence

The geological sequence is as follows:

Solid formations

Jurassic

Oxford Clay Formation

Upper Oxford Clay Member

Only the highest 5.61m of the Upper Oxford Clay were proved in the Brill No.1 Borehole in which it consists of pale to mid grey silty calcareous mudstone with thin siltstone beds, pyritic fossils and burrows.

West Walton Formation

The West Walton Formation is 15.3 m thick in the Brill No.1 Borehole and comprises alternating dark, mid and pale grey calcareous mudstone with much shell debris, some pyritised, thin pale grey siltstone and argillaceous limestone beds, interburrowed horizons and many non- sequences, but only the uppermost thirteen metres crop out in the district. The outcrops are on the sides of shallow valleys in the north-east and the extreme south-west, where the strata produce a pale grey brown, slightly silty clay soil.

In his account of an excursion to Dorton, Brill and Arngrove, Davies (1907b, p.184) refers to the Dorton railway cutting [SP 678 148] as exposing 'inliers of Oxford Clay in the midst of the Corallian outcrops'. It seems likely that it was the beds of the West Walton Formation which were exposed, for they were not distinguished from the Oxford Clay at that time. No evidence of the inlier recorded by Davies under the Dorton road bridge was seen during the present survey; the West Walton Formation crops out a little to the south and could easily have been exposed in an excavation for the bridge.

The Formation was also penetrated under the alluvium in two gas pipe-line boreholes (SP61SE/1B) and (SP61SE/2B) in which up to 2.00 m of pale greenish grey and pale and dark grey silty laminated clay was proved.

Cumnor Formation

Arngrove Spiculite Member

The Arngrove Spiculite Member (previously the Arngrove Stone of Davies (1907a)) was proved only in the Brill No.1 Borehole in which it consists of 1.64 m of muddy grey sandstone, siltstone and silty mudstone with sponge spicules and shell debris, probably a marginal development of the Member seen in the type area. No lithological evidence for it was seen at the outcrop of the West Walton Formation/ Oakley Member boundary although in a few places east of Dorton [SP 687 145], [SP 694 146], and [SP 693 144] the boundary is marked by a weak convex feature. The Member is thought to die out beneath Brill Hill. Davies (1907a) gives a detailed description of its field characteristics.

Oakley Member

The Oakley Member, previously the Oakley Clay (Buckman, 1927, p.49) or Oakley Beds (Arkell, 1933, p.409) crops out over large tracts of the low-lying ground in the north-east and south-west. In the Brill No.1 Borehole it consists of 4.20 m of alternating beds of pale grey siltstone or silty nodular limestone and grey marl and mudstone, bioturbated and rich in the small bivalve Nanogyra. The limestones weather away and are not seen at the surface, where the strata produce a dark brown silty clay soil with abundant Nanogyra. The thickness of the Member is thought to vary between three and five metres in this area.

In his description of the Dorton railway cutting, Davies (1907b, p.184) notes 'clay varying in colour from light grey to yellowish- white, with two bands of impure limestone' which he ascribes to the Ampthill Clay. These beds are now referred to the Oakley Member which was not separated from the Ampthill Clay at that time. The bivalve Nanogyra nana (then known as Exogyra nana) was recorded as extremely abundant. In a footnote to the report (p.185), Davies records 'E.nana beds' at the southern end of the cutting and also mentions 'Unexpected complexities of structure' (see Structure p.11, and Sumbler, 1988). Specimens collected by Davies and now in the B.G.S. collection (Zn 2483–2497) include Nanogyra nana, Lopha and Perisphinctes sp. Davies describes a railway cutting section in the B.G.S. fossil register as 5.15 m of light grey and brown clay with brown ooids, on thin white marl beds and a 0.2 m limestone bed, on 1.5 m of stiff black to grey clay with large Gryphaea dilatata. This section, which comprises the Oakley Member on the West Walton Formation, is probably located at Ashendon Junction [SP 692 138]. A specimen collected by Davies from the Oakley Member at Ashendon Junction (ZK 3953) comprises forty-four loose valves and clusters of Nanogyra nana, some encrusted with serpulids and foraminifera. H.B. Woodward, in unpublished field notes, recorded 'Exogyra band' at Ashendon Junction [SP 693 135] and about 800 m south-east along the railway line [SP 696 128] indicating the presence of the Oakley Member.

Ampthill Clay Formation

The Ampthill Clay Formation crops out over a large area of the district on the lower ground and on the lower slopes of the hills. In the Brill No.1 Borehole it consists of 20.25 m of mid to dark grey mudstone with softer, paler layers and thin marl and siltstone beds, fossiliferous throughout. Two thin cementstone beds occur within 2 m of the top of the Formation. The thickness at outcrop is difficult to estimate because of cambering and valley bulging (see Superficial Structures) and the uncertainty of the boundary with the overlying Kimmeridge Clay, but it is thought to vary between 15 and 23 m. The strata produce a heavy grey brown clay soil, silty in places, with rare oyster shell debris, including Deltoideum delta (see (Appendix 4), [SP 6501 1255]). Below about 0.3 m from the surface, the deposit is smooth grey clay. Several weak convex features were traced across the Ampthill Clay outcrop. These may be formed by the cementstones or harder mudstone beds. At the boundary with the overlying Kimmeridge Clay Formation, pale grey cementstone and shell debris is common at the surface; for instance in a stream bed 350 m south-east of Chiltonpark Farm [SP 6711 1282], 600 m south of Leatherslade Farm [SP 6526 1184], 150 m south of Buttermilk Hall [SP 6657 1192], around Grove Spinney [SP 672 119] and 900 m south-west of Chilton [SP 6810 1095].

Kimmeridge Clay Formation

The Kimmeridge Clay Formation underlies the middle slopes of the larger hills and forms two small outliers. It consists of mid to dark grey mudstones with very fine-grained sand and silt beds in the upper part and thin impersistent limestones in the lower part. It was 55.6 m thick in the Brill No.1 Borehole, including about 9.69 m of very fine-grained sand and silt at the top, 1.35 m of fine-grained sand and sandy limestone from 43.76 m and 1.57 and 0.74 m of very fine-grained sand from 51.12 m and 53.30 m respectively; the last bed was found only in the Borehole, and only three sand, beds were traced at outcrop. Between 30 and 56 m of the Formation are thought to be present in this district but the thickness has apparently been substantially reduced in places by cambering.

At outcrop the clay beds weather to a smooth grey brown clay soil; a little grey fine-grained limestone debris may be found in the lower part, for instance by Spa Wood [SP 6688 1363], south of Buttermilk Hall [SP 6655 1195] and on the north-east side of Dorton Hill [SP 6830 1327] and [SP 6856 1284]. Specimens collected from near Chilton (Appendix 4), [SP 6810 1093] and [SP 6792 1016] contain Nannocardioceras, Aspidoceras and Nanogyra virgula, indicating the Lower Kimmeridge Clay. In the upper part, the fine to medium-grained sand makes a lighter loamy soil. The lowest outcropping sand bed was traced only around the middle slopes of Rid's Hill [SP 664 149]. The middle bed varies from 0 to 10 m thick and occurs only in the north- west around Brill Hill, where it is impersistent; it is seen to die out, for instance, around Chiltonpark Farm [SP 668 130] and north of Leatherslade Farm [SP 655 126]. It was present in the Leap Hill No.2 Borehole ((SP61SE/15) [SP 6667 1237]). At the base of the bed, and at its horizon where absent, small black pebbles or 'lydites' are common. These are thought to be abraded relics of phosphatised ammonite body chambers derived from the underlying clay formations and were seen north of Chiltonpark Farm [SP 6669 1333], east of Parkpale Farm [SP 6637 1304] and north-east of Leatherslade Farm [SP 6567 1261].

The upper sandy unit (Wheatley Sand of Arkell (1947)) is continuous along its outcrop except in the extreme south-east [SP 696 103] where it is absent over about 500 m. It was formerly exposed in the Brill Brickyard probably [SP 6501 1422] beneath the Portland Beds and was described by Arkell (1947, p.110) as 'three feet of brown and greenish marly sand'. In the Brill No.1 Borehole it consists of very fine-grained grey sand and grey sandy silt beds with common lydite pebbles in the top 0.5 m, and is shell debris-rich and finely bioturbated throughout. Both main sand beds are sufficiently resistant to form outliers and spurs such as those at Chiltonpark Farm [SP 668 130], Leatherslade Farm [SP 6535 1240], east of the railway [SP 699 134] and at Rid's Hill [SP 664 149], the summit of which was previously mapped as Portland Sand by the Survey. Davies (1907a, p.31) found no lydite pebbles there but did not suggest that it might not be Portland. Elsewhere the weathering products of the upper sand bed and overlying Portland Sand can be difficult to distinguish. Springs may arise at the base of either (see Water Supply).

The Brill Serpulite Bed first described by Davies (1907a) from the bottom of the Rid's Hill Brickpit, 150 m off this district to the north [SP 666 152] (Sumbler, 1988), is thought to lie at or near the base of the Kimmeridge Clay (Pringle, 1926, p.63, Arkell, 1933, p.462) but no direct evidence of it was seen in this district. Cementstone and shell debris, mentioned earlier (see Ampthill Clay Formation) as occurring near the base, may be material from the Bed.

Portland Beds

The outcrop of the Portland Beds between Oxford and Aylesbury is one of the largest in Britain and has received much attention in the literature. Fitton (1836) see (Appendix 3) described many sections in the area, Brodie (1857) recorded an unlocated section at Brill and Blake (1880) described the local sequence in his account of the Portland Beds of England and listed fossils found on an excursion to Brill (1893). Davies (1899) reported on the Portland Beds in his account of the geology of the area and Buckman (1922 and 1927), who lived in the area for the later part of his life and examined and collected from many localities, provided much valuable data. Arkell (1933) knew the area well and drew together much of the knowledge of the Jurassic and, more recently, Ballance (1963) elucidated the stratigraphy of the Thame-Aylesbury area. Neaverson (1925) studied the petrography of the Portland Sand from several localities in the vicinity but not from the subject area.

In this district, except where extensively cambered, the Portland Beds do not form a very broad outcrop, but they have a profound influence on the topography. They form the crest of the steeper slopes of the main hills by virtue of the resistance to weathering of the limestone beds. Previously, there were many small pits in the Beds, but these are now almost entirely overgrown or backfilled and exposure is very poor. The lowermost beds were formely exposed in the Brill Brickyard probably [SP 6502 1422] where Arkell (1947, p.110) described them as seven-and-a-half feet (2.3 m) of 'shelly, chalky, earthy, oolitic and marly beds' on 7 to 8 feet (2.1 to 2.4 m) of 'glauconitic, sandy and marly beds and limestone' overlying 1 foot (0.3 m) of 'Lydite Bed: brown clay with pebbles'.

It has not been possible so far to relate the sequence seen in this area to that of Dorset and so no formation names have been used. No division on the map beyond 'Portland Sand' and 'Portland Stone' has been attempted, and these are not shown where there is severe cambering (see Superficial Structure) or lack of surface evidence in areas of pasture.

The Portland Beds consist of between two and six metres of inter-bedded limestone, marl and sand (Portland Stone) on between three and six metres of fossiliferous sand, cemented in parts (Portland Sand). The thickness at outcrop is reduced in places to as little as two metres by the erosive base of the Whitchurch Sand.

In the Brill No.1 Borehole the recovery from the Beds was not good. Of the interval 16.5 to 24.0 which the Beds are thought to have occupied, only 3.32 m of core was obtained, which consisted of brown, orange-brown and greenish brown, fine-grained, glauconitic sandstone, grey sandy and shell detrital limestone and grey marl. The Brill No.3 Borehole terminated in the Portland Beds, proving 0.03 m of brown sandy clay on 0.19 m of khaki fine to medium-grained sand above 0.15 m of khaki, shelly, marly sand and marl (see Appendix 1).

At the surface, the Portland Sand weathers to a brown to greenish brown, finely sandy clay soil, with limestone rubble in places, the product of the greenish grey glauconitic sand and sandy limestone which was seen in several sections around Chilton ((Appendix 2), localities E, F, K, N and P) at as little as 0.3 m from the surface. Weathering of the basal 'lydite' bed leaves scattered black pebbles in the soil in places. In an excavation for a garage in Easington (locality N), 1.20 m of Portland Sand were exposed very near the base of the Beds. Bivalves and very large ammonites are common in these strata (see Appendix 4).

A variety of lithologies was seen on the outcrop of the Portland Stone, the most common being a cream, fine-grained, semi-porcellanous bored limestone, a shell-detrital limestone and a marly oolite. All these were observed in brash in a field [SP 6911 1051] north-east of Easington. In a section in an old stone pit [SP 6512 1333] south-west of Brill, 0.5 m of cream, shelly, rubbly oolite is exposed. Debris of the semi-porcellanous limestone is very common in fields near Ryman's Farm [SP 6600 1244], south of Chilton [SP 690 110] and 1200 m east of Easington [SP 699 101].

A large number of springs arise within and at the base of the Beds (see Water Supply).

Purbeck Beds

The beds lying between the top of the Portland Stone and the base of the Whitchurch Sand over much of this district are called here the Purbeck Beds, although there is some evidence that the Whitchurch Sand is also of Purbeck age.

The Purbeck Beds underlie a considerable part of the slopes of Brill Hill and smaller parts of hill slopes around Chilton and to the east of the railway. Elsewhere they are absent due to non-deposition or the unconformity at the base of the Whitchurch Sand.

The Purbeck Beds are now very poorly exposed. Sections in this area were first described by Fitton (1836) (see (Appendix 3) of this report) but the Beds on Brill Hill were first identified by Brodie (1867) when they were a great deal better exposed, notably on the north side of the hill, where they were worked for brick clay and stone. Brodie noted the non-marine character of the fauna of the limestone. A generalised section in the pits is given by Phillips (1871, 418). The section described by Fitton ((Appendix 3), locality A) of the beds on the north-west side of the hill appears to be slipped, but his locality B on the west side proved a total of about 7.85 m of grey clay and ochre now tentatively assigned to the Purbeck, and his locality C showed c2.83 m of clay and ochre.

Pringle (1926, 85–86) gives a section described by G.W. Lamplugh in 1904 as 'tough bright green clay with rusty spots...earthy fragments and ironstone concretions...6 to 9 inches, on brecciated white stone and marl with broken lumps of hard calcareous grit...Portland limestone below... seen for 3 feet'. Pringle estimates the thickness of the beds on Brill Hill to be not more than 'five or six feet...may have been locally thicker' and both he and Blake (1893, 73) note that the sections are obscured by landslipping. The present map of the area shows an apparent thickness at outcrop of up to 23 m of Purbeck Beds; it is not suggested that the actual thickness is anywhere near this, but it is substantially thicker (up to 8 m) than Pringle's estimate. The very extensive cambering and dip-and-faulting of the beds, especially on the north and west sides of Brill Hill (see Superficial Structures), has lowered the top of the Portland Beds by as much as 20 m. The area of Brill Common is shown as Purbeck Beds outcrop but it is thought that there may be many places where Portland rocks may be found on it in small slipped and tilted blocks.

In the Brill No.1 Borehole ((Appendix 1), (SP61SE/10)) the Purbeck Beds are thought to be represented by the interval c14.7 to c16.5 m; 1.26 m of core were obtained, consisting of grey marl and clay, with iron staining and shell debris. The Brill No.2 Borehole (SP61SE/11) proved 1.54 m of Whitchurch Sand on 4.41 m of pale grey and buff clay and marl without reaching the Portland Beds. The Brill No.3 Borehole (SP61SE/12) started in the Purbeck Beds and proved about 2.7 m of interbedded pale grey to khaki clay, off-white, cream and buff marl, pale grey limestone and cream to khaki sand, on brown clay and khaki sand thought to represent the Portland Beds. The Brae Hill Close No.2 Borehole (SP61SE/29) proved 0.51 m of made ground, topsoil and hill-wash on 0.58 m of brown and grey mottled and grey brown clay.

At the surface the Beds appear as pale grey clay, pale marl and limestone debris. Around Chilton where they are very thin they are distinguished from the Whitchurch Sand clay facies by their pale colour and from the Portland Beds by the presence of clay.

I.P. Wilkinson examined five samples of clay for calcareous micro- fossils from the Purbeck Beds of the Brill No.1 Borehole (see (Appendix 5)). One sample (16.0–16.1 m) contained a fauna consistent with the middle and upper parts of the Lower 'Purbeck' and indicating brackish, estuarine conditions. A second (16.4 m) contained a fauna characteristic of the Lower 'Purbeck', indicating a more marine environment than the succeeding sample.

Whitchurch Sand Formation

The name Whitchurch Sands was proposed by Casey and Bristow (1964) for deposits of ferruginous sand recorded between the Portland or Lower Purbeck Beds and Lower Greensand or Gault, and cropping out from Wiltshire to Buckinghamshire; in this district they were previously called Shotover Sands (Blake, 1893) or Shotover Ironsands (Davies, 1899, p.40). They are referred to here as the Whitchurch Sand Formation. The strata have generally been considered to be of freshwater origin; Pringle (1928, pp.87–89) gives the earlier history of research. However, Casey and Bristow concluded that the Sand is marine-brackish Middle Purbeck (Cretaceous) age and equated it with the Cinder Bed of Dorset. Morter (1984) disagreed and placed it lower in the sequence 'close to the top of the 'Lower Purbeck' and base of the 'Middle Purbeck' i.e. within the Portlandian...stage as defined by Wimbledon and Cope (1978)'. The position of the Formation is not yet resolved, so for the purposes of this report it is taken to be in the Portlandian and therefore in the Jurassic.

The Formation caps Brill Hill, Dorton Hill and hills north-west of Chilton [SP 684 117] and [SP 681 122], east of the railway [SP 699 140], and forms a faulted outlier near Ryman's Farm [SP 6595 1235]. It also underlies the Gault in the area south-east of Chilton. The Formation oversteps the underlying Purbeck Beds and the upper part of the Portland Beds to lie in places on the Portland Sand. It consists of two distinct facies; a very fine- to coarse-grained sand (the sand facies) over most of the district, and to the south-east of Chilton an orange-brown and grey very ferruginous silty clay (the clay facies), apart from the area [SP 697 111] south-east of Chilton Grounds which is in the sand facies. The basal erosive and unconformable relationship of the Formation may in part account, for its variation in thickness from two to eighteen metres. The total Formation (i.e. where overlain by Gault Clay) is thinnest in the south-east where the clay facies predominates. This may be due partly to the unconformity beneath the Gault and also to the finer facies comprising a condensed sequence.

At the surface, the sand facies of the Formation produces a brown sandy soil with ironstone debris. The clay facies produces a brown and grey-brown, silty, ironstone-rich clay soil.

The Brill No.1 Borehole begins in the Whitchurch Sand and the Formation here consists of c14.7 m of buff, ochreous and brown very fine to medium-grained sand with pale grey and brown clay and mudstone beds up to 0.50 m thick, mainly below 10 m, thin pale buff to ochreous siltstone and silt beds up to 0.2 m thick and thin dark purplish brown sandy ironstone beds. The sand is weakly to well-bedded and cross-bedded in places, and generally consists of well-sorted sub-angular quartz grains, with a little ironstone detritus. In places it is weakly cemented, and a few sandstone pebbles, burrows and clay pockets were observed. Shear planes and organic debris were seen in the clay beds which are sandy in parts. The clay beds were examined for calcareous microfossils by I.P. Wilkinson but were found to be barren (see Appendix 5).

The Brill Church Borehole (Appendix 1), (SP61SE/13) proved 5.86 m of orange and grey medium to coarse-grained sand interbedded with thin grey clay and ironstone beds without reaching the base of the Formation and a borehole at Brae Hill Close, Brill (SP61SE/31) proved 0.15 m of made ground on 1.07 m of 'loam (understood to indicate a more-or-less equal mixture of sand, silt and clay) with some ironstone lumps, becoming more sandy with depth and increasing in ironstone content' without proving the base.

The Whitchurch Sand Formation is presently exposed at several localities. In the area known as The Walks [SP 6540 1424] in Brill, 2.6 m of orange and brown fine to medium-grained, poorly sorted sand with a 0.6 m dark orange-brown bedded ironstone band was exposed. About 150 m to the south-east [SP 655 141] it was reported (1986) that excavations up to 18' (5.5 m) deep at a building site had proved nothing but sand. A borehole at this site (Appendix 1), (SP61SE/26) proved 2.0 m of brown, grey and yellow sand with ironstone. In a section in the farmyard [SP 6806 1256] at Dorton Hill (see Appendix 2, locality B) 0.4 m of brown and orange-brown sandy soil on 0.4 m of orange to dark purple sandy ironstone, on 0.4 m of orange and dark green clay with ironstone was observed. A ditch section [SP 6880 1080] (locality L) by Ash Tree Cottage exposed yellow-brown to pale brown silty clay with rusty limonitic nodules and, 450 m to the south, about 0.7 m of mid grey and brown mottled silty clay under stony topsoil was seen in a section [SP 6871 1034] (locality M) in Easington.

Sections described by Fitton (1836, pp.280–281) are summarised in Appendix 3. A general sequence on Brill Common of about 14 m of white, brown and ochreous fine-grained sand with grey and brown clay beds and ironstone is given by Pringle (1928, p.93).

Cretaceous

Gault

Up to 15 m of Gault caps the hilltops in the extreme south-east of the district, forming three outliers, one extending onto the ground to the south. It is not exposed in this district, but in sections in the vicinity it consists of grey silty clay with phosphatic nodules. At the surface the strata give rise to a very heavy grey brown to orange brown silty clay soil. Where the underlying Whitchurch Sand is not sandy, the Gault is distinguished from it by its greyer colour and lack of ironstone.

Both Upper and Lower Gault were thought to be present at a reservoir site 200 m south of this district ((SP60NE/49) [SP 6912 0985], see Ambrose, in prep.), where a borehole (No.6) proved 11.40 m of pale to dark grey silty mudstone and siltstone without reaching the base. The uppermost 9.76 m were shown to be Upper Gault and the bottom 0.20 m were thought to be Lower Gault (determinations by A.A. Morter). The core was lost between 9.76 to 11.20 m. It is probable that both divisions are present in the subject area.

The Gault is unconformable on the underlying Whitchurch Sand which may in part account for the variation in thickness of the latter.

Structure

The regional structure of this district has a consistent low dip to the south-east of less than one degree. This is modified by a minor anticlinal structure [SP 677 148] north of Dorton and, in the higher strata of the hill areas, by profound superficial structures (see Superficial Structures) and minor faulting.

The zone of folding north of Dorton was first detected by Davies (1907b and 1910) in the railway cutting. The most complicated part lies on the ground to the north (see Sumbler, in prep.). It is no longer exposed but surface mapping reveals that the structure does not extend far to the south-west, probably dying out in the inlier of Oakley Member [SP 674 143] by Dorton Park Farm.

The minor faulting in the area south-west [SP 660 124] of Ryman's Farm preserves two small outcrops of Portland Stone and one of Whitchurch Sands. However, the apparent steep dips of the strata (up to three degrees) owe more to cambering and non-diastrophic dip-and-fault structure than to tectonic faulting.

The fault to the south-east [SP 693 102] of Chilton has a maximum down- throw of about 12 m to the north, although it is thought that this has been accentuated by local non-diastrophic processes.

No evidence was seen of systematic jointing in the strata of the district.

Superficial structures

The disposition of all the strata which crop out on the higher ground has been profoundly altered by superficial cambering and dip-and- faulting. This is most apparent from the dips of the strata which slope outward in all directions from Brill Hill and the hills around Chilton. For instance, the base of the Gault has been lowered about 25 m to the north of Chapel Lane, Chilton [SP 692 114], and the base of the Portland by nearly 30 m near Home Farm [SP 692 119] and about 50 m near Ryman's Farm [SP 658 121]. The cambers are thought to be largely composed of strata which are intensely faulted normal to the downhill direction with the intervening strata dipping downhill steeper than the overall dip of the camber in the manner described by Hollingworth and others (1944) and Horswill and Horton (1976). A section behind a barn at Home Farm [SP 6876 1197] displays dip-and-fault structure in the Portland Beds; there is a normal fault with a downthrow of about one metre to the west separating blocks with easterly valleyward dips steeper than the camber (see (Appendix 2), Locality D). A dip observed in a section at Dorton Hill [SP 6806 1256] ((Appendix 2), Locality B) probably results from cambering. Any existing jointing will have influenced the early stages of cambering. The poor recovery of the Purbeck and Portland Beds in the Brill No.1 Borehole indicates that even well away from the outcrop the strata may be considerably disrupted by joints and fissures. Jointing and fissuring will also be a major factor in water movement.

Another effect of cambering is the change in the apparent thickness of formations at outcrop. Increases in thickness are most noticeable in the Purbeck Beds on Brill Common and around Brill Hill and in the Portland Beds near Home Farm, Chilton [SP 687 121] and south-west of Wombwell's Farm [SP 695 106]. This is due to downhill dip and repetition of the strata by faulting. The apparent increase in the thickness of the upper formations is compensated for by the decrease in thickness by squeezing of the Kimmeridge and the Ampthill Clays, most noticeably north-east of Chilton [SP 693 122] and north of Meads Farm [SP 65 11]. The timing of the cambering event is not precisely known but it must partly predate the erosion of some of the minor valleys because the sand beds of the Upper Kimmeridge outlier at Chiltonpark Farm are at much the same level as the beds of the adjacent cambered spur of Brill Hill.

The position of the outcrop of the Oakley Member north-east of Chilton [SP 69 12] suggests that valley bulging (see Horswill and Horton, 1976) may also have taken place.

Landslip

In this district, landslips have been produced by the lubrication of existing shear planes in clay formations causing the detachment and downslope mass movement of the outer parts of spurs. Only six areas of landslipped strata are shown but it is likely that there are older inactive slips elsewhere which have degraded sufficiently as a result of erosion and cultivation to become unrecognisable.

The largest area of landslipped terrain is on the north-west flank [SP 698 143] of the hill east of the railway, the upper part being sandy, the lower part clayey and apparently forming a mud flow. Around Brill Hill only one slip is outlined [SP 654 131]. The western side of the hill at Chiltonpark Farm [SP 667 130] is extensively slipped and three minor slips are shown near Chilton. There are landslips at Canoncourt Farm [SP 686 114], west of Ash Tree Cottages [SP 6824 1076] and near Springhill Cottages [SP 682 115], the last of which showed signs of recent movement where the weight of a tractor had caused a fissure about l m deep to open in 1986.

Landslipping in these areas is likely to be triggered by water seepage at the base of the Whitchurch Sand, Portland Beds or sand beds in the Kimmeridge Formation (see Water Supply) lubricating shear planes in the underlying clay formations.

Drift

Quaternary

Scattered flints were found in two areas of this district; on outcrops of Gault [SP 694 111], [SP 690 106] and [SP 690 101] and on the Ampthill Clay outcrop [SP 67 10] and [SP 67 11] to the east and north-east of Hornage Farm. The former group of deposits may be a relic of a high level drift deposit, the latter probably the remains of a degraded tract of Head.

Head

Seven tracts of brown-sandy clay, some stony, are described as Head deposits. Most cap low knolls or subdued spurs on the lower hillsides, and are likely to be products of both hillwash and solifluxion.

A minor deposit of sandy clay [SP 6576 1477] lies in a small valley 500 m west of Coldharbour Farm. A tract flooring a col [SP 665 135] 600 m north-east of Parkpale Farm consists of less than l m of orange-brown sandy clay with a few black pebbles. Two small springs arise from this deposit. The Leap Hill No.2 Borehole (SP61SE/15) proved about two metres of grey and orange-brown, very sandy clay and clayey sand on a Kimmeridge Formation sand bed in the centre of a minor tract of Head [SP 6667 1237] 200 m east of Leap Hill. A substantial deposit of brown, stony, very sandy clay overlies Portland Beds on a spur [SP 689 121] 500 m north of Chilton. At the surface, much ironstone debris is apparent. This deposit is exposed in a bank at Home Farm [SP 6876 1198] ((Appendix 2), Section D), where 1.5 m of orange-brown to khaki mottled, fine to medium-grained sand can be seen. The other three Head deposits [SP 690 133], [SP 692 126] and [SP 698 119] lie on the lower valley slopes of the stream running southwards alongside the railway and consist of up to about two metres of brown, stony, sandy clay with flint and ironstone clasts.

Alluvium

Tracts of alluvium up to 250 m wide flank several streams, mostly on the low-lying ground in the north-east and south-west of the district. The narrower spreads in the south-west consist of probably not much more than 2.5 m of grey and brown sandy silty clay with gravelly lenses and a basal gravel. About two metres of grey and orange-brown mottled, silty, sandy clay on gravel was seen in a stream bank [SP 6502 1190] 600 m south-west of Leatherslade Farm. The broader tract [SP 690 140] 1 km east of Dorton is considerably thicker; two boreholes ((Appendix 1), 1B and 2B) near its margin prove 3.00 m of silty, sandy, gravelly, alluvial clay. In one borehole 1.00 m of gravelly sand underlies the clay.

Three degraded dams, probably intended to impound water for fishponds [SP 6658 1303], [SP 6700 1269] and [SP 6711 1280] near Chiltonpark Farm, have modified the deposition of alluvium in this area.

The ornamental lake [SP 6700 1353] in Spa Wood, which is now only partly flooded, may also have a thin alluvial deposit.

Made Ground

Four small areas of made ground have been delineated on the map. Two are sludge tips surrounded by earth banks at Dorton Hill [SP 6800 1255] and at Canoncourt Farm [SP 6872 1130]. A bank below a plantation [SP 6785 1273] 300 m north-west of Dorton Hill was reported as man-made. A small bank by the B4011 road [SP 6750 1063] near Hornage Farm is thought to be spoil from an excavation at the gas pipeline valve house.

Elsewhere there are many minor pits which have been partially backfilled. On the north and west sides of Brill village on Brill Common, North Hill and South Hills the ground has been intensively excavated, mainly for brick clay from the Purbeck Beds, and the spoil heaps from these activities, together with village refuse, now grassed over, have given rise to an area of inextricable pits and tips.

Water supply

The natural water supply of the district is dominated by the numerous springs arising within and from the bases of minor aquifers such as the Whitchurch Sand and Portland Formations and the sand beds in the Kimmeridge Clay. Several are shown on the Ordnance Survey map but many more were located in the course of the present survey. It is not known which are 'permanent' and which ephemeral. Whitaker (1921) gives details of some of the springs on the south side of Brill Hill (pp.45 and 270) and comments generally on the water supply of the area; a short account (pp.66 and 271) is given of the chalybeate well [SP 6693 1358] at Dorton. Once a moderately fashionable spa, which gave its name to Spa Wood and Spa Farm, it is now entirely disused and over-grown, the water level is about 0.3 m below ground level.

Whitaker also notes wells at Coldharbour Farm [SP 6630 1483] (p.124), Sebastopol Farm, now Ixhill [SP 6550 1036] (p.153), and Chilton Grove Farm [SP 6745 1234] (p.95), water from which caused an outbreak of gastro-enteritis in Marylebone in 1873.

Appendix 1 Boreholes

1B. Gas pipeline borehole 42A

c. [SP 688 140] S.L.+72.1 m OD

2B. Gas pipeline borehole 43A

c. [SP 688 139] S.L.+72.1 m OD

3. Gas pipeline borehole 44

c. [SP 681 132] S.L.+122.50 m OD

4. Gas pipeline borehole 45

c. [SP 680 132] S.L.+122.20 m OD

5. Gas pipeline borehole 46

[SP 675 114] S.L.+81.30 m OD

6. Gas pipeline borehole 47

[SP 675 113] S.L.+81.05 m OD

7A. Gas pipeline valve house borehole 1

c. [SP 675 107] S.L.+76.00 m OD

7E. Gas pipeline borehole 48

c. [SP 675 107] S.L.+74.70 m OD

8. Gas pipeline borehole 49

c.[SP 675 106] S.L.+76.35 m OD

9. Coldharbour Farm

[SP 6630 1483] S.L.c+114 m OD

10. B.G.S. Brill No.1 (SP61SE/10)

[SP 6570 1412] S.L.+184.65 m OD

11. B.G.S. Brill No.2 (SP61SE/11)

[SP 6606 1403] S.L. c+165 m OD

12. B.G.S. Brill No.3 (SP61SE/12)

[SP 6629 1402] S.L. c+146 m OD

13. B.G.S. Brill Church (SP61SE/13)

[SP 6552 1392] S.L. c+186 m OD

14. B.G.S. Leap Hill No.1 (SE61SE/14)

[SP 6661 1240] S.L. c+120 m OD

15. B.G.S. Leap Hill No.2 (SP61SE/15)

[SP 6667 1237] S.L. c+116 m OD

16. B.G.S. Ryman's Farm (SP61SE/16)

[SP 6600 1268] S.L. c+148 m OD

18. Clarke's Field Close (SP61SE/18)

[SP 6561 1368] S.L. c+182 m OD

26. Temple Street No.7 (SP61SE/26)

[SP 6553 1413] S.L. c+186 m OD

29. Brae Hill Close No.2 (SP61SE/29)

[SP 6520 1386] S.L. c+178 m OD

31. Brae Hill Close No.4 (SP61SE/31)

[SP 6526 1385] S.L. c+183 m OD

Appendix 2 Localities

The classification abbreviations used below are as follows:

Oa Oakley Member

P1S Portland Sand

WhS Whitchurch Sand Formation

P1 Portland Beds

P1 St Portland Stone

A. The Walks, Brill. [SP 6540 1424] Section exposed by landslip

B. Dorton Hill Farm. [SP 6806 1256] Section in farmyard

C. Dorton Hill Farm [SP 6802 1255] Section exposed by slip

D. Home Farm, Chilton [SP 6876 1197]

Section behind barn, showing dip-and-fault

E. Chilton [SP 6873 1152]

Temporary excavation behind house

F. Chilton Grounds [SP 6940 1146]

Section by barn

G. Section in bank 600m ESE of Easington [SP 6934 1012]

H. Dorton Park Farm [SP 6741 1441]

Ditch section

J. Section in abandoned stone pit 300 m S of Manor Farm [SP 6512 1333]

K. Plantation near Ash Tree Cottages [SP 6845 1082]

L. Ash Tree Cottages [SP 6880 1082]–[SP 6879 1078]

Ditch section

M. Easington [SP 6871 1034]

N. Easington [SP 6860 1033]

Excavation for garage

P. Chilton [SP 6890 1124]

Appendix 3 Previously described localities

Sections open previously around Brill were examined by Fitton (1836). Unfortunately they cannot be accurately located, but an indication is given of where they are thought to have been. The sections are summarised below. Fitton's original classification is given in braces.

A. Brill

A pit open in 1827. The note that the beds are inclined to the north- west suggests that the section is likely to have been on that side of the hill.

As no intervening Purbeck strata are recorded but they are mapped all around Brill, it is thought that the section might be faulted or slipped, or is a composite of several exposures.

B. Brill

'A clay pit 20 feet below the road on the south-west of Brill, the beds declining to the west and south.' Probably at Manor Farm [SP 6511 1367].

A large whole tree preserved in lignite and pyrite was found about 1.8 m below the base of this section, beneath which about another 3.7 m of dark clay was proved by boring.

C. Brill

Another pit not far from B. (Possibly one 80 m to the west [SP 6502 1366])

Appendix 4 Palaeontological Determinations by B.M. Cox

Specimens AMB 323–328

[SP 661 130] 70 m on 360 from Parkpale Farm, Brill.

Glauconitic shelly limestone with bivalves including Protocardia dissimilis (J de C Sowerby) and trigoniid fragments, serpulids and ammonite whorl fragments presumed to be Glaucolithites (some encrusted with small oysters). A single lydite pebble is present.

Horizon: Glauconitic Beds (basal) or ?Upper Lydite Bed Portlandian, Glaucolithus Zone.

Specimens AMB 329–330

[SP 659 122] 600 m on 280 from Buttermilk Hall, Brill.

Glauconitic limestone with small oysters and other bivalve fragments and ammonite fragments (Glaucolithites?) with an encrusting fauna.

Horizon: Glauconitic Beds.

Specimens AMB 331–332

[SP 662 129] 50 m on 170 from Parkpale Farm, Brill.

Cream-coloured, slightly glauconitic limestone with?Camptonectes lamellosus (J Sowerby), Pleuromya sp. and a trigoniid internal mould.

Horizon: ?Glauconitic Beds.

Specimens AMB 333–334

[SP 6860 1033] 1250 m on 183 from Chilton Church, Chilton.

Slightly glauconitic limestone with an ammonite whorl fragment (Glaucolithites?) with a poorly preserved encrusting fauna.

Horizon: ?Glauconitic Beds.

Specimens AMB 335–339

[SP 6845 1082] 800 m on 197 from Chilton Church, Chilton.

Shelly, glauconitic limestone with lydite pebbles and Pleuromya?, trigoniids, oysters, serpulids, a gastropod and an ammonite (?Glaucolithites).

Horizon: Upper Lydite Bed

Portlandian, Glaucolithus Zone

Specimen AMB 340

[SP 6810 1093] 870 m on 221 from Chilton Church, Chilton.

Worn nodule of shelly cementstone with poorly preserved impressions of a Nannocardioceras and Nanogyra virgula (Defrance).

Horizon: Lower Kimmeridge Clay (KC30 of Gallois and Cox, 1976)

Lower Kimmeridgian, Eudoxus Zone

Specimens AMB 341–344

[SP 6501 1255] 380 m on 247 from Leatherslade Farm, Oakley.

Fragments of the oyster Deltoideum delta (Wm. Smith).

Horizon: Ampthill Clay Formation (although D. delta occurs in the lowest Kimmeridge Clay, the relative thickness of strata over which they can range in the Brill No.1 Borehole strongly suggests that loose specimens will be from the Ampthill Clay).

There is other material in the BGS collections from this area obtained or collected during previous work. Identifications of this can be seen in BGS files.

Appendix 5 Calcareous micropalaeontology of the 3.0–16.4 m Interval of the Brill no. 1 borehole (SP61SE/10) By I.P. Wilkinson

Preparation: L Glew. Examination: I P Wilkinson

Introduction

Nine samples from the 3.0 to 16.4 m interval of the Brill No.1 Borehole were examined for Foraminiferida and Ostracoda from a biostratigraphical and palaeoecological viewpoint, in order to determine the age of the deposit and whether it is Whitchurch Sand or Shotover Sands.

Sample details and fauna

Conclusions

The fauna at 16.4 m is characteristic of the Lower 'Purbeck'. The characteristic Portlandian to Lower Purbeck species P. rimosa and H. serpentina, together with common K. alata and M. rugulata and rare R. jurassica, are consistent with the Quainton to Stair faunicycles of Anderson (i.e. 'assemblage 1'). The absence of Anderson's 'C-phase' ostracod species substantiates this age.

At 16.0–16.1 m a similar fauna was recorded, although H. serpentina, P. rimosa and P. cf. brodiei were not found. This is consistent with the middle and upper part of the Lower Purbeck, particularly as 'C-phase' species were not found.

The 15.70–3.0 m interval was barren and biostratigraphical conclusions cannot be made.

The sands at the base of the examined part of the Brill No.1 Borehole cannot be assigned to the Shotover Sands, which are Hauterivian in age and stratigraphically too young. However the suggestion that the deposit may be Whitchurch Sand Formation cannot be wholly substantiated. This latter was generally considered to be at approximately the Upper/Middle Purbeck boundary and of earliest Cretaceous age (Casey and Bristow, 1964). The Ostracoda recovered from the Brill No.1 Borehole are of Jurassic age (i.e. Portlandian and basal 'Purbeck') aspect and therefore stratigraphically older than would be expected. (This is consistent with the conclusions of Morter, 1984. A.J.M.B.).

Palaeoecologically the lowest sample examined has more 'S-phase' species in it than at 16.0–16.1 m, and the marine influence appears to be greater than in the succeeding sample. Chara oogonia were present in very small numbers at 16.4 m, but in greater numbers at 16.0–16.1 m at which level more brackish, estuarine conditions seem to be indicated. Water depths, particularly when the sample at 16.0–16.1 m was deposited, were shallow, probably no more than 10 or 12 m. No palaeoecological conclusions are possible for the 15.70–3.0 interval.

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