54 ME. G. W. LAMPLUGH OX THE JrXCTIOX OF [vol. Ixxviii^ 



The general dip of the Cretaceous sequence is south-eastwards at 

 a low angle ; the plotting of continuous sections brings out, how- 

 ever, some local anomalies due, not to tectonic structure, but to the 

 mode of formation of the deposits. Thus, a long section drawn 

 north-eastwards, on the general strike of the Grault,^ from Chamber- 

 lain Barn pit through the Shenley Hill pits to Poplars pit, a 

 distance of nearly 2 miles, shows a rise of 55 feet in the floor of 

 the Basement beds in the flrst 1350 yards (between Chamberlain 

 Barn and Harris's, Shenley) and then a fall of about 20 feet in 

 the further mile (between Harris's and Poplars). If the initial 

 rate of rise had been continued to the north-eastern end of the 

 section, the base of the Gault would have been some 70 feet 

 higher at Poplars than at Harris's, instead of being 20 feet 

 lower; so that the floor at this end of the section falls about 

 90 feet below the plane of its southern part. Calculating by 

 the same method in the reverse direction, from north-east to south- 

 west, we find that the flooi- at Chamberlain Barn is about 70 feet 

 lower than the plane prolonged from the northern half of the 

 section. The gentle slopes of the low arch thus indicated are 

 somewhat accentuated on both sides towards its crest, which is 

 situated under Shenley Hill and has been cut across by the north- 

 and-south range of sandpits. The top of the arch shows distinctly 

 in the combined section of these pits on the natural scale (fig. 12^ 

 p. 22) ; but, owing to the vertical component being so small in 

 comparison with the horizontal, it has been found impracticable to 

 reproduce the full-length section from Chamberlain Barn to 

 Poplars on a small scale, without such excessive vertical exag- 

 geration as to destro}' its utility.- 



It is clear that the Basement beds and Gault were laid down on 

 an uneven surface, which may partly have reflected the original 

 heaping-up of the Lower Cretaceous sand-banks, but appears to 

 have owed still more of its irregularity to the erosive action of sea- 

 currents during the early stages of the Upper Cretaceous trans- 

 gression. The conditions, from late-Lower Cretaceous times 



^ The conjectural boundary of the Gault on the Old Series 1-inch Geological 

 Survey map (Quarter- Sheet 46 N.W.) in the area north-east of Leighton has 

 proved to be incorrect in many j)laces, as was inevitable from the obscurity 

 of the ground, the slipping of the clays, and the difficulty of distinguishing 

 between Boulder Clay and Gault from surface-indications only. The Gault 

 boundary near Heath is now known to lie at least half a mile farther west 

 than shown. The district was partly re-surveyed on the 6-inch scale some 

 30 years ago, and the boundary corrected so far as was then possible; the 

 results have not been published, but are available for reference on manuscript 

 6 -inch maps in the Geological Survey Library at Jermyn Street. 



^ For the same reason, a still longer section, from the Grovebury pits to 

 Poplars, intended to illustrate the upward curve of the Gault and the 

 accompanying changes in the character of the Basement beds, has been found 

 tinadaptable for reprodiiction. Its 'purpose can be served by a combination of 

 the small figured sections, in the following order (south-west to north-east) : — 

 figs. 17, 16, 4, 3, 6, & 14. All these are to the same scale, and their relative 

 position is shown on the sketch-map (fig. 1, p. 2). 



