III.—The Unconformity between the Bagshot Beds and the London Clay

Abstract

In the paper on the well-section at Brookwood in the August Number of the GEOLOGICAL MAGAZINE, the author pointed out that a comparison of the lithological facts brought to light in that section with those furnished by two other deep-well sections (at Wokingham and Aldershot respectively) lead to an inference in favour of a considerable amount of denudation of the London Clay having taken place during the deposition of the Bagshot Beds; that is to say, irrespectively of the particular horizon in the Bagshot Series to which certain marginal portions of the Bagshot Beds may be ultimately assigned, there seemed to be pretty clear evidence furnished, from the London Clay itself, of the fact, that along their northern and southern flanks the Bagshot Beds rest on beds belonging to lower horizons in the London Clay formation than those on which the Bagshot Beds rest in the more central portions of the area. The present brief paper is an attempt to show that the above conclusion is borne out by a much wider induction from data gathered from the area of the London Basin in general. I have carefully gone through the 448 well-sections appended to Mr. Whitaker's Memoir on the London Basin, and have selected from among them such instances as seemed to throw any light upon this question. A large majority of those sections include the London Clay; but since, in a great number of them, the thickness of that formation in feet is given only, unaccompanied by any lithological notes on the strata, they have been omitted, as affording no help to us in the present investigation. There are, however, as many as 29 sections, in which such notes appear. In every one of these the Beading Beds or the Thanet Sands, and in most cases both these formations, were proved below the London Clay, so that we are quite sure of a correct basal horizon. In many instances also the Basement Bed of the London Clay is noted. In 22 of these 29 sections, the thickness of the London Clay is less than 200 feet, and in all these there is mention of such inclusions in the strata as ‘green sand,’ septaria (‘claystones’), pyrites, flint pebbles, and ‘shells.’ These therefore agree in this respect (without a single exception) with the lower half or rather more of the London Clay of the Brookwood section, as shown in the deep well at the Asylum on Knap Hill, which also (as shown in my last paper) agrees with the London Clay strata pierced at Wokingham on the northern, and at Aldershot on the southern, margin of the Bagshot area. From this I conclude that in all these 22 sections we have (within their several limits) homotaxial equivalents of strata of the lower half or rather more of the London Clay, as that formation is proved in the Brookwood section.