342 ART H UR L. LEACH, north-east, and commented upon the difficulty of conn ecting this with the Badshot knoll without assuming a fault in between ; but no such fault is shown on the map of the Geological Survey, nor has any decisive proof of its existence yet been found. After tea at Messrs. Ransom's, the relief map was again studied , and the question discussed of the causes which led to the diversion of the river, the Director maintaining that earth movement s had played no part in it, but that it was a simple case of river capture by an obsequent stream growing out from the sou th along the line of an old consequent river-the power of capture resting with the Wey by reason of its shorter course to the sea, and conse quent lower level at the critical point. Some Paleeoliths from T errace D were also shown, all in a water-worn condition, except two of a late St. Acheul type found on no other terrac e. Le Moustier, though present at a higher level, is apparently absent here. As there was no time left for further work in the field, the party proceeded directly to the station in time for the 7. 15 train. Mr. A. H. Williams acted as Excursion Secretary.
REFER EN CES. Ordnance Surv ey Map, I in. scale . No . 26g. Ord nance Su rvey Map, 6 in. scale, Surrey xxx, N .W. & N .E . Geo logical Su rvey Ma p, O ld Ser ies, No .8. Ig08. Y OUNG , G. W.-"The Chalk Area of Western Surrey." Proc, Grot. Assoc., vol. xx, part 6, pp. 435-8. Ig08. BURY. H._HNotes on t he River Wey." Quart. :J0ur1l . Geoi, Soc., vol. lxiv , p. 318. . IgIO. - - - -.-" The Relations of the R iver Wey to the Blackwater and the Arun." 7ran s. •'iouth-E astr1'n Union oj Sci. Soc., p. 14. 1913. ._H The Gra vel Beds of Farn ha m in Rela tion to Palreolith ic Man " Proc, Ceol A ssoc., vol, xxiv, p. 178.
GE OLOGICAL AND GEOGR APHICAL NOTES ON WELL HILL, KENT.
S EPT EM BER 11TH, 1915.
By A RT H UR L. LEACH, F.G.S., D ine/or oj the Excursion.
F ROM Chelsfield station the party walked over Well Hill into Shoreham, halting near Chelsfield Church, in the gravel-pit on Well Hill, beside a swallow-hole a little to the east of this pit, in Timberden Bottom, and on the eastern slope of Meenfold Hill, to observe various points of interest. After tea, at the" George Hotel," Mr. H. W. Monckton, on behalf of the party, thanked the Director for his services. T he Director's remarks on the geological and geographi cal features of the district are summarised below. GEOLOGICAL AND GEOGRAPHICAL NOTES ON WELL HI LL. 3-B
WELL HILL. Geographically this hill forms the summit of a ridge west of the Darent and between that river and its tributary, the river Cray. In its present stage the Darent, beginning as a strike-stream in the Vale of Holmesdale, becomes a consequent or dip-slope river only after its sudden northward deflection and passage through the North Downs. The consequent Cray, which rises now at Orpington, has a dry valley extending several miles above the stream-head to a gentle depression barely a mile west of the Darent Valley, and on the northern edge of the Clay-with-Flints area of the North Downs. From this point the dry valley of the Cray bends westward to Green Street Green, takes thence a
TrihuTory (dry)uolleys . R. Dare"r Magpie Bolio'" A CraY{dry}vo/ley Well Hill CarJerhursf HDarent 8
FtG. n.-PARALLEL SECTlQNS ACROSS THE VALLEYS OF THE CRAY AN[) DARENT.-A. 1. Leach.
Section A, through the plateau I mile south of Shoreham. Section B, I mile north of Shoreham. Section C, ro miles north of E. The letter M indicates the Maplescombe dry-valley.
Vertical scale, about r o times the horizontal. Length of section, 8 miles. northerly course to Orpington, and the stream, which here is formed by a group of springs, flows through St. Mary Cray, St. Paul's Cray, Foot's Cray, North Cray, Bexley, Crayford, and Barnes Cray, to join the Darent in the marshes below Dartford. The two valleys present many interesting geological and geo graphical features. The inter-stream district, an undulating and diversified area of about 50 square miles, falls northward from Well Hill (610 O.D.) to the Dartford Marshes (6 O.D.). In its geological aspect Well Hill may be described as an outlier of Eocene formations lying on the northern flank of the \V ealden anticline, To the south rises the feature-conveniently termed the plateau-formed by Chalk uplands (North Downs) overlain by Clay-with-Flints. The Chalk has a general dip north ward towards the synclinal axis of the London Basin. In the vicinity of the deep transverse valley of the Darent the Eocene formations, which west of the Cray have a broad outcrop and a 344 ARTHUR L. LEACH, well-defined southern margin here and there resembling an escarpment in character, have undergone great denudation, and in the inter-stream district Eocene deposits remain only as out liers, ranging from small patches of sands and pebbles to areas of more than a square mile in which Thanet, Woolwich, and Black heath Beds may be recognised. The larger outliers are well wooded in contrast with comparatively treeless adjacent areas of Chalk. At Well Hill, Eocene deposits control the cultivation and impart to the landscape a distinctive character. A narrow, steep-sided outlier of Thanet Sand extends for three miles north and south, with a narrower strip of Woolwich Beds on its central and higher parts. The cap of plateau gravel remains only in small disconnected patches, which, although probably occupying an area greater than that shown on the Geological Survey Map, are yet too limited in extent to affect directly the aspect of the ridge. From either side Well Hill appears thickly clothed with woods, orchards, and fruit gardens, the margin of the tree-clad area agreeing roughly with the boundary of the outlier. Well Hill, where the sub-Eocene Chalk might have been expected to show the characteristics of a high zone, supplies an instance of the value of zonal stratigraphy. Marsupites occurs on the west at Farnborough, and Uintacrinus at Orpington, to the north-west; but according to Mr. H. Dewey, of the Geological Survey, the Chalk at Well Hill is low down in the zone of M. cor-anguinum, or even in that of Holaster planus These zonal features indicate pre-Thanet Sand movement of the Chalk.'*' The Thanet Sand appears in several road-side sections, notably in Hollows (Hollard's) Wood. Broken shells of Cyrena, a characteristic Woolwich Beds species, occur in the north bank, where the road to Cockerhurst crosses the ridge, but these beds are poorly exposed. A brickyard, where Woolwich clays were worked half-a-century ago, has disappeared. Diversity of soils and good natural drainage have favoured the high cultivation of Well Hill and its surrounding district. Pears, apples, plums, bush fruit and strawberries, thrive on the outlier. Lavender is grown at Chelsfield, potatoes in large quantities on the Thanet Sand areas, grain on the chalk soils, and an important centre of cultivation under glass has developed near Swanley. The rainfall decreases with the northward fall of the ground from 35-30 in. on the adjacent plateau to 35'2-30 near Wen Hill, 30-27 near Swanley, 27-25 about Dartford Heath, 25-22 around Dartford. t
• Mem. Geol, Survey. "Summary of Progress of the Geological Survey for '9'4," p.28. t See Rainfall Map in Me... Geol, Survey," The Water Supply of Kent," by W. Whitaker. 'g08. CEOLOGICAL AND GEOGRAPHICAL NOTES ON WELL HILL. 345
THE WELL HILL GRAVEL. On the highest ground of Well Hill are several patches of coarse gravel. The greatest thickness of the deposit is unknown, but Woolwich Beds come within a few feet of the ground surface on the summit of the hill. Possibly, as at Shooter's Hill, the gravel rests on an irregularly convex floor, and thickens along the sides of the ridge. The constituents are large battered chalk flints, subangular and angular flint fragments, white and deep brown in colour, flint pebbles (probably from Eocene beds), small quartz pebbles, rolled and angular pieces of chert from the Hythe Beds (Lower Greensand). These materials lie confusedly in a coarse red sand. The isolated position of this gravel, the absence of northern rocks (such as occur abundantly in the High Terrace gravels of the Thames), and its elevation above the adjacent river-valleys, indicate a remote period for its formation, but its mode of origin and age remain unknown. Mr. J. R. Larkby" treated it as an old deposit of the Darent; Prestwich,t at least in his earlier view, regarded it as a relic of "a marine gravel, of Glacial age, which once stretched over Kent" All its materials may have come from the south, and two modes of transport may be suggested-ice and water. Evidence is altogether lacking that ice carried Hythe Beds cherts from the Lower Greensand outcrop, over the Holmesdale and across the Plateau to Well Hill, and under the present geographical con ditions water could not have done so. The gravel must have been deposited while a land surface, unbroken by a depression along the Gault outcrop, connected the Lower Greensand source of the cherts with Well Hill. Again, either ice or water may have borne the gravel down such a slope. There is no convincing evidence of ice-action, and under the conditions indicated, water carriage by streams appears an efficient and sufficient mode of transport. In its present state, however, the gravel may not he an undisturbed river-drift, for its materials, even if brought down and deposited by streams, may have been re-arranged by the sea upon an old plane of marine action. Nothing in the character of the gravel allows any light on this point. No palseoliths have been found in the gravel, but Mr. J. R. Larkby, in his interesting and well-illustrated report.] figures as eoliths several flints obtained from the pit on the summit of the hill. As the origin of the" eolithic" edge-chipping on flint remains in debate, the occurrence of eoliths cannot at present be accepted as evidence of a definite geological age. But an idea of the great antiquity of the deposit may be gained by considering
,; Proc, Geol, Assoc., vol, xix, '905-6. Excursion to Chelsfield and Well Hill. t lbid.; vol. tv, See the report by H. Walker, F.G.S., of an excursion to Well Hill in 18i4. I See above. PROC. GEOL. Assoc., VOL. XXVI. PART 5. 1915.] 25 ARTHUR L. LEACH, what great geographical changes have occurred since Lower Greensand cherts were incorporated in the Well Hill gravel. The Vale of Holmesdale and the escarpment of the North Downs are important land-forms developed after the deposition of the gravel, which thus must belong to a very early, and probably pre-Pleistocene, cycle of surface changes.
SWALLOW-HoLES ON "YELL HILL. Springs issue at the margin of the gravel-cap on each side of the Well Hill ridge, giving rise to streamlets, which in some instances have incised steep-sided valleys on the Eocene formations. On reaching the Chalk outcrop these valleys widen considerably, but no water now flows through them. The whole rainfall on Well Hill is absorbed directly or indirectly, and the ridge thus forms a watershed from which no visible streams proceed. An explanation of this anomaly lies in the geological conditions. Rainwater falling on the Thanet Sand or on the Chalk becomes at once absorbed; that which descends upon the highly permeable gravel, although thrown out as springs by the clayey Woolwich Beds, is finally and completely re-absorbed by the Thanet Sand and Chalk. The diagram illustrates the conditions :- Highly permeable Gravel. Impermeable Woolwich Beds. Permeable Thanet Sand. Highly permeable Chalk.
The re-absorption of the spring waters may be gradual, the streamlet merely dying away, or sudden, as when the drainage disappears in a swallow-hole. Two of these are mentioned by Mr. Whitaker," but others are known. One example, visited on September I r th, receives surface drainage only after heavy rains. A few years ago the bottom of the funnel-shaped hollow sub sided about 20 ft., but the pit has since been filled up with refuse from the adjacent farm. The streamlets in their courses towards the swallow-holes erode their channels and carry sand and silt down into under ground water-channels. Hence the upper rim of a swallow-hole must extend up-stream. The lower rim tends to be raised by soil-drift, and thus the funnel-shaped aperture may gradually move up the valley. By this process swallow-holes reach positions considerably within the boundary of the Thanet Sand. The diagram given above shows that the conditions are favourable for the shallow wells,whence, doubtless, the hill received its name. " "Water Supply of Kent," p. 50. GEO LOGICA L AND GEOGRAPHICAL NOTES ON WELL HILL. 347
MEEl\"FOLD HILLAND THE DARENT V ALLEY. Meenfold H ill, a steep-sided ridge of Chalk, cap ped by Clay-with-flints, is separated from Well Hill by a dry valley kno wn as Timberden Bottom. A pat h runs from Timberd en Farm through the woods on the ridge and emerges on the steep hillside above Shoreham in the valley of the Darent. T he view tha t opens suddenly to the south is very fine in its merely picturesq ue aspect, and when the geological significance of the physical features can be appreciated the landscape acqui res a high inte rest. In the distance, to the south, through the wide and deep gap by which the Darent breaks th rough the North Downs, lies the wooded ridge of Lower Greensand rocks. The gap in the Chalk contracts and becomes a steep- sided valley, through which the river goes northward towards the T hames. The diagram (Fig. 27) indicates the remark able change in the form of the valley as it passes from the high Chalk Plateau district to the neighbourhood of the Th ames, where loose Eocene sands and pebble-beds border the river. The gorge-like character of the passage through the North Downs contrasts strongly with the subdue d relief of the valley near Dartford Heath_ T he intermediate character of Section B, although somewhat obsc ured by the Well Hill ridge, brings into prom inence the geo graphical character of this relic of the high platea u. A shelf like feature at Cockerhu rst (Section B), describe d by Mr. Larkby as a "flood-plain," is connected by him with an early stage of the Daren t. There seems but slight evidence of flood-plain dep osits on this shelf, but it clearly lies with in the valley of the Darent, and dou btless represents an old base-level of river erosio n transformed into a shelf or rock-terrace by subsequent deepening of the valley.
RE FERENCES. Geolog ical Survey Map, I inch . London District (Drift) Sheet 4. Ordnance Survey Ma p, I inch, Sh eet 271. 18go. PRESTWICH, J. "On the Relation of the We stle ton Shing le," etc. Pa rt 3. Qllart. J ourn. Grot. Soc., vol, xlvi. pr. 155. 157. 170. rq ro, LEACH ,A. L. " N orth Kent," etc . Ceology in the Fidd, pr. 236, 246.