Field meeting in the Chilterns: 17 June 1956

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  • Field Meeting in the Chilterns17 June 1956

    Report by the Directors: B. W. AVERY and A. J. THOMASSONReceived 16 July 1956

    THE OBJECTIVES of this excursion were to study certain problems of thestratigraphy and geomorphology of the Chiltern area covered by Geo-logical Survey, New Series, 1 in. Sheets 238 and 255.

    The first stop was at Chesham Bois (Grid Ref. 973996), where anexposure of 5-6 ft. of Clay-with-f1ints, overlying Upper Chalk, wasexamined in a new road-cutting on the south side of the Chess Valley about50 ft. below the general level of the adjoining plateau. The section showedthe true Clay-with-flints, as described by Whitaker (1889)-a reddishunctuous clay containing black-coated unworn flints in immediate contactwith the irregular and piped surface of the chalk. Above this was a hetero-geneous clay containing shattered flints and flint pebbles and enclosinglenticles of stoneless fine sandy clay.

    At this point Mr. Avery gave the party a brief review of the publishedwork relating to the Chiltern superficial deposits, citing the contributionsof Whitaker, Jukes-Browne (1906), Sherlock & Noble (1912), and Barrow(1919). The results of detailed soil surveys, accompanied by mechanical andmineralogical analyses of samples from selected soil profiles, suggested thatboth plateau and valley drifts were modified by the addition of loess-likesilt (0.002-0.05 mm. diameter) and were themselves the product of twointeracting processes, viz:

    (i) Solution of chalk beneath a pervious cover of remanie Eocene,Pliocene or drift materials, and accumulation above the junction of infil-trated clay.

    (ii) Solifluction and frost action.Slow formation of basal Clay-with-f1ints by the first process was initiated

    in mid-Tertiary times and continued during the Pleistocene, underfavourable conditions, but much of the earlier-formed Clay-with-flints wasincorporated in the stony Head which covers most of the plateau, over-riding the remains of Eocene and Pliocene sediments and extending downthe gentler valley sides.

    At Cowcroft (985018) a shallow sand-pit on the north side of the hillshowed the following succession:

    3. Pebble bed2. Brown sandy clayI. Pale and brown sands with occasional thin,

    but continuous, grey clay bands and a fewblack-coated flint pebbles ...

    168

    ft.1 ? base of London Clay2 Reading Beds

    6 Reading Beds

  • FIELD MEETING IN THE CHILTERNS 169

    All the beds dipped towards the north-east. The attention of the party wascalled to the work of George Barrow, who described sections through themiddle of the Cowcroft outlier, unfortunately now obscured, whichshowed considerable disturbances in both Chalk and Reading Beds. Bycontrast, the present section, although nearer to the surface, was compara-tively undisturbed. Barrow attributed the deeper-lying disturbances to alarge ice-sheet moving from the west during some period before thedeposition of Chalky Boulder Clay. The absence of erratics on this part ofthe Chilterns makes this proposition difficult to sustain.

    Members pointed out that the upper layers of the section, containingfestoons of pebbles, had been affected by frost-heaving under periglacialconditions.

    After lunch at the 'Blue Ball', Asheridge, the party examined a sand-pitnear Dundridge Farm, St. Leonards (Grid Ref. 916061), sited on a narrow,flat-topped ridge bounded by dry valleys and exposing 30-35 ft. of whitishsand with grey clay bands beneath the superficial flinty clay Head. TheDirectors recalled that, originally, while the pit had been excavated to adepth of 50 ft. without reaching Chalk, exposures in the western dry valleyrevealed Chalk scarcely 20 ft. below the plateau surface. The beds examinedin the pit resembled Reading Beds sands at Lane End, Buckinghamshirein structure, lithology and mineral assemblage, and were believed to beof similar age, the evident features of disturbance, including minor faults,folds and clay breccias, being attributed to slumping soon after deposition.A full account of this section will appear in a subsequent communication.

    The party then drove by way of the Lee and Great Missenden toPrestwood brick-works (862014) where a fresh 30 ft. exposure of plateaudeposits was examined. At this point a superficial Head of flinty clayaveraging 5 ft. in thickness is channelled into weakly laminated 'brick-earth' containing occasional flint flakes, pebbles and large sarsens. Thewhole succession closely resembles that described by Sherlock & Noble(1912) at Walters Ash, three miles to the south-west. The possible correla-tion of the 'brick-earth' with similar laminated deposits at Gaddesden Rowand Caddington, in which W. G. Smith found Acheulian implements(Oakley, 1947), was discussed. The Directors drew attention to Barrow's(1919) contention that apparently water-laid brick-earths of this type wereassociated with 'pre-glacial' sink-holes in the Chalk, and suggested thatthey could conceivably represent a local facies of Reading Beds. Neither ofthese views was generally accepted and it appeared from the ensuing dis-cussion that the nature and origin of these deposits might provide a subjectfor further investigation.

    Finally the party walked across the dry valley of Little Hampden(864037), taking the road towards Cobblers Hill from Hampden BottomFarm, and thence through the woods on the east side of Little Hampden,

  • 170 B. W. AVERY AND A. J. THOMASSON

    turning due west across the fields to Manor Farm. The attention of theparty was drawn to the distinct asymmetry of many of the dip-slopevalleys, in which the steeper slope tends to face west or south-west. LittleHampden Valleyis a particularly good example ofthis. Copies ofgeneralisedslope maps of the Chesham district on a scale of 1:25,000 (constructedby C. D. Ollier, now of the Uganda Soil Survey), which showed thatthe phenomenon is widely developed in this part of the Chilterns, weredistributed to the party. The pattern of slope deposits and related soils ismarkedly different on the two sides of the valleys. These were studied indetail during 1955 and it is expected that the results of this work will bepublished elsewhere. Calcareous soils are associated with the steeper slopes,while naturally acid soils occur on north- and east-facing slopes. Siltymaterial is an important constituent of the soils on the lower parts of thegentler slopes, and may be wind-deposited (Soil Survey Report No.8,H.M.S.O., 1955). It is believed that this pattern of slopes and soils is aresult ofuniclinal shifting of a down-cutting stream, the gentler slope beingvirtually a slip-off slope. This shift is not structurally controlled, and as theChalk surface under the deposits is also strongly asymmetrical, the pro-cesses involved in the shaping of such a valley are erosional rather thandepositional. The steeper slope would seem to have undergone parallelretreat in the manner described by L. C. King (1951). The exact reason forthe difference in weathering and erosion between the opposite sides ofthese valleys is not known. Insolation is probably important and may haveaffected the rate of weathering. Thus, under peri-glacial conditions, slopesfacing south-west may have experienced more frequent changes of tem-perature than those facing north-east, resulting in corresponding freezingand thawing, while the north-east facing slope remained permanentlyfrozen, or thawed out for only a short period during the summer. Solifluc-tion activity would have been more rapid and turbulent on the slopesfacing south-west. However, S. C. A. Holmes commented that he knew ofessentially similar valleys in the North Downs, where the chalky head at thefoot of the west-facing steeper slope contained fossil land-snails indicatinga temperate climate not appreciably different from that of the present-day.Thus the asymmetry may be a result of inter-glacial rather than of glacialconditions. The presence of similar valleys in both arid and temperateregions of North America seems to indicate that differential weathering asa function of aspect can occur in a wide variety of climates.

    The party had tea in Chesham after which the Acting President, Dr.G. W. Himus, thanked the Directors and Secretary.

  • FIELD MEETING IN THE CHILTERNS

    REFERENCES

    171

    BARROW, G. 1919. Some future work for the Geologists' Association. Proc. Geol. Ass.,Lond.,30,1.

    JUKES-BROWNE, A. J. 1906. The Clay-with-flints: its Origin and Distribution. Quart. J.geol, Soc. Lond., 63, 139.

    KING, L. C. 1951. South African Scenery. London.OAKLEY, K. P. 1947. Early Man in Hertfordshire. Trans. Herts. nat. Hist, Soc., 22, 248.SHERLOCK, R. L. & A. NOBLE. 1912. On the Glacial Origin of the Clay-with-flints of

    Buckinghamshire and a Former Course of the Thames. Quart. J. geol. Soc.Lond., 68, 199.

    WHITAKER, W. 1889. Geology of London. Mem. geol. Surv. U.K.

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