|)art 1] SECTION AT WOEMS HEATH. 17 



Passing into Surrey, a fair- sized mass was recorded on the old 

 map (Sheet 7) by a boundary-line and the word ' Pebbles,' near 

 Egham ; but the new survey has wiped this out from Bagshot, and 

 included it with Drift. To make up for this, however, the new 

 map (269, not }^et published) will show a larger mass of pebble- 

 beds, just south, at New Egham. Judging from the Index of 

 the 6-inch map (Surrey, 4 S.E.), this is in the lower part of the 

 Bagshot Sand. In the Memoir on the new map quartz-pebbles 

 have been recorded from this neighbourhood. 1 



At St. Ann's Hill, near Chertsey, a pebble-bed is again shown : 

 at first it was classed with the Bracklesham Beds, and Mr. C. N. 

 Bromehead looks upon some of the beds here as of that age and 

 some as Bagshot. 2 



It is in Essex that the Bagshot Pebble-Beds are best developed, 

 .and there they overlie the sand, which is not always the case in 

 Surrey, etc. They have been mapped at Havering, between Kel- 

 vedon Hatch and South Weald, at Brentwood and Warley, at 

 Erierning, south-west of Writtle, at Billericay, Stock, and Galley- 

 wood (all in Sheet 1 of the old map), and were described by 

 H. B. Woodward. 3 



Now, some of these Essex pebble-beds are only 9 or 10 miles 

 from the broad Kentish tract of the Blackheath Beds, though 

 .some are about 18 miles away. We are, therefore, prepared for the 

 probability of the newer beds having been derived from the older. 



It is not, however, with the nearest point of outcrop of the 

 Blackheath Beds that we are concerned, but with the probable 

 former occurrence of those pebble-beds miles beyond the present 

 main mass, even beyond the farthest outliers. It is in the western 

 part of Kent and in the neighbouring border of Surrey that we find 

 .such extension, right out to the crest of the North Downs, and we 

 have every reason to infer that it continued farther south over part 

 of what is known as the Wealden area, including therein all the 

 beds below the Chalk. 



Clearly the Bagshot sea must have crossed over the London Clay 

 until it reached the underlying Blackheath Beds, and most likely 

 passed still farther to the Chalk. We may take it, therefore, that 

 the Bagshot Beds of Essex got their pebbles from the Blackheath 

 Beds on the south, and partly perhaps direct from the Chalk a 

 little farther south. 



This leads to the question whether Ave have any evidence of the 

 thinning of the London Clay southward from Essex that is thus 

 pointed to ? It is in Essex that the London Clay reaches its greatest 

 thickness, up to well over 500 feet, and on the northern margin 

 of Kent there is a considerable thickness, though the very top is 



1 Mem. Geol. Surv. vol. iv (1872) pp.315, 316; and C. N. Bromehead, 

 ' Geology of the Country around Windsor ' Mem. Geol. Surv. 1915, p. 36. 



2 ' Geology of the Country around Windsor ' Mem. Geol. Surv. 1915. 

 pp. 37, 50. 



3 Mem. Geol. Surv. vol. iv (1872) pp. 322 -27 ; and ' The Geology of London ' 

 &c.' vol. i (1889) pp. 272-79. 



Q. J. G. S. No. 297. c 



