On the Structure of the Strata between the London Clay and the Chalk in the London and Hampshire Tertiary Systems

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Abstract
The first part of this paper, containing a description of that portion of the Lower Tertiaries immediately under the London clay, and which I termed the “ Basement Bed of the London Clay ,” was read before the Society in January 1850. On that occasion my object was to show, that over the whole of the south of England tertiary area, a stratum forming a distinct and constant geological horizon clearly separated the London clay from the group of strata beneath it. The variable and interesting set of deposits between the Chalk and the London clay, including the “Basement Bed” of the latter, form the group hitherto called “the Plastic Clay Formation,” which has been described “as composed of an indefinite number of sand, clay, and pebble beds, irregularly alternating ,” and as being “members of one great series of nearly contemporaneous deposits,” and essentially of fresh and brackish water origin. A careful examination of these strata has led me to believe, on the contrary, that a regular and definite order of superposition does exist, and that, instead of one series of alternating and intercalated strata, the conditions of structure and changes in the fauna show that there are five well-marked and distinct groups. Three, however, of these groups are apparently synchronous. Therefore the number of consecutive and separate divisions of the Lower Tertiaries may be reduced to three.

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