255

FIELD MEETING IN THE SOUTHERNCHILTERNS

Saturday, 28 April 1951

Report by the Director: H. L. Hawkins, D.Sc., F.R.S., F.G.S.

[Received 29 June 1951)

THIRTY members assembled at Reading Station and thencetravelled by motor-coach.

The first stop was at Taplow, where the South Lodge pit, famousfor its exposure of Phosphatic Chalk, was visited. Mr. N. R. Willcox,who is making an intensive study of the problem, demonstrated thesequence. The Chalk is completely flintless, and shows a remarkablecondensation of zones, which range from that of M. coranguinum tothat of O. pi/ula in a total thickness of less than thirty feet. There aretwo main seams of brown Phosphatic Chalk. The lower one, aboutfour feet thick, rests on an indurated and eroded surface of Coran­guinum Chalk riddled by 'borings' to a considerable depth. This seamrepresents the greater part of the Uintacrinus Zone. Above it, andsharply defined from it, come some twelve feet of very white chalk,'marbled' throughout by tubular and irregular inclusions of brownphosphatic material. Most of this peculiar Chalk belongs to the zoneof Marsupites. Its top is indurated, and coated with a veneer ofglossy phosphate, which is encrusted by oysters. Then follows aneight-foot seam of Phosphatic Chalk (slightly less rich than the lowerseam), in which plates of Marsupites occur near the bottom, andOffaster pi/ula higher up. This seam passes upwards gradually intowhite Chalk which is very unfossiliferous, but can be confidentlyascribed to the zone of A. quadratus s.l.

The fauna of this Chalk is remarkable for the great abundance ofbelemnites (mostly referable to Gonioteuthis granulata) and thepresence of large ammonites akin to Parapuzosia. Echinocorys, in abewildering variety of forms, is common, and Ostrea crowds thehorizons of non-sequence. The phosphatic seams contain enormousnumbers of Foraminifera (mostly minute types), many of which arefilled with phosphate but still retain their calcareous shells.

The reason for the abnormal and condensed development of theChalk here is still obscure, especially since the available evidencepoints to a very limited lateral extent of the peculiarity. The con­centration of high-zonal Chalk is preserved in a shallow bowl­syncline with a faulted eastern margin; but its production wouldseem to have been the result of a shoal-like elevation of the Cre­taceous sea-floor, where currents inhibited the free accumulation ofchalk. The whole succession is full of non-sequences and surfaces ofcontemporaneous erosion; and, to judge from the distribution of the

256 FIELD MEETING IN THE SOUTHERN CHILTERNS

zonal fossils, seems to have been frequently stirred up and re­arranged.

Some time was spent in collecting samples of this remarkableChalk, and before the party left Mr. Willcox gave a brief account ofthe present stage of his research.

The drive was then resumed across the plateau above Cliveden,from which a steep descent down the river-bluff led to Bourne Endand Wooburn Green. Here the old chalk-pit near the railway stationwas examined. Most, if not all, of the Chalk exposed belongs to thezone of M. cortestudinarium; but in addition to the usual micrastersit reveals peculiarities in lithology and sedimentation. Several well­marked seams of marl traverse the Chalk in a disposition thatappears to imply current-bedding. This presumptive evidence ofshallow-water conditions may perhaps be linked with the moredefinite signs of shoal-water at Taplow; and it is consistent with thepresence of an abnormal thinness and 'rockiness' of the zone of H.planus at Cliveden, mid-way between Taplow and Wooburn Green.If this interpretation is correct, the tendency to the maintenance ofshallow water in the district (which culminated in the developmentof the Taplow phosphate) would seem to have persisted throughoutthe whole time of the Upper Chalk.

The journey was continued along the valley-road through Marlowto Medmenham, where a truly magnificent exposure of the ChalkRock was seen in a roadside quarry. The rock is traversed by widefissures that produce huge rectangular blocks; it is capped by a seamof tabular flint.

From Medmenham the party proceeded along the valley to Hen­ley, and thence up the long dip-slope to Nettlebed, where tea wastaken.

The return drive to Reading was by way of Peppard, and a briefhalt was made at the Kennylands gravel-pit, where the relation of thegravel to the underlying Chalk, with differential solution causingirregular subsidence, showed the danger inherent in too precise cal­culations of terrace-levels. The great variety of pebbles occurring inthe gravel was duly appreciated; but unfortunately the pit has beenused recently as a dumping-ground for miscellaneous rubbish, and aconsiderable proportion of unnatural occurrences (notably iron­stone from the Middle Lias) tends to confuse the record. In itspresent state the pit is of more value as a warning to the young thanas a reliable guide to the Pleistocene.