Field Meeting in the Chilterns,

near Tring21 July 1963

Report by the Director: E. H. BROWN

Received 25 September 1963

APPROXIMATEL y thirty members and friends assembled at Tring station on ahot and sunny morning. The object of the meeting was to demonstrate theprincipal items of geomorphological interest in the gap through the Chil­tern scarp between Tring and Ivinghoe. The first vantage point at thesouthern end of Pitstone Hill (SP 952131)gave a view across the mouth ofthe funnel-shaped gap. The Director suggested that it was probably cut bya southward-flowing river consequent upon the withdrawal southwards ofthe Calabrian sea (Wooldridge and Linton, 1955), the degraded shorelineand floor of which were clearly visible in profile on the western flank of thegap at Wiggington. The present River Bulbourne rises in the neck of thegap but is a beheaded stream, its northward extension has probably beenreversed by glacial action to form the northward-flowing Ouzel. That icerested in the mouth of the gap is witnessed by the presence of the Tringboulder clay, on the subject of which Mr. Avery of the Soil Survey spokea few words. Doubtless meltwater from this ice flowed through the gapand modified its form to some degree. The wide funnel entrance is attri­buted to the low angle of dip of the chalk in this locality « I 0).

On the walk up to the summit of Pitstone Hill, a tree-framed view of theBridgwater monument, Ashridge, was seen across the Aldbury valley. Themonument stands on the Chiltern plateau at 725 feet 0.0., overlookingthe inner margin of the Calabrian platform, here approximately 600 feet.A small spur flat on the south side of Pitstone Hill at a height of 600 feetmay be an eroded remnant of the same platform and the steep slope fromthis to the summit of the hill could be the associated degraded cliff line, thecounterpart of the breaks of slope at Wiggington, west of the Tring gap,and below the Bridgwater monument, east of it.

From the summit of the hill a fine view was obtained across the Middleand Lower Chalk bench at the foot of the Chiltern scarp and the Gault valeof Aylesbury, to an horizon which comprised, from east to west, theplateau summit of the Lower Greensand scarp in the Woburn area, the gapmade by the Ouzel at Leighton Buzzard, the Boulder Clay plateau west ofthe Ouzel and the Portlandian hills south of Aylesbury. The President

341

342 E. H. BROWN

commented upon this fine view and made reference to an earlier visit bythe Association to that area (Bristow & Kirkaldy, 1962).

After crossing two small cols which notch the summit of the hill andhead down into small, steep-sided scarp-face coombes the party crossed thegap, probably caused by scarp recession, which separates Pitstone Hillfrom Ivinghoe Beacon at the head of the Aldbury valley. A stop was madeat the head oflncoombe Hole, a large dry valley cut in the western flank ofSteps Hill. The Director outlined the several opinions which have beenexpressed over the years concerning the origin of its steep-sided, flat­floored cross-section: and its zigzag plan (Barrow & Green, 1921; Earle,1928; Oakley, 1936; Evans & Oakley, 1952). He made comparisons withsimilar coombes investigated by the late W. V. Lewis and B. W. Sparks atPegsdon, 1957, by Mr. R. J. Small (1958) in Hampshire and Dr. Kerney,Mr. Chandler and himself at Brook, Kent. The latter was demonstrated tothe Association by Dr. Kerney in a recent visit (Sparks & Lewis, 1957). Adiscussion ensued in which it seemed to be agreed that jointing was prob­ably an important factor in determining the zigzag plan of the coombe.Views on the relative efficaciesof spring sapping and niveofluvial processesas factors in the fashioning of this and similar coombes were expressed.The Director stressed the fact that all the coombes had what appeared to bewater-eroded channels heading into them.

After lunch a visit was made to the quarries of the Tunnel Cement Co.Ltd. at Pitstone by kind permission of the Manager, who accompanied theparty. In the upper quarry, interest focused both on the well-exposed,strike-faulted Belemnite Marl and Melbourn Rock and on a section insuperficial deposits at the head of a small scarp-face coombe. The strati­graphy of the latter corresponds very closely to that observed in LateGlacial sections on the Chalk south of London (Kerney, 1963). A sampletaken by Dr. Kerney from the dark band in the deposit has yielded manycharcoal fragments and an assemblage of land Mollusca of a distinctivelyLate Glacial type, including Cochlicapa Lubriccela (Porro), Pupilla muscor­um (Linne), Vallonia costata (Muller), Vallonia pulchella (Muller), Helicellaitala (Linne), Punctum pygmacum (Draparnaud), Euconulusfuerus (Muller),Rectinella radiatula (Alder) and Vitrina pellucida (Muller). There seems tobe a good a priori case for regarding this dark band as reflecting Zone IIof the late Glacial Period (Allered Oscillation). The dark band has inplaces been disturbed and broken by movements which it may be surmisedwere due to frost-heaving and solifluction during the ensuing Zone IIIcold period.

In the lower quarry later exposures, after continued quarrying, of anumber of periglacial features, described on an earlier visit by Evans andOakley, were inspected. Members dispersed after the President hadproposed a vote of thanks to the Director.

FIELD MEETING IN THE CHILTERNS 343

REFERENCESSoil Survey of England and Wales, Sheet 238, Aylesbury.

BARROW, G. & J. F. N. GREEN. 1921. Excursion to the Tring Gap and Steps Hill. Proc.Geol. Ass., Lond., 32, 215-7.

BRISTOW, C. R. & J. F. KIRKALDY. 1962. Field Meeting to the Leighton Buzzard­Aylesbury Area. Proc, Geol. Ass., Lond., 73, 455-9.

EARLE, K. W. 1928. Excursion to Ivinghoe and Cheddington; Proc. Geol. Ass., Lond.,39,492-7.

EVANS, P. & K. P. OAKLEY. 1952. Field Meeting in the Central Chilterns. Proc. Geol.Ass., Lond., 63, 59-62.

KERNEY, M. P., E. H. BROWN & T. J. CHANDLER. Phil. Trans. (In press).---. 1963. Late-Glacial Deposits on the Chalk of South-East England. Phil.

Trans., 246, pI. 9.OAKLEY, K. P. 1936. Field Meeting at Cheddington, Ivinghoe and Gubblecote. Proc.

Geol. Ass., Lond., 47, 38-41.SMALL, R. J. 1958. The Origin of Rake Bottom, Butser Hill. Proc. Hants. Fld. Clb., 21,

22-30.SPARKS, B. W. & W. V. LEWIS. 1957. Escarpment Dry Valleys near Pegsdon, Hertford­

shire. Proc. Geol. Ass., Lond., 68, 26-38.WOOLDRIDGE, S. W. & D. L. LINTON. 1955. Structure, Surface and Drainage in

South-East England. London.