Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/204

104 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Dec. 8. wards beyond Leicester — whereas the chalky clay extends to the southern parts of Essex and Middlesex*, and the sea depositing it extended still further, as is proved by the way in which the clay is cut off from denudation at elevations reaching up to 350 feet on the northern brow of the Thames valley; but to what distance it may have gone, the denudation has been too complete in that direction to justify an assertion, though, from the structure of the Thames gravel, I believe it to have stretched beyond the limits of England.

The long triple section, No. 3, traverses the entire length of this clay from its southern termination to its overlie in Holderness by the purple clay with chalk, and thence to the purple clay without chalk, which begins about Flamborough, and, crossing one of the lowest parts of the dividing ridge at Stainmoor, terminates at the Westmoreland Fells. The upper representation of this triple section is drawn to the existing sea-level, and shows, as nearly as the small scale will allow, the relative elevations along the line taken, the older formations forming the floor of the deposit, and the distribution of the clays in question along the line taken. The Lower Glacial series (which together forms a separate and unconformable deposit) lies to the east of the line of section taken, and may be altogether omitted from consideration in this question. The Middle Glacial, although it was a deposit formed under a marine climate very different from that obtaining at the time of the chalky clay which overlies it, yet appears to have been a deposit formed during the commencement of the submergence under which the chalky clay was accumulated, since in many parts it passes up by inter-bedding into that clay. Features in its structure and position, which I hope to enter upon in detail on a future occasion, indicate, moreover, as it seems to me, that although the marine climate was so different from that obtaining when the chalky clay was deposited as to have permitted neither the formation of Boulder-clay† nor the transport of rock-boulders, yet the land was occupied with ice in places which were afterwards covered deep below the chalky clay, these places being far south of the ice-limit indicated in the outline map. The line of section crosses this deposit at one part only, the bulk of it lying to the east of the line, though a considerable development of it also occurs to the west of it.

The middle representation of this triple section indicates what I conceive to be the conditions obtaining when the deposit of the chalky clay was taking place. This representation supposes a depression of from 600 to 700 feet below the present level, and it shows this clay as forming southwards from the foot of the ice-sheet on the western side of the Lincolnshire Wold, and supplied by material

that overlaps a little the unshaded part, and also overlies there the chalky clay which the shading represents. See sections 1, 5, and 11 of the paper of Mr. Rome and myself (Quart. Journ. Geol. Soc. vol. xxiv. pp. 148, 160, and 169.)
 * The purple clay with chalk forms a belt in Holderness and East Lincolnshire

† A band of Boulder-clay (chalky) does occur in the Middle Glacial gravels at one or two localities, and serves as an exception to prove the truth of this rule.