The structure of Bower Hill, Nutfield (Surrey)

THE

STRUCTURE OF BOWER NUTFIELD (SURREY).

HILL,

By F. GOSSLING, B.Sc., F.G.S., F.C.S.

(Weald Research Committee Report No. 20). [Received 8th October, 1934.] [Read 1St June, 1935.]

CONTENTS. PAGE

I. II. III. IV.

V. VI. VII. VIII.

IX. X.

INTRODUCTION AND PREVIOUS LITERATURE

3 60

INVESTIGATION BY BORINGS

36 1 36 1 3 67 3 68 37 8

LITHOLOGY GEOLOGICAL MAP AND SECTIONS DESCRIPTION OF THE OUTCROPS DISLOCATIONS A

SUB-HYTHE PERICLINE

3 82

SUMMARY OF RESULTS OF THE INVESTIGATION

384

INTERPRETATION

3 84 3 87

ACKNOWLEDGMENTS

I.

INTRODUCTION AND PREVIOUS LITERATURE. object of the investigation here described was to elucidate the geological structure of the outlier of Hythe Beds at Bower Hill which lies to the south of the Lower Greensand escarpment between Redhill and Nutfield. This hill is the most westerly of a series of six outliers fonning ridges and knolls (of which Tilburstow Hill is the most easterly) near the foot of the escarpment and approximately parallel to it, between Redhill and Godstone. Generally, the scarp face in this district consists entirely of Hythe Beds resting conformably on Atherfield Clay, although between Redhill and N utfield the summit is composed of Sandgate Beds, and near Blechingley even the Folkestone Beds extend as far south as the top of the escarpment. The outliers attain to an altitude of about 100-130 feet lower than the escarpment, and, in the case of the three larger ridges, about 100 feet higher than the Weald Clay at the base of the escarpment. These ridges and knolls are composed of Hythe Beds, of which the whole or only a part of the succession may be present, and the strata are characterised by a high northerly dip which may reach 50 degrees or more on the north flank. The structure of these outliers presents a problem the solution of which is by no means obvious. In the case of the Tilburstow outlier it has been stated' that the highly inclined

THE

I Cox) A. H. xxvi,

1915. Excursion to Tilburstow and Godstone.

Prot. Geol, Assoc., vol,

STRUCTURE OF BOWER HILL, NUTFIELD, SURREY.

361

strata probably indicated a reversed fault which is illustrated It has also been in a section showing a thrust from the north. suggested> that the dislocation might be of the nature of a landslip. In the case of the Bower Hill outlier, the landslip hypothesis was suggested- at a field meeting of the Association in 1929. The recently published Reigate sheet of the one-inch map of the Geological Survey shows, tentatively, a fault line separating five of the outliers from the escarpment, and another accounting for the Tilburstow outlier. II. INVESTIGATION BY BORINGS. The author has had these outliers under observation from time to time since 1927. That of Bower Hill has yielded much information derived from a study of two deep lane sections which cross parts of it, and of several long-abandoned stone pits along its crest. An attempt was made in the spring of 1931 to ascertain directly by boring whether the steeply inclined Hythe Beds rest upon a sole of Weald or Atherfield Clay, but running sand and the prohibitive cost of coping with it stopped the operations. Success in a direct attack on the problem having proved impracticable, it remained to explore the hill in detail by means of borings on the flanks of the hill. Two primary objects were kept in view, viz.: (I) The mapping of the outcrops of the Weald Clay, of the subdivisions of the Atherfield Clay and of the subdivisions of the Hythe Beds; and (2) the determination of the positions of any lines of fracture or discontinuity, together with the direction and inclination of any planes or surfaces of dislocation and the stratigraphical horizons of the strata brought into juxtaposition at such surfaces. About 190 borings have been made, ranging in depth from about 3 feet to about 25 feet in some instances, and four trenches were dug and parts of two old stone pits were cleared. Two augers, 3t inches and 5 inches respectively in diameter, were used. The positions, a.D. levels, and details of the strata passed through in each case have been recorded. III. LITHOLOGY. The succession, which is exposed in the lane running down the escarpment southward from Nutfield Schools past Little Cormongers Farm in Mid Street, i.e., immediately to the east of Bower Hill, furnished the necessary criteria for the correlation of the beds present in the hill. I Stamp, L. D. xxxii, 2 Gossling, F. Asso';'J vol. xli,

1921. 1930.

Excursion to Tilburstow and Nutfield,

Proc. Geol. Assoc., vel,

Field Meeting at Redhill, S. Merstham and Nutfield,

Proc, Geot.

F. GOSSLING,

Omitting minor details, the succession in both the outlier and the escarpment, which is also illustrated in column form may be described as follows:-

6.

HYTHE BEDS. feet. (b) Pale buff to whitish clayey, glauconitic silt, with 20% of fine sand and much sponge debris. The highest foot contains abundant Pteria pectinata, Sow. about 4 (This bed is not exposed in the escarpment section.) . (a) Bedded soft to hard sandstone, glauconitic, more or less silicified, varying in grade from very coarse to medium, with a whitish silty matrix. The corrugated spicules (usually as casts) of Geodites are abundant throughout about 20 (b) Pale buff and whitish, silty, medium and coarse hard sand, or soft sandstone, with much glauconite and limonite and pebbles to i in. and! in., mainly lydite a bout 18 (a) Red-brown sand, medium to coarse and very coarse, with some glauconite and limonite, and with pebbles of quartz and lydite to ! in. and ~ in., getting finer in the lower part about 52 Greyish to whitish fine sand, with much glauconite, the lower portion passing into soft stone (Lower Hythe Stone) .. about 38 Very pale buff and fine white sand with very little glauconite about 68

1 5.

4. 3·

Total

200

The thicknesses given for the subdivision (6) above were obtained from Bower Hill, the exposure at the top of Mid Street being incomplete. The remaining thicknesses were estimated from the section in Mid Street. The corresponding beds occur in Bower Lane, but they are there steeply inclined, and appear to be more or less crushed and broken. Consequently they are not accurately measurable, but they appear to be of thicknesses comparable with those in ~id Street. ATHEH.FIELD CLAY. 2. I.

Feet. Bright to dull chocolate clay, very tough and homogeneous. about 7 Pure grey to buff coarse silt a bout r6-18 Total

25

These very marked characters dearly distinguish the Atherfield Clay from the Weald Clay and shale. The thicknesses given for the Atherfield Clay were obtained from borings at the foot of the escarpment in South Park,

STRUCTURE OF BOWER HILL, NUTFIELD, SURREY.

363

i.e., immediately north of Bower Hill. As will be seen later, the thickness of the Chocolate Clay round the Outlier varies greatly. The thicknesses in Mid Street appear to be of the same order as in the escarpment. Mechanical analysis and microscopical examination have been freely used in establishing the characteristics of the various beds described above. BOWER HILL

MID STREET

FEEr

iie,

Cloyey glauconitIC silt with $ponge debri$ and Pteria 1Nctlnata. 22<>

Closely bedded soft to hard glauconitic sandstone with white s,/ty matrix and more or less sdic.lfied In parts Geodi1a sp'c.uJes common

rap st(Jne beds Closelybedded soft to hard glaucombc sandstone with .Jutish sJty 1tJDtn..

210

and more or less S/lk:'l!ed '" ports.

Geodil:.t!a 5piclJl~s COfTImOfl.

Two thin

btNtth

Q~ colcore/JI,IA

Whitish silty

/70

Dnd medium hard sand or

.... "

........: .. • :. " .: ';:~ RfNi·brown sand. mtld,um to CQOrSf! ottd • .'. very coarse, some glouconite and Ivnol'IIte. With pebhlt!s of quart3 and Iydfte co •: •• '. .Yt Inch, gel:tmgliner mthe IOwt:r part.

160 : : '.5~·.:..

/4.0

·.,.. . .. :: .... ••.•••

130

'.

'SO

Whlbsh to bufr sdty fine to IMdlum 9tInd."n, much gJaucolttte and Ilm()(IIte O«i pdlbks to J1 mcb, mamly Jydlte

soft sondstOlH! wrth MUch glovconite and limomte oad pebbles ta ~ f/1ch,mom/y Iydite..

'90

ISO

fln~

..

R~·bro.,1t sand. medium bJ coarse and~ coarse, some glauconite and hmomte. With pebbles 01 quart] and lydlte to M11nc:h. getbng tAe lower-port.

.>.:~::: . .....

rifler In

~

/20

110

Greyish to whitish fine sand, with much

g!ouconitej:he lowerporbon passing into soft 9~yi$h stone ( Lower Hythe Stone) which is usuallY moreor lesscolCOreDus.

~ G~Ylsh burr to whibsh fine sooa. With much t

glauconite, In part passing ".,/;0bard sand Of'

';: soft stone

~ (This occurs O/)(y on the south flank 01 the hill)

~

/00

'0 80

70

Lower HM sands. Very pole to bright

60

buFF soft Nnesand with very little glauconite..

L()~,. fitte sol'Kis. Very pole buff to whitish soft fIne sand With very little glauconite. (Only lowest part ts present an the narth

Flank of the hill).

50

40

30

to dull, tough and homogeneous

Chocolate Clay band. Bright 2IJ

v~ry

10

Coarse silt hand. coarse SIlt.

Pu~

grey

to buff

Coarse silt band. MOWN light Qnd brig!, burt COarse Silt.

0

COMPARISON OF STRATA IN MID STREET AND BOWER HILL. NUTFIELD.

F.

GOSSLI~G,

It will be noted that the Stone Beds (6a) are here included in the Hythe Beds. They appear first on the east side of RedhiIl and were included in the Sandgate beds in the table in the Reigate paper. I But the study of the Lower Greensand between RcdhiIl and Oxted has shown that these Stone Beds, which always overlie the top Hythe Sand containing black pebbles, increase in thickness eastward and gradually develop much massive chert in their upper part, until at Tilburstow the chert beds are I I ft. thick. It has always been customary to regard the definite chert beds as belonging to the top of the Hythe Beds, and it is therefore most convenient to regard their representatives westward as of the same horizon. The thick Fuller's Earth of the Sandgate Beds in the N utfield district is therefore here regarded as the base of the Sandgate Beds, the beds below it being allotted to the Hythe Series. It may be mentioned, too, that the corrugated spicules of Geodiles always occur in the Stone Beds in question, even in the thin bands which are found occasionally some way down in the Hythe Sands, but they have not been observed in the Sandgate Stone Beds which overlie the thick Fuller's Earth. The appearance of these spicules is consequently a useful guide in the field for distinguishing between Hythe Beds and Sandgate Beds. In this connection it seems to be necessary to refer to the discussion of the Stone Beds in question which occurs in the recently published- memoir of the Geological Survey. For the reasons stated earlier in this paper, the author has had occasion to make many careful and repeated detail comparisons between the Stone Beds at the top of the outliers and those at or near the top of the escarpment. In his experience the so-called " chert beds" of the outliers at Bower Hill and eastward of Lyttel Hall (perhaps more accurately described as more or less silicified sandstones) are not distinguishable from most of the stone in the main escarpment. Moreover, it can readily be seen in several places, e.g., in the Bower Hill Lane section and in the footpath section N.E. of Lyttel Hall, that these chert beds in the outliers overlie the" pebbly top bed" as they do in the escarpment, as shown in the Column Section, and therefore should not be correlated with them. It may be further stated that these relative positions are maintained eastward. The pebbles become progressively smaller in that direction, but the Stone Beds rarely contain pebbles.

Characteristics of Atherfield Clay and Weald Clay. It may be desirable to give an account of the Atherfield Clay and Weald Clay in this district. 1 Gossling, F. 1929.

vol, "I., fig. 33. p. 227.

The Geology of the Country around Reigate.

Proc. Geol, Assoc.,

2 Dines, H. G., and Edmunds, F. H., 1933. The Country around Reigate and Dorking, MM. Gecl. Surv., p. 33.

PROC. GEOL. As soc., VOL. XLVI. (I93 5).

A .-

P LATE

30.

H I GHL Y INCLINED STON E BEDS (,lA I N "ASS), SEE N FR O ~I T H E \V EST .

Qu arry west o f Bow er Hill L anc .

[Pltotos : A .]. B ull.

H.

-- P A R T S OF THE

EAS T A N D \V E S T EIDGES I N SANDY LANE.

SEEN

FRO M

T HE

N .E .

T he to p of the W est Ri d ge is formed of Top Stone Beds of th e Main Slipped Mas s , while m ost o f t he slop e bel ow is compos ed of the ea st ern ex t re m ity of the Second Slip ped Ma ss . T he Main Mas s con t i nu es east wa rd behind the East R id ge , whi ch co ns ist s of Lower Hytlle Sand i ll situ.

STRUCTURE OF BOWER HILL, XCTFIELD, SURREY.

365

The Chocolate Clay Band of the Atherfield Clay, when unweathered, is usually of a milk-chocolate colour, but it may be nearly as dark as ordinary chocolate, and it closely resembles these substances in general appearance. When much weathered, however, the colour brightens towards orange, and becomes more or less mottled with light fawn shades. In a number of places where the auger passed through the slip surface, the first few inches of the clay were very hard and difficult to penetrate, the colour was darker and greyer, and the material was streaked or laminated with pale grey or white shades. The clay is strikingly homogeneous and very tough. When disintegrated by methodical stirring in water, settling, and decanting in such a way that only clay and the finest silt are carried away, the loss is usually about 70 per cent. The residue is a pinkish pale buff silt, which, when mounted in water and magnified about 60 diameters, appears as a mixture of quartz grains from 0.02 to 0.05 mm. diameter with a small percentage of larger grains. Some grains are stained with limonite, but composite grains are rare or absent. The Coarse Silt band of the Atherfield Clay, as seen in road sections and most of the bores, seems to be weathered to mottled bright fawn and pale buff colours. Occasionally in deep bores it is found to be of a full grey colour. After careful washing, the loss (clay and finest silt) is usually about 30 per cent. and the residue is mainly coarse silt (0.06-r.o mm.) with a small proportion of the finest sand grade (up to .14 mm.). Under the microscope the grains seem to be almost entirely clean quartz, with a variable proportion stained with limonite. Composite grains are rare or absent. The Weald Clay in this district generally presents itself in road sections and bores as a more or less mottled clay, most commonly of a whitish or very pale grey or buff colour, streaked with yellow, orange or brown shades. Only in a few cases where it has been protected from weathering is definite shale found. Under weathering the shale, which is normally composed of alternating thin lamina: of grey clay or mudstone and very fine whitish silt, becomes coloured with limonite either in the clay or silt lamina: or both, and eventually it disintegrates into a mottled whitish and brown or buff material. After careful washing as described above, the 103s of clay and very fine silt is round about 80 per cent. When it is much less than this the residue consists very largely of platy grains of undisintegrated mudstone. Microscopic examination of the residue reveals, in addition to clean or stained quartz silt, a variable proportion-IS to 60 per cent. of composite grains composed of fine silt or clay with a limonite cement. The grade is less than 0.01 to 0.035 mm. with some larger grains among the composite plates. That is, it is very much finer

SCO ll' d' r I'pt

1~ -Jet. ~O'~ II Y1H (

:60"~-V()P et coe brl/s... dh l'/a) 't:v ~~I/t bmllf 1'. 5~~b :h;,..r) coa r-se and rtM": h bl)' sands .!..:..A:w....u

B .ul.

t4PD'" th, O,d'"UlC t St4'-Jt)'

.\f IJp Al'ith

... THCRFIC.LO

HC1J .i

,II, Sli ll et / O", . ,/ 11" F IG . 4 6 .-

C LA Y

m4-®9;:;~~~~(~::~:J;"lI'

"_Chocolot~---;h.!¥ balla'

~ lO ~'+t'r r;nt'! sands

~ Coorse SIlt bOlld

~

C~lltrolft1 of H.•\I , St.,io ,,">, Ollcr.

G E OL OGI CAL MAP OF Til E B OW ER HILL OUTLIER , }; UT FI E I. D .

- -- floufldury

ormom moss

- - - - JJoundar:r orsccood m03'$ ( ....,..... ., . L"

C_. "· ~ .." I

STRUCTURE OF BOWER HILL, KUTFIELD, SURREY.

367

than the Coarse Silt of the Atherfield Clay and usually finer than the Chocolate Clay. The composite grains are more or less opaque, of a dark brown to yellow colour, and when sufficiently transparent they show their composite character clearly under crossed nicols. Treatment with hot dilute hydrochloric acid breaks up some of them and renders the remainder more transparent. Composite grains of this character are rare or absent ill the Chocolate Clay and in the Coarse Silt band of the Atherfield Clay, and their presence in considerable quantity is fairly decisive evidence of Weald Clay in cases where other characters are of doubtful significance. In practice in the field, the colour and texture of the Chocolate Clay and the coarseness of the Coarse Silt band serve to distinguish the Atherfield Clay from the Weald Clay. The most positive characters of the Weald Clay are-indications of shale, when this is present, and the grade of the clay, when this is extremely fine. In cases of doubt, recourse must be had to the washing procedure and microscopic examination as described above. IV. GEOLOGICAL MAP AND SECTIONS. The geological map (Fig. 46) shows the outcrops of the subdivisions of the Hythe Beds and of the Atherfield Clay, which are numbered I to 6 as described above. Sections on the lines A, E, C, cr, D, E, F, and G, are shown in Figs. 47 and 48. The map is based on the 25 in. (1;00) m l.p of the Ordnance Survey, and the positions of the outcrops are drawn as nearly as may be at the distances indicated by the scale of the map from the features which have been copied from the Ordnance map. The distances were usually estimated. by pacing from the surface features a'sove me.. itioned and the elevations were measured by levelling with an Abney level from the nearest available bench mark or spot rmrk, or from the contour lines of the 6 in. map. The data from which the lines were drawn were obtained (I) in part from existing exposures of the strata on the ground. such as road-banks, sunken lanes, footpaths, and ancient stone quarries and sandpits, and (2) in part from trenches and borings made in the course of the investigation. The latter numbered about 190, and as in many areas they are rather crowded, it has not been attempted to show them on the map. On the sections and in some other places they are indicated, and in certain cases particulars are given in the sequel as to their positions, O.D. levels and thicknesses of the strata encountered. Similar information was recorded with regard to the remainder, and the lines and details given in the map and sections represent in the author's judgment the facts which have been ascertained. Two main results are brought out, viz. : (r) the outcrop of

368

F. GOSSLING,

the Atherfield Clay-Weald Clay junction is continuous in unbroken curves from the foot of the escarpment round both ends of the hill and on the south side, except where the displaced mass over-rides it. It will be noticed also that the neck of Hythe Beds connecting the hill with the foot of the escarpment is composed entirely of the lowest subdivision of the Hythe Beds; and (2), there are two dislocated masses, the boundaries of which form closed curves. These are shown on the map by full and broken heavy lines, respectively. V.

A.

DESCRIPTION OF THE OUTCROPS.

The Principal Surface Exposures. (I) Bower Hill Lane. The section in this deeply sunken lane is exposed on the west side, the east side being obscured by vegetation. The succession has been described on page 362 and in the Column Section. At the junction with Hogtrough Lane, the clayey glauconitic silt (6b) is exposed on the top of the bank or cliff on the west side. Under it, there is seen in the cliff and on the north side of the old quarry which opens into the lane, the upper part of the Stone Beds (6a) in an approximately horizontal position. Trenching at the back (west side) of the quarry also exposed the horizontal stone beds. In the southwest corner of the pit a rubble-filled gap about 7 feet across separates the horizontal beds from those on the south side. The latter are exposed in the lane-cliff south of the pit, and are seen to be composed of closely bedded Stone Beds (6a) dipping north at 60-70 degrees. Old sandpits follow southwards down the lane, showing in succession rather oxidised glauconitic Pebbly Sand (Sb) and red-brown Coarse Pebbly Sand (sa) getting finer in the lower part. The cliff next shows the Grey Fine Sand (4) with soft stone in the lower part. The stone appears to have a strong northerly dip, possibly as much as 28 degrees, but it is in a crushed condition, and the true dip is very uncertain. From this point the Lower Fine Sand (3) follows to the end of the cliff, but the southern half is much obscured. There are no lines of stratification in the sands which might indicate the dip of the beds. On the opposite side of the lane, at 10 feet north of the fence running eastward therefrom, rather impure Chocolate Clay was found by boring, as described later. There does not appear to be room for the whole thickness of Lower Fine Sand (3) between the Grey Sand with soft Stone (4), and the Chocolate Clay band (2). It will be seen later that the lowest part of the Lower Fine Sand (3) has been left behind beneath the presumed slip surface above its northern outcrop.

STRUCTt:RE OF BOWER HILL, NUTFIELD, SURREY.

369

(2) The Quarries. In addition to the old quarry west of Bower Hill Lane already described, there are others along the top of the hill. One on the cast side of the lane, opposite to that on the west side, is entirely obscured by rubbish and vegetation. Another is situated about 160 feet east of Bower Hill Lane and about 40-50 feet south of Hogtrough Lane. The section at the east end of this quarry shows a thickness of about 19 feet of closely bedded Top Stone Beds (6a), dipping northward at 65-75 degrees (see Plate 30. B). At the north-east corner there is a marked break and the beds on the north side of the pit are crushed and broken up. To the south of the inclined beds the normal succession of Coarse Pebbly Sand (5b) occurs. Along the highest part of the hill, a little east of Spot 392, another old quarry occurs which is now much overgrown. But in several places on the north side, Top Stone Beds (6a) are to be found dipping northward at about 52 degrees, while the south side is composed of Coarse Pebbly Sand. (3) 11ogtrough Lane and Sandy Lane. Going eastward from Spot 274 the surface of the lane rises as far as the corner of Bower Hill Lane, where it reaches O.D. 343. The north bank is walled up, but in the bank on the south side, which varies in height from about 5 feet to 10 or 12 feet, the following sequence is disclosed, viz., Weald Clay, Atherficld Coarse Silt (1), Atherfield Chocolate Clay (2), part of the Lower Fine Sand (3), part of the Coarse Pebbly Sand (5), and part of the Top Stone Beds (6a), capped at the end with the Clayey Silt Band (6b). Only the lowest part of the Lower Fine Sand (3) is present (see Sections D to G). The thickness of this bed between the Coarse Pebbly Sand (5) and the Chocolate Clay (2) as measured in a bore shown in Section G was only 10 feet. There is thus a gap in the succession in which much the largest part of the Lower Fine Sand (3), the whole of the Grey Fine Sand (4), and most of the Coarse Pebbly Sand (5) are missing. The mechanical junction between the existing parts of the Lower Fine Sand (3) and the Coarse Pebbly Sand (5) is generally sharp and is marked in some places by a hard ironstone band 1 to 2 inches thick which dips sharply southward. The strata in this area, up to and including the Lower Fine Sand (3), have a westerly dip of about 4 degrees, but the junction between (3) and (5) is approximately horizontal. In a rather hard part of the Coarse Silt (1), casts of small lamellibranchs occur, which have been identified bv 1\1r. R. L. Cox as Astarte sub-costata d'Orb, and Pecten sp. 'The Clayey Silt band (6b) also contains small fossils which have been identified by Mr. J. F. Kirkaldy as Pteria (Oxytona) pectinata Sow. and Exogyra iuberculifera, Koch and Dunker.

37°

F. GOSSLING,

Continuing eastward from the comer of Bower Hill Lane to about Spot 36r, Hogborough Lane lies on more or less broken up Top Stone (6a) which can be found in the bank within South Park on the north side of the lane and in old workings between the lane and the old quarry on the south side. It is also visible in the footpath which leads up southeast from Spot 36r. As the lane (now Sandy Lane) descends eastward from Spot 36r and bends more northward, the Top Stone is left up above on the south side and both banks of the lane show Coarse Pebbly Sand (5) to near Spot 350. Then for a short distance Coarse Pebbly Sand (5)' rests again discordantly in the south bank on Lower Fine Sand (3). Beyond this point Chocolate Clay (2) comes out under the Lower Fine Sand (3), the outcrop of the junction crossing the lane at about the B.M. 343.9 into South Park. Eastward of this mark, Sandy Lane lies on Lower Fine Sand.

B. Outcrops located by Borings. (r) The west end of the hill. The mechanical junction between the Coarse Pebbly Sand and the Lower Fine Sand in Hogtrough Lane which passes nearly horizontally westward, reaches the top of the lane-bank near the point where the Section Line E crosses the lane. Its outcrop was traced thence south-westwards from the results obtained from numerous borings, some of which are shown in Sections D and E. (Fig. 47). The south-east part of the large area between Section lines D and E, mapped as Coarse Pebbly Sand, consists of a mixture of this sand and fragments of Top Stone which is impenetrable by the auger for more than a few feet, and a southward extension of the Section lines was thereby prevented. It will be noted in Section D that the Lower Fine Sand (3) thins out rapidly southward, so that the Coarse Pebbly Sand comes down within a foot or less of the Chocolate Clay. A number of bores between Sections D and E confirm the thinning of this bed, some of them having been carried down 7 to ro feet as far as the Chocolate Clay. Sections D and E also record the fact that the bores commonly encountered a seam of ironstone r to 2 inches thick, which here marks the sharp line of mechanical division between the Coarse Pebbly Sand and the Lower Fine Sand even more clearly than in the case of the Hogtrough Lane section. A comparison of the bores in Sections D and E with the trenches and bores in Section F shows that the Stone Beds, most of the Coarse Sand Beds, and the Grey Sand with the Lower Stone come to an end not far west of lines F and E.

STRUCTl:RE OF BOWER HILL,

~l:TFIELD,

371

SURREY.

D

]

f-- - ---\'V IL

r

A

l

--- - - - - - - - - ~ 0

(

.s~ aI r

$((0 ",0 SL./PP f.D

l

1\

",

1. r

,

~

--!•

t

MASS

FIG. 47.-SECTIONS ON

THE LINES

D, E, F and G.

372

F. GOSSLING,

The trenches on the line F itself, taking them in order from the north end, disclosed successively (i) Pebbly Glauconitic Sand (5b); (ii) Bedded Top Stone in courses dipping about 10 degrees south. (Horizontal Stone shows also some feet higher in the bank between the trenches I and 2) ; (iii) much rubbly stone, not horizontal; (iv) 8-9 feet of red to bright buff Coarse Sand with Pebbles, identified as Bed sa. The two bores larther south showed: (v) Red clayey Sand to 6 feet and then glauconitic Grey Sand; and (vi) glauconitic Fine Sand identified with Bed 4. In Section D, however, Bed 4 is absent. At the north end of this line the bores showed the juxtaposition of the Coarse Pebbly Sand (5) and the Lower Fine Sand (3), the thin ironstone band separating them; while the three bores on the southern part of the line showed the Coarse Pebbly Sand coming down near to and upon the Chocolate Clay. It is unfortunate that the thickness of the Chocolate Clay in these bores was not determined. They were among the earliest bores made and the importance of such information was not then realised. But it will be noted that the outcrop of Chocolate Clay in the lane which is in line with these bores has a thickness of less than 2 feet. {2) Outcrops west oj Section D and south oj the lane.

Bores in this area established the outcrops mapped and showed that the strata here have but a very slight southerly dip.

(3) Outcrops north oj Hogtrough Lane from Spot 274 to Section B. In this area the walled-up bank forms the boundary of South Park, and from this bank the land falls northward, at rust steeply, towards the Lake (see Sections D, E, G and B.) (Figs. 47 and 48). Across this slope, more or less parallel to the lane, as well as on several cross lines similar to Section E, upwards of 30 bores (not including those on Section B) were made and from these the outcrops were mapped. As an example, a description of (he bores north of the lane on Section E is given below.

(i) ~ii)

(iii)

Ncar the north end of the Section a bore, not shown, at 90 feet from the lane and at a.D. 30 <), showed white and orange Weald Clay under r It , of soil. The first bore shown, at 65 ft. north of the lane and a.D. 313. disclosed soil It ft.; Atherfield Clay Coarse Silt, mottled pale and dark buff, I~ ft.; Weald Clay, white and orange. The next bore, at 40 ft. from the lane and a.D. 317. yielded soil I ft.; Atherfield Coarse Silt, mottled pale and dark buff, 5 ft. ; more Coarse Silt of various colours ending in pale grey, 8 ft. ; black and white laminated clay (Weald shale), It ft.; variously coloured clay, ending in buff and white with fine larninze, It ft. The AC/\VC junction was drawn at a.D. 302t ft. The bore at the top of the bank, about 5 ft. from the lane and at a.D. 324, showed: Soil, It ft.; buff rather glauconitic Fine

STRCCTURE OF BOWER HILL,

~UTFIELD,

Sl:RREY.

373

Sand, 8! ft.; silicified and very hard Fine Sand, I inch; very dark grey, very tough clay (top of the Chocolate Clay), passing rapidly into Chocolate Clay I ft. ; Chocolate Clay 2! ft.; Atherfield Coarse Silt.

As far as a point somewhat beyond Section G the sequence continues normal, but the Chocolate Clay varies in thickness from about 3~ ft. to a few inches. The overlying Lower Fine Sand is usually only about 3 ft. thick, being cut off by the northward slope of the ground. As seen in Section G, it reaches a maximum of 10 ft. on the south side of the lane. Eastward of Section G, evidence of the gap between the Top Hythe Beds and the Lower Fine Sand, already described in the Hogtrough Lane Section, appeared also north of the lane. Very definite evidence was afforded by a section obtained at a point nearly opposite the corner of Hogtrough Lane and Bower Hill Lane and about 65 ft. K. of the former lane. Here the roots of a very large beech tree (which had been torn up by a gale) have brought up to a vertical position a mass of stone, roughly bedded horizontally, but much broken up. The hole was cleared and a bore made in it. The whole section, including the torn-up mass, of stone, was as follows:O.D. 341. Soil, 1ft.; irregular layers of broken stone. more or less bedded horizontally. 2 ft. ; stones and sandvz ft.; pale buff and nearly white Lower Fine Sand, I ft. 3 in.; Atherfield Chocolate Clay band. r It. 3 in.; Atherficld Coarse Silt band bored to I ft. .

It will be seen that the Top Stone Beds are separated from the Atherfield Clay only by a small remnant of Lower Fine Sand. At about 30 ft. north of this hole and east and west of it, several bores showed no stone, but about 3 ft. of dark, rather coarse sand over Chocolate Clay. Eastward of the bores above described, stone continues in the Park as far as a point opposite Spot 361 in the lane, where it crosses diagonally to the south side in consequence of the north-east trend of the lane and the falling level of the ground in that direction. North and north-east of the outcrop of the stone in the Park and at a lower elevation, Coarse Sand (5) either rests directly upon Chocolate Clay (2) or is separated from it by a foot or so of Lower Fine Sand. For example, at a point 55 feet north of the lane and rather west of Spot 361, a bore at O.D. 342 showed, under 6 inches of soil, reddish-brown Coarse Sand 5 ft. ; soft yellow and buff Fine Sand I ft. ; and Chocolate Clay bored to I ft. Further ~.N.W. of this point, at lower elevations, bores disclosed a normal succession of Lower Fine Sand (a few feet), Chocolate Clay and Atherfield Coarse Silt.

(4) Section E, about 365ft. west of E.M. 343·9. In consequence of its north-easterly trend, and the lower

F. GOSSLING,

374

DC TAIL

secno«

8

L------:::".--=c-::=====---'-----==-====:-::t",·

..

f.,urt .J6C

N

Seal.

01

Fef!'t

GENERALISED SECTION

A

_L..

W

£

A

L D

r

C L A

....J

Scale ,

orfeet so

V

FIG. 48.-SECTIONS ON THE LiNES

T B, C,

e'

and A.

STRl:CTURE OF BOWER HILL, XUTFIELD, SURREY.

375

elevation, the lane here lies wholly in Coarse Pebbly Sand (5). In addition to the five bore holes shown in Fig. 48 (Detail Section B) three others were made farther north-west. The series of eight holes gave the following results (omitting some minor lithological details), commencing at the north end: (i)

0.D.343. 120 feet from the lane; Atherfield Coarse Silt penetrated to 5 feet. (ii) O.D. 348. 90 feet from the lane; soil I foot. Atherfield Coarse Silt to feet. (iii) O.D. 351.75 feet from the lane; impure Coarse Sand 3~ feet; Silt with streaks of Chocolate Clay I foot; buff Coarse Silt to I t feet. (iv) O.D. 352. 67 feet from the lane (first in Fig. 48); clayey, dark Coarse Sand I t feet; Chocolate Clay I foot; Atherfield Coarse Silt to 2 feet. (v) O.D. 353. 60 feet from the lane; dark clayey Coarse Sand 3 feet; hard Chocolate Clay to 6 feet. (vi) O.D. 354. 53 feet from' the lane; dark clayey glauconitic Coarse Sand 5t feet; streaked white and dark buff and grey Fine Sand q feet; Chocolate Clay 15 feet; Atherfield Coarse Silt to I foot. (vii) O.D. 355. 40 feet from the lane; dark brown and red glauconitic Coarse Sand I I feet; buff clayey, extremely hard Lower Fine Sand t to I foot; Chocolate Clay to 2! feet. (viii) a.D. 357. 32 feet from the lane; red glauconitic \"Cry Coarse Sand with pebbles 15 feet; buff and white Lower Fine Sand 5 feet; hard Chocolate Clay to I foot.

3r

(5) Outcrops between Section line Hand B.M. 343.9. As already stated, the further fall in the level of the lane and its N.E. trend bring the Chocolate Clay across the lane eastward of Section B. On a line about 120 feet ~.E. of the line B, a bore south of and 5 feet above the lane disclosed 3 feet of Coarse Sand followed by 8 feet of Lower Fine Sand, and then dark Chocolate Clay. Another bore on the steep slope south of the lane, about 25 feet from the lane and about 30 feet south-west of the bore last mentioned, showed upwards of 7t feet of Coarse Sand, while the lower half of the slope below it consists of Lower Fine Sand. The outcrops of Lower Fine Sand, Chocolate Clay and Atherficld Coarse Silt in the remainder of this area and round to the north side of the Lake, as mapped, were ascertained by means of about 20 borings. Near the line where the Atherfield Clay-Weald Clay junction crosses the Lake, on a mound which was probably made of material thrown out in the excavation of the bed of the Lake, a number of large blocks of highly fossiliferous calcareous stone occurred. The fossils have not yet been determined, but they appear to be of Atherfield Clay age. Similar fossiliferous stone has been recorded at the base of the Atherfield Clay at Woodhatch, Reigate>, and at Brockharns : and it was also exposed near the north bank of the River Mole at Betchworth in 1932. I Butler, G. \V. 1922. The Perna Bed and Weald Clay. Proc. Geol. Assoc., vol. x xxii i, 2 Gossltng, F. '929. The Geology of the Country around Reigate. Proc. Grot. Assee" \01.

xl.

PROC. GEOL.

Assoc .. VOL, XLVI.,

PART

3, 1935.

37 6

F. GOSSLING,

(6) The North-East Outcrops. The outcrops of the Weald Clay, Atherfield Coarse Silt, Chocolate Clay and Lower Fine Sand in this area, i.e., east and north of the East Ridge, were determined by upwards of 30 borings. They do not call for description, as the succession is everywhere normal. (7) The East Ridge. . From a point at O.D. 350 near the letter" D " in " Ridge," a ridge rises westward to about O.D. 386 at about 120 feet short of the north-west fence on the map, and the crest narrows to about 10 to 20 feet across. Eastward and south-eastward of the point first mentioned, the ridge broadens and flattens into the east end of the hill, which falls with increasing rapidity to the road near Spot 274. On the north side of the ridge the descent is precipitous, while on the south side it falls sharply at first and then gently towards the footpath (see Section C). About a dozen borings along the crest of the ridge proved it to be wholly typical Lower Fine Sand, except at the west end, where the ridge broadens out south-west to join the west ridge and is composed of Coarse Sand with much broken Top Stone. Several short lines of bores down the north flank of the ridge proved the continuance of the Lower Fine Sand in that direction. But on the south side, starting from the west end of the ridge, the slope, south of a line gradually diverging from the ridge, is composed of broken Stone and Coarse Pebbly Sand. From the footpath the line is drawn south-westwards to the east-west fence. The significance of the juxtaposition of the Top Stone and Coarse Sand with Lower Fine Sand along this line is brought out by Sections C and ex (Fig. 48). (8) Sections C and cr. Section C is drawn from the outcrop of the Chocolate Clay on the north-east side of the hill, through the East Ridge and the east end of the dislocated mass. Section ex shows the critical borings on a larger scale. At the north end the Chocolate Clay was entered at O.D. 323 and penetrated for 8! feet without reaching the bottom, but Atherfield Coarse Silt crops out farther down the slope. Estimating from the ascertained levels of the Chocolate Clay to the east and west of line C, it is believed that the Clay continues to rise towards the East Ridge before falling southwards. As already stated, the Ridge and its north slope are composed of Lower Fine Sand. But on the south side several lines of bores, comparable to Section C, disclosed a sharp junction between the Fine Sand and Stoney Coarse Sand. Along that part of the Section line C, which is comprised between' the

STRUCTURE OF BOWER HILL, NUTFIELD, SURREY.

377

crest and the footpath, and is illustrated in more detail in Section ex (Fig. 48), the following data were obtained. The crest here is composed of hard, very pale, clean sand of the Lower Fine Sand type. At 30 feet south and at a.D. 370 under I foot of soil, the Fine Sand was so hard as to make penetration very difficult. At 15 feet farther south (45 feet from the crest). and at a.D. 367, one foot of sandy soil and dark buff, fine to very Coarse Sand with some pebbles, occupied the first 3 feet of the bore; then followed t foot of laminated clayey white and buff Fine Sand; then t foot of more uniform white and buff Fine Sand; and finally normal Lower Fine Sand was penetrated for 2 feet. At 12 feet farther south and at a.D. 365, under I ~ feet of soil, came 5! feet of dark buff fine to very Coarse Sand with lydite pebbles, and getting finer in the lower part; then t foot of orange and very pale buff clay with streaks of fine sand; t foot of pale buff silty sand, I foot of laminated orange and white Fine Sand, and finally normal very pale buff and white Fine Sand. At 25 feet farther south (82 feet from the crest). at a.D. 362, under 2 feet of sandy soil with stone and pebbles, came 5 feet of dark buff very Coarse Sand, with lydite pebbles, followed by 2 feet of similar sand, below which cherty stones stopped the auger. At another 25 feet south (107 feet from the crest) at a.D. 360, under I foot of sandy stoney soil, came zt feet of very Coarse glauconitic buff Sand. Finally, at the east-west fence and footpath, 132 feet from the crest, and at a.D. 360, clayey Coarse Sand with pebbles was bored to 3t feet. A little farther west of this point, bands of stone cross the footpath obliquely. Thence to the foot of the steep slope dark Coarse Sand with stones of Top Stone type and difficult to penetrate occupy the ground. The slope ends in a steep bank and a hedge. To the east of the section line at this point the junction of the Atherfield Coarse Silt and Weald Clay occurs, and to the west and a few feet lower, Weald Clay is obvious at the surface.

(g) The Outcrops on the South Side of the hill. Chocolate Clay was proved in the field west of Bower Hill Lane at about a.D. 288, under Lower Fine Sand. In the Lane, Fine Sand rests upon a variable thickness of Chocolate Clay mixed with some Fine Sand. East of the lane, however, the Fine Sand was found to come down, as mapped, across the Chocolate Clay, then across the Atherfield Coarse Silt, and finally, upon the Weald Clay. A little farther cast, where slipping due to springs has occurred, the surface of the Weald Clay at about 30r a.D. extends well under the steep slope of the hill and is covered by Coarse Pebbly Sand. From this point for about 700 ft. eastward, a broad sheet of Coarse Pebbly Sand containing numerous fragments of Top Stone, spreads some 200-3°0 ft. southwards over the Weald Clay. This sheet is continuous with the mass of the same mixture which covers the steep south slope of the hill between Bower Hill Lane and the eastern extremity of the displaced beds. It has apparently spread downwards from the Coarse Sand and Top Stone at the crest of the hill, and it may have been an original feature of the landslip. This mass completely

F . GOSS U l\ G,

obscures the outcrops of the lower beds as exposed in Bower Hill Lane, so that it is doubtful how far eastward from the lan e they are present beneath it . Still farther east of th e area ju st described , the stee p south slop e of the hill is composed of Lower Fine Sand , succeeded to the south by Atherfield Clay and Weald Clay . This normal succession is continued, as sh own , round the east end of the hill t o the north-east ar ea alrea dy described. VI. DISLOCATIONS. (1) The Outer Boundary. In tracing the outcrops at th e west end of th e hill it has been shown that in H ogtrough Lan e there is a brea k in th e st ra tigraphical su ccession bringing the upper part of t he Coarse Sand down upon the lowest part of the Lower Fine Sa nd. Th e lin e of discontinuity has been followed round the west end of the hill (Sections D, E and F ) [Fig. 47J. On the north side of the P ark the line, after crossi ng H ogtrough Lane, was picked up east of Section G and followed along eastward as far as Section B . In this area the remnant of Fine Sand is very thin and in part of it the Coarse Sand rest s upon th e Chocolate Clay. It has also been shown th at in Sandy Lan e the line of discon tinu ity ap pears again along the s tee p northern slope of th e hill. Frcrn th e west end of th e East Rid ge th e line has been traced along th e south side of the ridge round to th e sout h side of the hill , th ence along th e sou t hern bord er , and finally t o t he west across Bower Hill Lan e. The lin e thus encloses the whole displaced mass.

(2) The Inner Line. An oth er line of dislocati on has been referred to in the description of the tw o qu arries west and east of the north end of Bower Hill Lane. The app roximate ly horizontal St one Beds in th e west qu arry obviously cross the lane into th e park and have been definit ely proved there, for exa m ple, a t th e upturned tree. The crus hed beds north of the dislocation in the more easterly qu arry have been worked to a depth of about 5 ft. bet ween the lane and the quarry, and th e shallowness of the workings there sugges ts that the beds ha ve flattened out so that they have no great thickness north of the qu arry. In the Park they are soon cu t off by the slope of the ground at about G.D. 340-350, whi ch is consisten t with an approximately horizontal position northward of the quarry . It must be Em phasized here that th e sto ne in this northern block is identical lithologically with that in the highl y-inclined southern block, and cannot be confused with the Lower H ythe Sto ne or with the sto ne of th e Sandga te Series.

STRUCTURE OF BOWER HILL,

~UTFIELD,

SURREY.

379

The line of discontinuity through the two quarries obviously continues both east and west. If it is extended westward as far as Section F, it comes out there south of the horizontal stone at about the position of the" rubbly stone." Eastward. the line comes out on the top of the hill. At its north-eastern end the northern block of stone comes to an end above the slope of the ground in that direction, so that a considerable thickness of the Coarse Pebbly Sand is exposed beneath it. Thus, as seen in Plate 30 B, the northern displaced mass does not extend as far eastward as the East Ridge. There are thus two lines of dislocation, one surrounding the whole displaced mass, and another separating a western and northern subsidiary block from the main mass. An attempt is made later to account for the inner line of dislocation, for the existence of the northern horizontal block, and for its terminations, The Angles of Emergence of the Surfaces of Dislocation. The Northern Side. (a) As shown in Section D, the slip surface, marked by the ironstone band, emerges at about a.D. 320. This point is IS ft. from the top of the lane bank and about 142 ft. west of the ~.-S. fence. In the five following bores the slip surface falls to a.D. 305 at 57 ft. from the outcrop, or 15 ft. in 57, i.e., the slope is about 15 degrees southward. In the next two holes the surface falls to a.D. 296 at 135 ft. from the outcrop, i.e., 24 fr. in 135, or an average of 10 degrees southward. (b) In Section E the fall is from a.D. 328 at about 8 ft. from the lane bank and 24 ft. from the E.-W. fence to a.D. 318 in the fourth hole, 52 ft. from the outcrop, i.e., 10 ft. in 44. or at an angle of nearly 13 degrees. (c) The mechanical junction between the Coarse Sand and the Lower Fine Sand in Hogtrough Lane is seen to dip southward at a considerable angle, but in consequence of the narrowness of the lane and the steepness of the bank-cliff, it was not feasible without impeding the traffic, to make a trench sufficiently far into the bank to enable a reliable measurement of the dip to be made. (d) The Section at the upturned beech tree, already described {po 373), shows the mechanical junction of the Top Stone and Sand with the Fine Sand at a.D. 336. The horizontal line of the junction in Hogtrough Lane goes underground at about a.D. 327, about 200 ft. west of the upturned tree. If the line continues eastward horizontally, it will pass south of the upturned tree at a distance of about 40 ft., so that the fall of the junction southward from the tree will be 9 in 40, or nearly 13 degrees in this locality. (e) In Section B (p. 374). neglecting the superficial bores, the (3)

A.

F. GOSSLING,

junction of the Coarse Sand and Fine Sand is clearly shown by the last three bores. The distances between them are 7 and 14 ft., respectively, and the levels of the junction are 348.5, 344 and 342 a.D. There is thus between the first two a fall of 4! ft., in 7 ft., and an average of 6~ ft. in 21 ft. between the first and third holes. That is, the angle of emergence of the slip surface at its steepest is I in 1.55, or more than 32 degrees; or, taking the average of the three holes, the fall is I in 3.33 or more than 17 degrees. (f) Farther north-east, south of the lane, the junction of the Coarse Sand and the Fine Sand occurs across the slope of the hill, but the combined effects of the unusual hardness of the Coarse Sand, the steepness of the slope and the high angle of the junction rendered it impossible to reach the Fine Sand by bores beyond 25-30 ft. south of the lane. Several holes within these limits indicated a high angle of emergence of the slip surface. (g) In Section C' (p, 375) the first hole gave Fine Sand at a.D. 369 ; the second hole gave the junction of Coarse Sand and Fine Sand at a.D. 364 ; in the third hole the junction was at a.D. 358 ; and in the fourth hole the junction was not reached at O.D. 352. The figures arc tabulated below, together with the distances between the holes: a.D. of Bore. a.D. of Fine Sand. Distances. Bore I 37 0 369} 15) Bore

2

367

364}

12

f27

ft.

Bore 3 365 358 The dip of the surface of the ,Fine Sand between bores I and 2 is 5 in 15, or 18 degrees; that between bores 2 and 3 is 6 in 12, or 25 degrees; that between bores I and 3 is II in 27. or 22 degrees.

B.

The South Side. The borings on the south side were not primarily directed to ascertaining the angle of emergence of the slip surface, but from the data recorded some information may be culled. (a) At the south end of Section C, the rapid rise of the surface in the bank and up the slope and the stoney nature of the ground precluded boring deeply enough to disclose at what angle the Weald Clay or Atherfield Clay surface emerges. (b) In the large southward bulge of the Stoney Coarse Sand. near the line A, the bores indicated a southerly slope of the Weald Clay surface of about 6 degrees. But it is not probable. in view of the following paragraph, that so large an angle continues to the north under the steep slope of the hill.

STRUCTURE OF BOWER HILL, NlJTFIELD, SURREY.

381

(c). North of the next field to the west, in the re-entrant angle where, as already mentioned, slipping of the Coarse Sand has occur red, Weald Clay was found about a.D. 301, under 5 feet of Coarse Sand. Attempts to make bores farther up the slope to the north were frustrated by the stoney nature of the ground. But bores made along a line 40-50 feet farther south and 100-200 feet west showed Weald Clay above the 300-foot contour line. In this area, therefore, the Weald Clay surface appears to be approximately horizont al. (d) In Bower Hill Lane th e bores which prove th e outcrops of the Lower Fine Sand and the Chocolat e Clay were too shallow t o indicate the angle between them. But a bore at O.D. 308, 130 feet north of a Chocolat e Clay outcrop at O.D. 294. showed Fine Sand down to O.D. 284. Water prevented the Chocolate Clay being reached, but the character of the Fine Sand and the abundance of water indicated that day was very near. Using these figures, i.e.. a fall of 10 feet in 130, the minimum angle of the surface of th e Chocolate Clay comes out at 4k degrees north. Allowing for a possible 5 feet more of Fin e Sand, the angle would be about 6 degr ees north.

Th e Inner Line oj Dislocation. In the qu arries or pit s west and east of Bower Hill Lane , already described, th e hori zontal and crushed Stone Beds abut aga inst th e top sur face of the highly inclin ed Stone Beds, so that the surface between the tw o masses of stone coincides with the surface of th e inclin ed St one Beds, i.e., it dip s north at about 65-75 degrees.

C.

D . Th e F orm and Composition oj the Surface oj Dislocation. The form of the surface on which the displaced masses have moved can be reason abl y. inferred from the facts which have been ascertained , alt hough it has not been possible to reach it where it is most deepl y buri ed. It is a curved surface rising from south to north. On it s south border it varies from horizontality, but by a few degrees north or south. It is inferred that under the hill the slope must increase northward s until it joins the observed steep southerly dips on th e northern border, which range from 10-25 degrees or even 32 degrees. While the surface is composed of Weald Clay or Atherfield Clay at its south border, it passes up into the Lower Fine Sand under the hill. In the present denuded state of the Outlier it emerges into the air near the base of this stratum at th e west end of the hill and about 10 feet up in Hogtrough Lane. Along part of the north flank of th e hill the surface is within a foot or two of the base of the Fine Sand, and even cuts it out altogether so that it is on the Chocolate Clay. At Section B and eastward,

F. GOSSLDIG,

the surface is rising again in the Fine Sand, and south of the East Ridge it attains a high horizon therein. VII. A SUB-HYTHE PERICLINE. A consideration of the a.D. levels and dips of the Weald Clay and Atherfield Clay surrounding the hill brings to light a sub-Hythe structure of a periclinal nature, the longer axis of which lies under the northern slope of the hill in a direction a little north of east along the western half, and more nearly east along the eastern half. (1)

The Weald Clay. At the Lake the Weald Clay outcrops at about a.D. 297-300, and appears to have a very low northerly dip in this neighbourhood. The flat or region of low north dip appears again at the foot of the north-east end of the hill, and extends towards Little Cormongers Farm, the level of the outcrop in this area being about a.D. 290. an the south-east side of the hill the Weald Clay outcrops in Section C at a.D. 30r. Along the south border the outcrop is covered by the displaced mass, but it reappears at about 300 a.D. as Bower Hill Lane is approached. In the lane it occurs at a.D. 284 and at the west end of the hill at the same elevation. From the foot of the escarpment north of the lake to the outcrop in the north-west flank of the hill the Weald Clay outcrop rises southward at an average angle of 2~ degrees, but in the southern half of this stretch the average dip is 5 degrees north, so that the northerly dip is beginning to rise quickly as the hill is approached. A sudden reversal, however, of the dip is shown in Section E, Fig. 47, where the Weald Clay falls from 310 to 3()2~ O.D. between the second and third bores, in a distance of 25 feet, giving a southerly dip of about 17 degrees. The surface of the Weald Clay also rises from the west end eastwards, for it is at a.D. 286 at the west end of the hill, 310 on Section E, 321 north of the upturned tree, and it is estimated at 332 on Section B; but beyond this locality it falls again to a.D. 290 at the east end of the hill.

(2)

The Atherfield Clay. It has been already mentioned that the strata in Hogtrough Lane below the stratigraphical break have a westerly dip of about 4 degrees. At the extreme west end there is also a slight southerly dip. In Section D, the Chocolate Clay falls from about a.D. 315 in the lane to a.D. 295 in about 170 feet, or at more than degrees south.

6t

STRUCTURE OF BOWER HILL, NUTFIELD, SURREY.

383

In Section E, the Chocolate Clay falls from O.D. 318 to 315 m about IS feet, i.e., at about 10 degrees south. In Section G, the fall is from 0.0. 326 to 316 in about 40 feet, giving a dip of about I4 degrees to the south. In Section B the fall of the Chocolate Clay in holes (v) to (viii) gives angles of 23, 30 and 23 degrees, or an average of 25 degrees to the south. In the same section the fall of the Atherfield Coarse Silt in holes (iv), (v) and (vi) gives angles of 33 degrees and 60 degrees, or an average dip of 51 degrees south. On a line about 75 feet east of Section B, the Atherfield Coarse Silt falls at an angle greater than 34 degrees to the south. Across the extreme east end of the hill, outside the area of dislocation, the borings show that the Chocolate Clay has a southerly dip of about 2-3 degrees. In an cast-west or axial direction the outcrop levels and elevations in the borings of the Atherfield Clay show a rise eastward to the middle of the hill, followed by a fall to the east end. Thus, the Chocolate Clay rises from O.D. 306 at its western outcrop to O.D. 315 in Hogtrough Lane on Section D. It reaches 319 in Section E, 326 in Section G, 335 at the upturned tree, 355 N.W. of Spot 361, 350 on Section Band 356 east of Section B. On part of the north-east flank the Chocolate Clay is inaccessible, being covered by the neck of Lower Hythe Sand which connects the hill with the escarpment. At the north end of Section C, the Chocolate Clay occurs at O.D. 323, but as already stated it is believed to rise towards the East Ridge to at least 0.0. 333. From Section C it falls eastward to about 320. The observed maximum rise from the west end is therefore about 50 ft. and the fall to the east 26 ft. Summarising the foregoing observations, it appears that within a long oval area of Weald Clay, the boundary of which is at a level of 0.0. 290-300 ft., the Weald Clay and Atherfield Clay rise from the east and west ends of the hill to an elevation about 50 ft. higher along the middle portion of the north flank. In a direction approximately at right angles to the longer axis, the clays rise from the northern flat with an increasing northerly dip, and then experience a sharp reversal of inclination, giving dips to the south varying from low angles of the order of 2 degrees at the east and west ends, to 20, 30 and even 50-60 degrees, along the middle part of the longer axis. The steeper dips doubtless quickly diminish under the hill, so that the strata flatten out and emerge on the southern border approximately horizontally. But it was not possible to follow the steeply dipping beds down further than 25 ft. with manually operated augers, especially as the steep dips

F. GOSSLIKG,

coincide with a rapid increase in the slope of the ground in the opposite direction. VIII.

SUMMARY OF RESULTS OF THE INVESTIGATION. (i) Bower Hill on its south side below the steep southem slope, at its west end, on the lower part of the N.-W. slope, on the whole of its N.-E. slope, including the crest of the East Ridge, and in its east and south-east areas, is composed of the regional rocks £n situ, viz., Weald Clay, Atherfield Clay, and more or less of the Lower Fine Sand. (2) An outer boundary of discontinuity has been traced separating an outlier of Hythe Beds from the beds in situ. (3) The outlier consists of two masses, viz., (a) a main mass which in its central part presents almost the entire Hythe Series; and (b) a north to north-western mass which comprises only the Top Stone Beds and part of the Coarse Sand. (4) The strata of the main mass have a northerly dip, especially along their northern border, while the north-western mass is approximately horizontal and abuts on its south side against the steeply inclined surface of the main mass. (5) The surface on which the double outlier rests is curved in its N .-S. direction, rising from approximate horizontality on its south border to a slope of 10-32 degrees south at its. emergence on the north side. Here it is based on a variable thickness of the Lower Sand, or in part upon the Chocolate Clay, while on the south side it is based on Weald Clay or Atherfield Clay. (6) The line of emergence along its northern course lies to the south of the longer axis of a sub-Hythe peric1inal feature, which is asymmetric in the transverse direction, and presents dips to the south of as much as 30-50 degrees in the central part of the anticline. (7) The upper part of the Atherfield Clay has been found to be a chocolate-coloured clay of a homogeneous texture and a tough consistency. It appears to be much thicker in the region of the steep dip of the periclinal feature than in the main escarpment, but it is known to thin out westward and southward. IX. INTERPRETATION. In connection with the general description of the Map and Sections it has already been stated that unbroken curves mark the outcrops round and within the hill. In the author's view, such outcrops are inconsistent with the idea of a regional fault running across the district and cutting off a number of outliers, including that of Bower Hill, from. the escarpment. If, however, one takes into account other forms

STRUCTURE OF BOWER HILL, :-.:eTFIELD, SURREY.

385

of faulting which could theoretically be invoked, however remotely applicable to the present case, lag-faults I and very low-angle thrusts from the south may be mentioned. But the existence of such structures in the northern Weald has yet tobe demonstrated. A reversed fault of the ordinary form requires a northward hading thrust plane at a considerable angle, whereas it has been shown that the surface of dislocation hades to the south all along its north border. Few reversed faults in the northern Weald appear to have been described. The well-known fault at Quarry Hill, Tonbridge, brings Ashdown Sand up about ISO ft. against Wadhurst Clay. It has been described by S. Hall" as hading to the south at a fairly high angle, and it was figured by E. W. Handcock.s Such a structure can have no application at Bower Hill. There remains the hypothesis of a large scale landslip. One of the most interesting and complete investigations of the characteristics of this type of dislocation is that of the Warren, between Folkstone and Dover, which has been described in our Proceedings by C. W. Osman.s Osman's results may be summarised as follows:(I) It was found experimentally that although the Gault Clay, when supported on all four sides, has a practically infiniteresistance to pressure from above, it cannot sustain without flowing a pressure of more than 5-6 tons per sq. ft. when supported on three sides only. (z) The form of the surface of the soles beneath the slipped masses is a curve having a high angle on the inner side and a low or even reversed angle on the outer side of the slip. (3) The slipped masses have a backward tilt reaching 30 degrees to 43 degrees. (4) The width of the slipped masses varies from about zoo feet to 300 feet. (5) The resistance of the talus makes the outflow of the Gault and the settlement of the slipping masses a very slow process. (6) The slips. occurred notwithstanding a landward dip of the strata, being: hastened by the action of the sea in carrying away talus. Landslips of a similar form are described and illustrated in the Alresford Memoir.t Linton" has also given an informative description of landslips of the Malmstone over Gault between Binstead and I A thrust from the south was suggested during the discussion of the paper and is dealt with later in that connection. 2 Hall, Sidney. 1930. Field Meeting at Tonbr idge and Leigb. Proc, Geol. Assoc.~ vol. xli., p. 92. 3 Handcock, E. W. 1909. Excursion to Tonbridge. PrO&. Ged, Assoc., vol, xxi, p. 229. 4 Osman, C. W. 1917. Proc, Geol, Assoc., vol. xxviii. 3 White, H. J. Osborne. 1910. The Geology of the Country around Alresford. Mem. Geol. Surv. 6 Linton, D. L. 1930. Notes on the Western Part of tbe Wey Drainage System. Proc; Geol, Assoc., vol, ali ,

F. GOSSLING,

Langrish. These display a similar concave shape of sale, and a backward tilting of the slipped Stone Beds, which have settled down with little disturbance. It will be apparent that the Outlier of Bower Hill presents the main characteristics of landslips. Of great importance is the relatively high dip of the slip surface along its upper margin, the very low or possibly in part reversed dip on its lower boundary, and the backward tilt of the main slipped mass. The last-mentioned feature has been accentuated by the weight and thrust of the second mass. Even in the present denuded state of the Outlier, the slip surface on its northern margin shows angles of emergence up to more than 30 degrees. They are, moreover, lower where denudation has been most severe, as at the west end, and they therefore suggest that the original cliff or shear face formerly increased to higher angles, such as are shown in Osman's figures. The predisposing conditions for a landslip are to be seen in (I) the periclinal nature of the underground structure with its southerly dip increasing rapidly from the ends of the hill towards the middle; and (2) the mobile qualities of the homogeneous Chocolate Clay, which enable it under suitable conditions to flow and spread until it is only a few inches thick, Dr to thin out entirely. Granted the formation of a sufficiently steep scarp face by denudation of the Weald, and possibly a waterlogged condition of the strata due to snow caps during one of the major glaciations, the structural and lithological conditions, above referred to, would be adequate for the development of a fracture or fractures which would extend upwards. This would lead to a subsidence of a slice of cliff, possibly several hundred feet wide. The settlement was probably slow and intermittent, as is evidenced by the comparatively unbroken character of the main central mass, although there are many indications that this is much more broken up than appears at first sight. The existence of two lines of dislocation, as shown on the Map (Fig. 46), suggests that while the subsidence was in progress, a second but smaller slice of cliff, mainly from behind the western half of the original slip face, settled upon and partly covered the original mass. This appears to have involved only the higher beds of the Hythe Series (see Map and Sections), and is now represented by a fragment of the highest Hythe beds, by the northern block of more or less horizontal and broken Stone Beds, and by part of the Coarse Sand, all of which intervene between the highly inclined Stone Beds at the top of the hill and the Lower Fine Sand, and/or Chocolate Clay outcropping along the northern flank. A diagrammatic representation of an intermediate and the final stage of the two slips is illustrated in Figure 49. Landslips in recent times of considerable area along the

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387

Lower Greensand escarpment, on the border of Surrey and Kent, are known. Topley' refers to slips ncar Crockham Hill in 1596, one involving 9 acres of ground; and at Toys Hill in 1756. It may be mentioned in this connection that the quarrying in the Top Hythe Stone Beds on Limpsfield Common has revealed a fissure 3 to 4 ft. wide and of unknown depth, running some hundreds of feet parallel to the edge of the escarpment and several hundred feet from it. There is ample evidence on the slope and at the foot of the escarpment south of Kent Hatch of former considerable slips. Other slipped masses have been observed farther east, near Ide Hill, etc. There is also an ...... ...... 5b

3

W

E:

A

L

D

Origlnsl bedding plenes /nt;ermediate positions of beds 5 to 6b - - Final positions of beds 3 to 6b

c First: shde surrsc« Second shoe surtsce - - -

FIG. 49.-DIAGRA:-I OF II'TERMEDIATE AI'D FHiAL POSITIONS'OF THE SLIPS.

obvious fissure in the Stone Beds at the top of the lane, already referred to, running down the escarpment from the Schools at Nutfield from which water runs out in wet seasons. Section A is a generalised and somewhat diagrammatic representation of the structure of the hill. It has been compiled from data obtained from the north flank of the hill, from the quarries, from Bower Hill Lane, and from the southern flank in its most extended direction. It has been drawn approximately to a natural scale.

x. ACKNOWLEDGMENTS. In conclusion, the author records his sense of indebtedness for invaluable help in the prosecution of the investigation and ,

Topley, W.

1875.

The Geology of the Weald.

Me",. Geal. Su",., pp, 3,6 and 448.

F. GOSSLING,

in the presentation of the paper. Generous assistance in the form of grants by the Research Committee of the Geological Section of the British Association on Critical Sections in the Tertiary and Cretaceous rocks of the London area made it possible to meet the cost of labour and other expenses connected with the borings. Mr. Fairall, of Godstone, arranged for the provision of men and cartage, loaned some of the tools employed, and facilitated the work in other ways. Permission to carry out the excavations on his estate was kindly given by O. P. Davis, Esq., of Nutfie1d Priory. Much encouragement was afforded by members of the Weald Research Committee. To Mr. A. ]. Bull special thanks are due for the excellent scenic photographs and lantern slides which graced the presentation of the paper. Finally, it would be difficult to exaggerate the author's appreciation of the ever- ready help of Dr. S. W. Wooldridge in advice and discussion and of his kindness in reading the paper to the Association.

DISCUSSION. Mr. DI~ES stated that when surveying the Nutfield area he was aware ·of Mr. Gossling's view that the outlier was due to landslipping. He was unable to find definite evidence for this theory and would have left the matter open had this been possible. The necessity, however, of completing the map rendered a decision imperative. The bulk of evidence then available appeared to favour structural faulting. Mr. Gossling had brought out much new evidence, and the speaker agreed that he has clearly shown the true position of the outcrop of the fault plane, and also that the hypothetical section shown at the foot of the one-inch geological map was not applicable to Bower Hill. It was to be remembered, however, that the borings were chiefly shallow ones, merely proving the outcrop of the various divisions of the Hythe Beds, whereas the sections shown were carried to considerable depth and were constructed on the hypothesis that the mass was landslip. Using the same evidence it would probably be possible to constru.ct sections 'showing the dislocated mass to be due to a thrust from the south, and consequently the speaker was still of the opinion that the point was not -defi nitcly cleared up. The form of other landslips in the Weald, and the fact that the district is on one of the lines of disturbance are points that -demand consideration. He expressed his admiration of the enormous amount of detailed work NIr. Gossling had put into his survey, and thanked Dr. Wooldridge for his dear explanation of the paper. Mr. F. H. EDMCNDS congratulated the author on his interesting conelusions, and also Dr. Wooldridge on his clear presentation of the paper. He felt, however, that the landshp hypothesis did not adequately explain the formation of Bower Hill. It was an important fact that this topography was totally unlike that of known landslips in the Wealden area, and, indeed, of landslip topography generally. Elsewhere along the Lower Greensand escarpment slipped masses broke up into innumerable -smaller parts, in conformity with the general habit of landslips. Were Bower Hill a landslip, an extremely large mass appears to have slipped without disintegrating. With regard to the sand and clay at the base of the sand, he was not disposed to consider that necessarily to indicate a stratigraphical horizon.

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389

Such clays occurred in many places at the base of the sand, both in the Lower Greensand and in the Hastings Beds, including the Weald Clay. Dr. S. E. HOLLl:-lGWORTIl congratulated the Author on the completion of a very thorough and painstaking piece of work that had cleared up the outstanding questions concerning the mapping of the outcrops of the formations that make up Bower Hill. The ongin of the present distribution by the Author is clearly a matter of interpretation, and although the speaker claimed only a casual acquaintance with the region, the alternative explanation of thrusting from the south appeared to him worthy of further consideration. South of the low anticlinal fold in the undisturbed beds of Bower l Lill there must be a complementary syncline as the regional northerly dip off the Wealden axis comes in again farther south. A low angle thrust carrying northerly dipping Hythe Beds of the southern limbs of this syncline on to lower beds of the anticline to the north would with subsequent dissection account for the distribution indicated on Mr. Gosslings map and sections. The dips, perhaps originally gentle, in the overthrust mass would be expected to be steepened considerably by drag where seen near the thrust plane as at Bower Hill, Tilberstow, etc. The hard band encountered in boring at the plane of dislocation could be considered to favour a thrust rather than a slip; had the Author any evidence of such hard bands being associated with planes of slip' The absence of landslip topography and the unbroken character of the displaced mass, remarked upon by Mr. Edmunds, tended to support this alternative, while the slight complication introduced by the second" slipped" mass of the Author could, of course, be fitted into either theory. Dr. S. W. WOOLDRIDGE, in replying to the discussion, emphasized the fact that :VIr. Gossling had undertaken this work not as the exposition of a hypothesis, but with a view of ascertaining the facts of rock attitude or geometry as fully as was possible by boring. While opinions might differ as to the lithological details of the succession and the correct attribution of the material raised from the borings, the main fact merging appeared to be beyond question and was independent of any personal opinions on lithological detail. This fact was the essentially stratiform relation of the mass, sand above clay, and the clear demonstration that the high dips prevailing near the surface terminated downwards against a plane of dislocation. This plane might represent a slip from the north or a thrust from the south, though the former seemed a simpler and preferable hypothesis. In any case it appeared that no vertical or steeply inclined fault-plane existed. The speaker agreed with Mr. Edmunds that the" chocolate clay" might occur at more than one level in the Atherfield Clay when traced over a wide area, but he believed that over this limited tract it could be shown to be a most valuable and competent datum-horizon. He further suggested that the absence of the more overt indications of "l andsliptopography" reflected merely the great age of the slip, in virtue of which its shape had been smoothen and the valley separating it from the escarpment widened. Since slipping on a considerable scale was known to have taken place in fairly recent times along the Lower Greensand escarpment, we should be prepared to consider the possibility of older and even larger .. failles de glissement "-particularly in the" water-logged" phase of late Pleistocene times referred to by Mr. Gossling in his paper. Dr. Wooldridge added that a recent examination of the undercliff between Ventnor and Niton had impressed him with the manner in which the combined effects of aligned slips could simulate those of faulting, and he called attention to other places where comparable phenomena could be scen. The AUTHOR desires to add a few observations to the discussion, especially with regard to Dr. Hollingsworth's suggestion of the possibility of a thrust from the south. It may be remarked that while a slipped

390

STRl:CTl:RE OF BOWER HILL, Nl:TFIELD, SURREY.

mass is a local phenomenon, a thrust hypothesis would necessarily have to fit a much larger area. Now a low-angle thrust must be such as to provide for the transfer of say, 20 to 40 ft. of the uppermost beds of the Hythe Series on to the top of the sub-Hythe pericline at about O.D. 350 frem some considerable distance in the south. Before displacement the beds in question were presumably at a lower elevation, and this condition requires that the syncline south of the pericline was deep enough to acccmmodate at least the lower three-quarters of the Hythe Series, even if the south hade of the thrust surface was zero. The greater the angle of the thrust the deeper the syncline must have been. Such a syncline would have brought the base of the Hythe Series to a level considerably below O.D. 200. But the terrain south of Bower Hill, as far as and beyond the railway (half a mile to a mile) ranges fromO.D.300toabout a.D. 240-250, and it is composed of Weald Clay. The deep syncline postulated can therefore have had no existence, even ifno allowance be made for denudation of the Weald Clay to its present levels. It may also be observed that if imbrication of the thrust 111a,s is invoked to account for the surface of dislocation within the outlier, the hade should be to the south, whereas it is in the opposite direction. With regard to Mr. Edmund's objection on the ground of the absence of disintegration in the slipped mass, it may be recalled that there arc numerous references in the paper to the shattered condition of the Stone Beds genually, except in the middle of the main slipped mass, and to the large spreads of confused Coarse Sand and fragmentary Stone.