300 H. A. Baker — The Pre-Thanetian Erosion 



are dependent tipon the information forthcoming from workers "who 

 have studied the Chalk of neighbouring districts. It is fairly 

 certain, however, that except in the west the whole of the sub- 

 Eocene surface of the area under present consideration is occupied by 

 the outcrop of the thick zone of Micraster coranguinum. At Crossness 

 the Chalk proved to be 684 feet thick, but the Chalk exposed to 

 view in the Woolwich district, not far away, is, to the writer's 

 personal knowledge, clearly referable to the M. coranguinum zone. 

 Of the 654 feet of Chalk occurring at Mile End, 259 feet were 

 assigned to the Upper Chalk, and, since allowance must be made for 

 the Chalk Rock and the M, cortestudinarium zone, it is not likely 

 that any higher horizon than that of M. coranguinum occurs here. 

 Even in the case of the 700 feet of Chalk which occurs between 

 Kentish Town and Mile End the statement still holds good. If we 

 take the thicknesses of the Lower and Middle Chalk as recorded at 

 Mile End, allow only 10 feet for the Chalk Rock and 50 feet for the 

 zone of M. cortestudinarium, we are left with 225 feet of Clialk, and 

 it is hardly likely that the M. coranguinum zone is much less than 

 this in thickness. 



Where the Chalk decreases to 500 feet in thickness, as it does 

 between Loughton and Turnford, the question arises as to whether 

 the M. cortestudinarium zone there composes the sub-Eocene surface. 

 At Turnford 406 feet were assigned to the Low^er and Middle Chalk 

 and 15 feet to the Chalk Rock. Allowing 50 feet for the If. cor- 

 testudinarium zone, we are still left with about 30 feet of 

 M. coranguinum Chalk. Hence, as already remarked, it is fairly 

 certain that except in the west the whole of the sub-Eocene Chalk 

 surface of our area is occupied by the outcrop of If. coranguinum 

 Chalk. 



But in the west a different state of affairs obtains. Here the 

 Chalk reaches a thickness of as much as 950 feet in the neighbour- 

 hood of Beaconsfield, and 900 feet a little further south at Taplow. 

 In this neighbourhood two deep wells have completely pierced the 

 Chalk, one at Slough and the other at Winkfield. With regard to 

 the first, beyond the fact that the Chalk was found to have a thick- 

 ness of 778 ft. 10 in., no further particulars concerning the formation 

 appear to be available. But in the case of the Winkfield well we 

 are more fortunately situated. A full account of this has been 

 published,^ and Ave are in possession of the information that the 

 Lower Chalk was 219 feet thick, the Middle Chalk 169 feet, the 

 Chalk Rock 8 feet, and the Upper Chalk 329 feet. Turning now to 

 other authorities for information, we find that Mr. H. J. Osborne 

 White ^ suggests 50 feet as the thickness of the M. cortestuditiarium 

 zone and 220 feet as that of the M. coranguinum zone in this area. 

 Adopting these figures, we find that at Winkfield we get 666 feet as 

 the thickness of the Chalk from the base of the formation to the base 

 of the Marsupites zone, thus leaving 59 feet of Chalk for the latter 



^ Water Supply of Berkshire (Mem. Geol. Surv.), 1902, pp. 93-6. 

 ^ " Berkshu-e and Part of the Thames Valley" : Jub. Vol. Geol. Assoc, 

 pt. ii, 1910. 



