288

REPORT OF AN EXCURSION TO KNEBWORTH AND .

By W. HILL, F.G.S., Director if the Excursion.

NEARLY thirty members arrived at Knebworth station a little before mid-day, their object being the examination of the strata exposed in the cuttings of the new loop-line of the Great Northern Railway between Broadwater and Hertford; a continu­ ation, in fact, of the excursion from Cuffley to Hertford on July 5th last year. They proceeded along the Great North Road to a point where the new line cuts this road near Broadwater, and were met here by Mr. J. C. L. Train, who accompanied them to Hertford. The road is carried over the line, and there is here a cutting showing about 12 ft. of clay with gravel at its base, above it a thin layer of gravelly soil. The lower part of the clay was the greyish-blue chalky Boulder clay, but in the upper third the chalky material disappeared, and the clay assumed a brownish­ grey colour. About a third of a mile south of this a long low cutting exposed the upper part of the clay, the bluer portion being touched in places. In the overlying gravel a marked layer of ironstone boulders, probably derived from the Lower Greensand, was noted. On arriving at Knebworth Road, by the courtesy of Mr. Train, an engine and trucks, with comfortable seats, were found waiting for the party, and in this manner they travelled as far as Staple­ ford, stopping at the most interesting points. A good opportunity for the examination of the Boulder day was given at Oak's Cross Lane crossing, a long heap of this material being exposed to the weather. The colour was rather a dark bluish-grey, but mixed with it was a lighter and browner clay, presumably from near the surface and weathered. It contained comparatively few small chalk pebbles or flints, or any other boulders, a feature which characterises this clay between Broad­ water and Watton. Among the few boulders noted were a block of grey limestone full of Serputa: (Serpula tetragona, J. de C. Sow.), derived probably from the Cornbrash, a glaciated boulder of Carboniferous limestone, a compact yellowish limestone with scattered oolitic grains, possibly Portlandian. A nodule with round (?) phosphatic pellets and another, well glaciated, seemed to be a fragment of Septaria, both from the Oxford clay. The fossils EXCURSION TO KNEBWORTH AND HERTFORD. 289 obtained were Ammonite sp., and Cardioceras sp., Gryphcea di!atata from Oxford clay, Belemnites minimus, and B. attenuatus (?) from the Gault. The cutting at Watton was next reached. The Boulder clay, still comparatively free from boulders, was seen to be underlain by a clean sandy gravel markedly current-bedded. The gravel consisted chiefly of flints mingled with rolled fragments of chalk, some of which could be recognised as Chalk Rock, and much Jurassic debris, together with rolled pebbles of older and harder rocks. Amongst those collected from the gravel were Sarsen stones, Carboniferous encrinital limestone, a fragment of decom­ posed chert (probably Carboniferous), an older palseozoic chert, with small quartz veins, a fine-grained calcareous sandstone, probably from the Middle Lias. A deep red sandstone with white mica, not uncommon, may have been derived from the Trias. Well­ rounded pebbles of a coarse quartz-veined rock, with some white mica, a quartz schorl rock, a felspar schorl rock (altered granite), and others of quartzite may perhaps all be regarded as Bunter pebbles. Also a metamorphic rock, almost granitic in texture; this seems to be a calcareous banded sediment entirely meta­ morphosed, and in the opinion of Mr. George Barrow, it is certainly of-Scandinavian origin. The surface of the gravel was even, but appeared to slope slightly to the north. The Boulder clay following this slope descended below the level of the line at the north-west corner of the cutting, while at the east end its base was 6 ft. above the line, but following the contour of the ground it diminished in thickness and finally ended in a thin bed of rain wash. The cutting is now broken by a small transverse valley, out is resumed immediately. Dark bluish-grey Boulder clay was seen at the base at a distinctly lower level than that last described. It is, however, a continuation of the same bed. Overlying it was a brown, rather clayey, gravel. This cutting will eventually be of considerable length, and will probably disclose the relations of the Drift to the Chalk. An excavation, the face having been exposed to the weather, gave a fair section of the gravel overlying the Boulder clay. It was found .ro consist chiefly of flints and rolled fragments of the harder rocks found in the lower gravel, the softer Jurassic debris and chalk being absent. [A subsequent visit to the cutting, on May i Sth, showed [hat the Chalk had been exposed for some 20 yds, Overlying it was 3 ft. of coarse brown gravel. The contents of this were almost entirely large flints, rounded pebbles of a fine sandstone (? Sarsens) quartzite (? Bunter pebbles), and white quartz. There appeared to be no chalk or Jurassic debris. About 13 ft. or 14 ft. of greyIsh. blue Boulder clay was shown above this gravel. It con­ tained more chalky debris than that at the north end' of the W. HILL,

cutting; but no boulders were noticed. There were, however, isolated patches of sandy silt of very irregular outline, the largest being about 20 yds. in length by 5 ft. in thickness at the broadest part. A quarter of a mile beyond this another cutting had just been commenced which showed 6 ft. of gravel similar to that above described, while the appearance of the upper soil suggested that Boulder clay would be reached in a short distance.'] The train carried the party near the cutting at Stapleford, the service line ending here, the rest of the journey being accomplished on foot. The short cutting at Stapleford, just before the line crosses the Valley of the Beane, exposed a clean sharp sandy gravel, in which Jurassic debris was abundant, the sand predominating at the southern end. A bed of chalky Boulder clay about 3 ft. thick was seen near the bottom of this cutting at its northern end, but it thinned out rapidly to the south. Walking across the valley the party mounted its eastern slope to the cutting on the hill above Waterford. Here the sandy facies of the drift in this neighbourhood was evident. Alternating layers of white or greyish-white sand and gravel were exposed, all markedly current-bedded. Near the top the material was browner and more clayey. There seemed no distinct division, one passing to the other. The stones of this gravel were of comparatively small size. and there was a good deal of chalk as well as Jurassic debris. The upper browner gravel was rather coarser. Another incontinuous layer of Boulder clay, similar to that at Stapleford, was noted. At the southern end of this cutting, overlying the gravel, there occurred 4 ft. or 5 ft. of a brown loamy clay which suggested to some members of the party the brick-earth of the Thames Valley. The gravel at its junction was iron-stained. A little farther on the bed seemed to pass rapidly down into a pale silty sand. Walking down the hill, a long low cutting was on the left in gravelly clay, in which rounded pebbles, possibly derived from the Reading beds, were in places not inconspicuous. An isolated patch of boulder clay was seen about half-way down the slope. lOn May rSth further excavation had nearly cleared this patch away.] The party had now arrived at the tunnel about 330 yds. long, about a mile north of Hertford. The hill through which the tunnel is carried is of Chalk capped with Drift. Though the walls of the tunnel are in the Chalk, the crown of the arch will be in the Drift the whole way. A heading has been driven through, and along this the party were conducted. At the southern end of it an interesting though con­ fused section was seen. A mass of Boulder clay, some 8 ft. thick, appeared to have slipped down the hill slope, dragging down with PROC. GEOL. Assoc., VOL. XXV. PLATE 38.

A.-SOUTHERN ENTRANCE TO THE TUNNEL

Photo by T. W. Reader. B.-GRAVEL BENEATH THE BOULDER CLAY SHOWING CURRENT BEDDING AND LENTICLE OF WHITE SAND, NORTH END OF WATTON CUTTING.

To face page 290. EXCURSION TO KNEBWORTH AND HERTFORD. 291 it the underlying layer of coarse gravel originally resting on the chalk. The last cutting examined was that at the new Hertford station. This disclosed a somewhat puzzling section. The lowest bed seemed to be the usual blue-grey Boulder clay, passing up­ wards to that of browner colour; but so free was it of chalk or other boulders that it required a search before a few small frag­ ments of chalk were obtained. Some 25 ft. of gravel and sand was seen above the Boulder clay. Tusks of Elephas have been obtained here, buttheir position is doubtful. During the walk Mr. Train most kindly gave members of the party details of sections obscured during the progress of the work, and any other information in his power. His courtesy was much appreciated by all present, and hearty thanks were conveyed to him by the President. Tea was taken at the Salisbury Arms, and after a hasty visit to the Hertford Museum under the guidance of Mr. Andrews, the party dispersed. Miss Pearse acted as Secretary for the excursion.

REFERENCES. r89[. MONCKTON and HERRIES.-" Hill Gravels N, of the Thames." Proc. Geoi. Assoc. vol. xii, P: r08. 1908• W. HILL.-" On a Deep Channel of Drift at Hitehin (Herts.}' Quart, :Nurn. Geo], Soc" vol. lxiv, P: 8. rg 0 9· A. E. SALTER.-" Sarsens and other Boulders in ." Trans. Herts, Nat. Hist, Soc., yolo xiv (Jan. J9Il). 19 12. W. H1LL.-" Excursion to and Gap." Prcc, Gcol. Assoc., vo!. xxiii, part 4 (r912). POCOCK, R. W._" Excursion to Cuflley and Hertford." Proc. Geo], Assoc., vol. xxv, p.77.