A . L. LEACH,

nata; C. clauigera ; B ourgue­ticrinus, Pentacrinus and I socri ­nus. (all columnars).

ANNELIDA.Serpul a am-pullacea ; S. plexus.

CRUSTACEA.Scal pellum maximum .

P OLYZOA. (Severa l species .)B RACHIOPODA .

T'erebratu lina striata; T erebra-

tu ta semiglobosa ; T . cornea,Rhvn chonella cuuieri : R . plica­W is .

MOLLUSCA .Ostrea oesic u laris ; O. lateralis ;Inoceramu s sp. ; Lima hoperi ;

Sbondy tus spi nosus ,FISHES.

Tooth of Lamna (?)

REFERENCES.

Geological Su rvey Maps, I -inch, Old Series, Sheet 8; London District(D r ift Map}, Sheet 4 .

Ordnance Survey 6-inch Map Su rrey.zo N. W ., a copy of which , geo logica llycoloured, may be seen in the Museum of Practical Ge ology,Jermyn ' Street.

1900. G. E . DIBLEY. -Zonal F eatures of the Chalk Pits in the Rochester,Gravesend, and Cro ydon Areas. Proc , Geol, A ssoc., vol ,xvi., p. 484.

EXCURSION TO THE NORTH DOWNS TN KENT.

SATURDAY, JULY 17TH, 1920.

Director :-ARTHUR L. LEACH, F .G.S.

Excursion Secretary:-Miss M. J. BASSETT.

(Report by the Director.)

THE party, 20 in number, left Otford at 12.30, walked to Becket'sWell and thence to a strong spring thrown out in the fields belowShore Hill. Turning then into a lane which led across thePilgrims' Way to Shore Hill (about one mile east of Otford),some time was spent examining the solution-pipes in the disusedchalk quarry. After an interval for lunch th e walk was resumedalong the Pilgrims' Way to a field-path running up the escarp­ment to Cotman's Ash and thence to Woodlands in the Maples­comb e valley. This track affords excellent views of the land­scape features of the Holm esdale, the Chalk escarpment, and thedip -slope towards the Thames. From Woodlands a lane ledpast Knockmill to the pit in Blackheath Beds on Oaklands estate,where the remarkable occurrence of whitened and disintegratedflint pebbles was examined and discussed at length. The partyreturned by the lanes to Otford Mount. On the way down by thefield-path into the village the interest and beauty of the prospectaroused general admiration.

After tea at The Bull Hotel, Mr. G. W. Young expressed thethanks of those present to the Director. The party left Otfordat 7.33 p.m.

At convenient points during the Excursion the Director-

EXCURSION TO THE NORTH DOWNS IN KENT. 37

drew attention to the geological conditions and to the verygreat interest and beauty of the landscape features, but as thebest point for a comprehensive view was not reached until theclose of the day, it seems desirable to gather together in one notethe substance of the remarks upon the physical features madeduring the Excursion.

LANDSCAPE FEATURES.

The route followed on this Excursion runs through a districtwhich affords excellent illustrations of the close relation whichmay exist between surface relief and geological structure.The east-and-west direction of the outcrops of rocks· of unequalresistance to denudation not only determines the east-and-westlines of the ridges and troughs, e.g., the Chalk and Lower Green­sand ridges bordering the Gault in the Holmesdale, but since thevegetation, self-sprung and cultivated, is related to the soils andthese in turn to the rock-outcrops, three very distinct zones maybe traced for many miles--the dry steep scarp slope of the Chalk,the meadows, hop-gardens and arable lands along the Vale,the tree-clad slopes upon the dip of the Lower Greensand. Thislong parallelism of outcrop, surface form and cultivation, makesthe Vale of Holmesdale probably unique in character. Similarfeatures continue beyond the limit to which the name is commonlyapplied.

From Otford eastward the field-track follows approximatelythe Chalk Marl; to the north rises the steep Chalk scarp,indented with many coombes; to the south the Gault crops outalong the lowest part of the Vale, and beyond, the ground risestowards the Lower Greensand hills. Along the outcrop of theChalk Marl many springs issue; one feeds Otford pond, anotherTUns to waste through the ruined mediseval conduit house(Becket's Well) which encloses it; below Shore Hill an unnamedspring issuing below a bank 8-ro feet high is clearly cutting backalong the saturation level and forming a miniature coombecomparable with those produced along the chalk scarp' aboveduring a remote period of higher saturation level.

All these Chalk Marl springs feed small anti-dip streamsflowing to the eastern strike-tributary of the Darent. NearOtford, where its eastern and western strike-tributaries unite,the Darent turns northward through a remarkable water-gapin the North Downs. All the Vale of Holmesdale is drainedin sections by strike tributaries of the Medway, Darent, orMole. and these streams all flow northward across the Chalkoutcrop to the Thames. But one part of the vale, east of Otford,exhibits exceptional drainage. A pronounced gap in the LowerGreensand hills (seen from the path running up the scarp toCotman's Ash), marks the valley cut by the Shade (Busty or

A. L. LEACH,

Buster), which flows southieard into the Medway. Of thevarious suggestions made to account for this abnormal coursethat of the late F. C. J. Spurrell has much to commend it. Hethought the Shode presented an instance of " reversed drainage."The stream flowed originally into the Darent down the dip-slopeof the Lower Greensand, but one of the head-streams of theMedway captured its head waters and thus changed the directionof drainage.

SHORE HILL ~HALK QUARRY.

This chalk-pit, first visited by the Association on July3rd, 1909, appears on the scarp of the Chalk at the head of acoombe which in part may have been formed by long-continuedworking for chalk. The quarry has been disused for manyyears, and all the slopes and rubbish heaps are much over-grown.Land shells, notably the large" Roman" snail (Helix pomatia),are very abundant. From the upper part of the pit the relationof the Chalk scarp to the Holmesdale can be studied. Thebeauty and variety of the flora make this place very delightfulat all times of the year. Few points along the North Downspresent so many aspects of interest: and flora, land molluscaand landscape features are all directly related to the geology.

Shore Hill quarry, on the 600 feet contour-line, lies probablyin the Holaster planus zone, according to Mr. G. W. Young, whoidentified a large specimen of T erebraiula semiglobosa (found by amember during the Excursion), and regarded it as characteristicof that zone. A good series of fossils could be collected fromloose blocks strewn about the pit.

The pipes, to which attention was chiefly drawn on thisoccasion, are four in number and three of them contain pocketsand lenticles of sand in addition to clay-with-flints. Startingon the western side of the pit, pipe No. I shows a broad funnel­shaped top narrowing to an ordinary cylindrical pipe, about25 feet across, lined on each side by 5-6 feet of brown clay-with­flints. The core consists of pale sand mixed with chalk mud.Over this lies a long lenticuIar section of clean, loose, pale andbrown sands, about It feet thick; layers of chalk rubble zfr.and clay-with-flints 4ft. thick then sweep across the whole pipe.Pipe NO.2 (going eastward), about zoft wide, divides in its lowerpart into two branches separated by a pinnacle of undissolvedchalk. The smaller branch contains only clay-with-flints; thewider branch has a core of sand. Pipe NO.3 appears to be fullof clay-with-flints. Pipe No. 4 has ill-defined margins, ob­scured by talus and contains much sand. An unusual mineral(? halloysite) occurs in strings and patches in the sands. Smalllumps of the fairly pure mineral may be found but generallyspeaking it is mixed with the sand and its peculiar greasy touchis thus disguised.

EXCURSION TO THE NORTH DOWNS IN KENT. 39

It is unnecessary to discuss the formation of pipes since theyare now generally accepted as solution phenomena. The featuresof immediate interest are the sands and the curious mineral(?) halloysite.

In colour the sands vary from pale yellow to brown, andpatches of pale pink sandy clay also occur. They are much lessferruginous than the (?) Pliocene sands along the North Downsand appear rather to resemble some yellow and pinkish sandsand clavs formerly to be seen below the Blackheath pebble­beds at 'Worms Heath on the Nore Hill side of the gravel pit.No fossils or flint pebbles have been found. Superficially at leastthey resemble Eocene sands (Woolwich or Blackheath Beds),but their age, like that of many other patches along the NorthDowns, has not been definitely ascertained.

The white clayey mineral found in the fourth pipe would havebeen described as allophane a few years ago, but Mr. G. M. Davieshas recently shewn that a clay-like mineral found in the pipes atWorms Heath* is halloysite, a mineral which differs fromallophane in containing less silica and more combined water.The clay-like material of Shore Hill occurs under similar con­ditions and in much the same state as the halloysite of WormsHeath and may be the same mineral.

In all the pipes on Shore Hill many flint nodules in the clay­with-flints exhibit a broad external zone of bleaching. Althoughthis may be disguised by a thin layer of cortex stained red byinfiltration of iron oxide from the surrounding red clay, whenthese nodules are broken open many are found to be whiteredalmost to the centre. That this white cortex is a solution effect,and is not an original condition, can be demonstrated by com­paring these nodules with the bleached and disintegratedpebbles exposed in the Blackheath Beds at Oaklands pit asdescribed below.

The layers of chalk rubble and clay-with-f1ints overlying thepipes of Shore Hill are due to the sub-soil creeping and slidingdown the steep slopes above the quarry. The period when muchlocal superficial movement occurred was after the formation ofthe Pleistocene brickearths in the Thames valley, as is shewn bythe deposits called "trail" at Crayford and "coombe rock"near Swanscombe.

DISINTEGRATED FLINTS AT OAKLANDS, KNOCKMILL.

The section at Oaklands, first visited by the Association inJune 1908, is in a small outlier of pebble-beds, one mile south ofKingsdown. The 700 O.D. contour-line runs through thisoutlier and the highest point marked on its surface is 722 O.D.

• W. Whitaker and G. M. Davies." The Section at Worms Heath (Surrey)." Qua,·t .Jour n , Geot Soc., vol. txxv. (1919). This paper gtves the mineral and chemical characters ofhaltoysi te.

40 A. L. LEACH,

The pit shews 20 feet of pebbles and sand more or less disturbedby sagging. The pebbles, when unaltered, are of normal sizeand form for the Blackheath Beds. One quartzite was recordedin the Report of the former Excursion and also the occurrenceof siliceous conglomerates in an old pit near the open section.The features which require explanation are (r) the sagging of thepebble-beds, (2) the presence of completely whitened and disin­tegrated flint pebbles in a compact mass which at first glanceresembles a pinnacle of chalk rising from the floor of the pit.

Since the date of the former Excursion the face of the sectionhas been cut back and the view illustrated in the former Reportcan no longer be seen. The disintegrated flint now appears some­what higher in the section, but is still clearly exposed and open toexamination. The notes that follow recapitulate (with someslight additions) what was stated in the former Report.*

(r) The sagging of the pebble-beds suggests subsidence intopipes in the Chalk which cannot be far below the floor of the pit.Such pipes may also be interred from the present drainage ofsurface water into a " water-sink" in one corner of the pit, fromthe irregular surface of the Chalk exposed in excavations atOaklands some years ago; from comparison with the sectionsat Worms Heath, Caterham, Tandridge Hill, etc. Oaklandspit has not been sunk deep enough to show whether the pebble­beds lie on older Eocene deposits or art the Chalk.

(2) The disintegrated flint forms a very compact mass whichwhen broken into is found to consist largely of flint pebbles, allpure white outside, and mostly whitened throughout, embeddedin a stiff gritty siliceous paste. These whitened pebbles can becracked easily with a hammer. Many are even as white andalmost as soft throughout as Chalk, and may be cut with aknife or crushed with the fingers, Some have even crumbled topowder. Those which retain their shape may show, when splitopen, patches or perhaps small cores of black or gi ey flint shadingoff gradually into the surrounding white cortex. The mass ofdisintegrated pebbles does not merge gradually into the sur­rounding pebbles. Its boundary (as may be well seen in theillustration accompanying the former Report), is fairly sharp.But nearly all the pebbles in the pit show some alteration andspecimens may therefore be selected which range from darkalmost black flint through every shade of grey and greyish white,to the pure white pebbles and silica powder in the most highlyaltered mass. The compact white paste enclosing the whitepebbles can be separated by levigation into (a) milk-white quartzsand; (b) pure white siliceous powder, so fine that it will remainfor hours suspended in water; (c) a still finer powder, whichwill remain in suspension for many days. The white paste istherefore composed of powdery silica. derived from the decayed

• Proc. Geol A.'Isoc vol. ::I[x:(1909), plate xxix,

PROC. GEOL. Assoc., vor., XXXII. PLATE 1.

Plu)!o, R. H. Ch andlcr,

A MASS OF DISINTEGRATED FLIJ"T PEBBLES IN BLACKHEATH BEDS,

KNOCKMILL.

Reproduced from Proc, Geol. Assoc., vol. XX., 1907-8.

EXCURSION TO THE NORTH DOWNS IN KENT. 41

flints, mixed with quartz sand (originally present with the pebbles)which has lost its usual yellow tint by the same action that haswhitened and disintegrated the flint pebbles.

Whitened flint pebbles are common in many localities butthis occurrence of a mass of completely disintegrated flint iscertainly very unusual if not unique. There seems no escapefrom the suggestion made in the former Report, that the altera­tion is a solution phenomenon, the final stage of that processwhich, by removing the soluble colloidal constituent of flint,produces the white cortex frequently observed on flint nodulesstill embedded in the Chalk and on nodules, fragments and flakesexposed on the ground surface or buried in the soil or in depositsin caves.

The late B. C. Polkinghorne* who described the constitutionand alteration of flint very clearly and concisely, attributed thewhite cortex on flint nodules in the Chalk to the removal of col­loidal silica from the surface of the nodules "by the alkalinecarbonates of soda and potash which are present in covered Chalkto-day (e.g. when capped by London Clay) though washed outof the upland Chalk by long percolation." Flakes, fragmentsand nodules on or in the soil (and flint flakes in cave deposits)become similarly whitened or patinated by the removal ofcolloidal silica by rain water charged with carbon dioxide. The" cortex" of all such whitened flints consists, according toPolkinghorne, of pure anhydrous silica.

At Oaklands pit the alteration seems to differ only in degreefrom that involved in the ordinary production of the white cortex.Here the " cortex" extends inwards to the core of the pebbles,and thus complete disintegration becomes possible. But it isdifficult to explain why this intense alteration occurs only in thisparticular pit, why it is lirriited to certain parts of the pit, andwhy the boundary between the completely and the slightly alteredpebbles is so sharp.

Since the pebble-beds are very porous one would naturally,and perhaps rightly. regard descending surface water, chargedwith carbonic acid, as the agent which has dissolved the colloidalsilica from the pebbles, but the facts that the alteration doesnot appear to extend to the ground surface, but dewlaps at adepth of a few feet and becomes more intense and extends laterallywith increasing depth led to the suggestion made in the forrnerReport, that water rising from the Chalk below might have beenthe agent responsible for this remarkable change. It would benecessary to postulate: (1) hydrostatic conditions which ledto the formation of a pipe; (2) rise of water (charged with alkalinecarbonates) through this pipe from a lower saturation in theChalk. Such conditions may be imagined but there is no avail-

• See "Chapter xviii. of Sl\Jlilllic Mall ill Nortti-East Surrey, by W. Johnson andW. Wr ight (1903).

G. BARROW AND J. F. N. GREEN,

able evidence that such have actually existed. An ingenioussuggestion made by one of the party during this present visitthat alkaline salts, washed out of decaying vegetation, may havebrought about the change in the flint is open to the objection thatno trace of this intense alteration has been detected in otheroutliers and outcrops of the pebble-beds equally covered byvegetation.

Although the cause of this complete alteration remains indoubt it may safely be stated that the change is due to solution,that it occurred in the pebble-beds and not on the surface of theground, i.e., that the pebbles were not whitened on the groundsurface and afterwards washed down into a pipe; that it is aprocess of partial de-silicification of ordinary flint.

REFERENCES.

1908. CHANDLER, R. H. and LEACH, A. L.-" Excursion to Knock-mill (Oaklands), and Cotman's Ash." Proc, Geol, Assoc., vol.xx., p. 532 and plate xxix.

1909. CHANDLER, R. H. and LEACH, A. L.-" Excursion to Otford andthe Darent Valley." Proc, Geol, Assoc., vol. xxi., p. 236.

EXCURSION TO WENDOVER AND BUCKLAND COMMON,NEAR CHOLESBURY.

SATURDAY, JULY 24TH, 1920.

REPORT BY GEORGE BARROW, F.G.S. AND J. F. N. GREEN, B.A.,F.G.S., Directors of the Excursion.

LEAVING Wendover at 12 o'clock a start was at once made forthe gravel pit, situated in the flat at the base of the gap in theChiltern Hills, near Wendover. This flat is the northern termi­nation of the Valley Gravel of the Misbourne valley. Southwardof these pits this gravel lies at the bottom of the valley; butnorthward it forms the crest of a rather steep slope, the formercontinuation of the valley in this direction having been entirelyobliterated by denudation. The base of the gravel at its ter­mination is 500 feet above sea-level. The gravel is formed ofunworn Dr little worn flints from the Upper Chalk, set in a matrixof sandy clay in the upper part and chalky clay in the lower.At the base of this flint-gravel is a very thin layer of small pebbles,the majority of which are of well rounded white quartz; lyditeis fairly common, while very small rounded nodules of ironstone,or sand cemented into minute balls by iron-oxide, are met within small numbers. This association clearly shows that theyhave been derived from the Lower Greensand northward of theChalk escarpment, and the Directors believed their depositionto be of early Pliocene age.

Boddington Hill was then ascended; this is the south-west