59

FIELD MEETING IN THE CENTRAL CHILTERNS7 May 1949

Report by the Director and Secretary: P. Evans, M.A., F.G.S., andK. P. Oakley, Ph.D., F.G.S.

[Received I November 1951]

MEMBERS travelled from Wendover by coach along the IcknieldWay, noting, in passing. the Melbourn Rock platform below

the steep slope capped by the Upper Chalk. North of Folly Farm(941141) in the large chalk pit, visited by kind permission of theTunnel Cement Co. (Pitstone) Ltd., sections were seen showing theTotternhoe Stone and overlying Holaster subglobosus Zone of theLower Chalk. This is grey and marly in the lower part, but purer,greyish-white and more blocky above. These beds are moderatelyfossiliferous, and a number of ammonites (including Acanthoceras)were found by members. Of much more interest are the overlyingPleistocene deposits, which have not hitherto been described.

At the south-western end of the pit the chalk is overlain by abouttwenty feet of greyish-white coombe rock consisting oflarge and smallblocks of chalk in a matrix of fragments of chalk from the size ofpeas down to fine powder. At the south-eastern end of the pit (point1, Fig. 1) the coombe rock (G) consists of sub-angular lumps ofchalk, averaging about two inches across, embedded in a matrix ofchalk-powder, pea-sized fragments being much rarer. The largerblocks here are about eight inches across, but farther north blocks upto two feet across may be seen. Some of the large blocks appear to beof Melbourn Rock. The coombe rock shows impersistent lines with aslight inclination. The chalk below the coombe rock is shattered forabout two feet (H); there is consequently no sharp junction betweenthe coombe rock and the undisturbed chalk. The underlying chalkrises north-eastwards and the lower part of the coarse coombe rock(G) is replaced by disturbed chalk which gradually passes into nor­mal chalk (6, 7, 8).

At the top of the section, resting on an irregular surface of thecoombe rock (G), is a thin band of light brown chalky clay (F) withangular flint fragments and small chalk pellets. This is overlain by athin covering of dark brown clayey soil with numerous flints. A shortdistance northwards (point 2) the light brown flinty and chalky clayis festooned down into the coombe rock (G), and farther north itthickens; still farther north. about 3, bed F gradually passes into afine, buff-coloured coombe rock (E) consisting of small angular andsub-angular flints and small fragments and pebbles of chalk withsome chalky matrix. Some of the broken flints are fresh grey, othersare brown and patinated, derived from a previously weathered sur­face.

60 P. EVANS AND K. P. OAKLEY

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FIG. I.-The north-east face of Folly Farm quarry near Pitstone, Bucks. (Forexplanation see text.)

At point 9, towards the northern end of the cutting leading fromthe quarry to the cement works, this finecoombe rock passes laterallyinto stratified gravel (D). This shows much variation but a measuredsection (10) gave, below 'made ground':

A. 1 ft. of fine, cream-coloured, flintless gravel consisting of small sub­angular chalk pebbles (up to ! in. diameter) in a chalky matrix.

B. 1!- ft. of coarser brownish gravel consisting of angular splintered piecesof flint, up to an inch in length, together with sub-angular and roundedchalk fragments ctin. to ! in.) in a scanty matrix of powdered chalk.

C. It ft. of fine, nearly white sediment consisting almost entirely of pow­dered chalk. This grades down into a basal gravel similar to the over­lying bed, but only a few inches thick.

At this measured section a small fault with a throw of three inchescan be identified in this deposit and traced for at least four feet down

FIELD MEETING IN THE CENTRAL CHILTERNS 61

into the chalk below, the fault-plane being marked by traces ofgravel. The deposit shows impersistent lines of iron pan. In generalthe fine beds occur as lenses within the coarser beds.

For some distance northwards from point 2 there are a number offestoons of darker material in the coombe rock; this includes brownclay with broken flints and the fine flinty coombe rock of beds F andE. Near point 5 (where a track runs down to the quarry) the verticalsections show festoons to a depth of almost four feet, with anamplitude of one to two feet. Clearing of the overburden has hererevealed in plan several well-formed frost polygons exceeding threefeet across.

On the western face of the quarry there are good sections showingat the northern end about six feet of very flinty fine coombe rockoverlying the coarse coombe rock. Brown clay with numerous flintsoccupies large festoons in the fine, pale brown or buff coombe rock.Farther south on this face, similar festoons are well seen in the upperpart of the coarse greyish-white coombe rock. These have beenslightly disturbed or pulled by later flow of the coombe rock.

The sequence of events seems to have been as follows. During aperiod of intensely cold winters broken by short possibly fairly hotsummers, the chalk from the lower parts of the adjacent scarp wasbroken up and flowed gradually northwards. This solifluxion did notcarry down to this area any great thickness of the chalk above theMelbourn Rock. A later solifluxion brought down a sludge of chalkwith a large number of flint fragments. This seems to have reached aflooded area, possibly the margin of a lake held up by the glacier ofthe Vale of Aylesbury, and there to have formed the stratified de­posits of chalk-powder and chalky gravel seen at the northern end ofthe cutting; the layer of chalk-powder suggests deposition on a lakebed. The spread of poorly stratified chalky gravel on the floor of theVale at Gubblecote (906150) (Oakley, K. P., 1936) was probablyformed at this time. Still later there was brought into the area a thinbed of flinty clay which was subjected to arctic temperatures leadingto festooning and the formation offrost-polygons.

After detailed inspection of these Pleistocene deposits the partypaid a brief visit to the quarry (948148) about 600 yards ESE. ofPitstone church which provides sections slightly higher in the suc­cession. Belemnite Marl (Actinocamax plenus marl) and the overlyingMelboum Rock were seen to dip appreciably to the south-east andto be slightly disturbed by minor faulting.

After lunch members inspected Coombe Hole (967174) and In­combe Hole (959156), two typical scarp valleys, the latter showingclearly the sharp right-angled bends so well seen in the Hextonvalleys. The Director mentioned various hypotheses put forward toaccount for these deep narrow valleys, and a discussion followed inwhich Dr. A. J. Bull, Dr. Oakley, and other members took part.From Ivinghoe Beacon (960168) it was possible to get a view across

62 FIELD MEETING IN THE CENTRAL CHIL TERNS

the clay vale to the Lower Greensand beyond. On the way back toWendover the opportunity was taken to get views over the Pliocenemarine bench at about 650 feet above sea-level and of the higherground to the north which was not submerged during the Pliocenetransgression. After tea, which was taken at Tring, the thanks of theparty were given to the Director and Secretary.

REFERENCEOAKLEY, K. P. 1936. Field Meeting at Cheddington, Ivinghoe and Gubblecote.

Proc. Geol. Assoc., 47, 38-41 (and references).