Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/199

1869.] SEARLES WOOD BOULDER-CLAY. 99 flicting with this hypothesis, that purple clay entirely destitute of chalk, but identical in most other respects with the purple clay containing chalk, extends for many miles over the extreme north-east of the Wold, ranging there from the sea-level up to altitudes of 450 feet — and that at intervals along the Holderness coast-section as far as Dimlington, and 42 miles south of the northern limit of the Wold, outliers of this purple clay without any chalk cap the purple clay with chalk that diminishes in quantity upwards.

If we merely examine the position of the clay where it lies at the Wold-foot near Speeton, more than 500 feet below the contiguous Wold-summit, without even enlisting into the argument the fact that the same clay extends over the Wold itself*, we shall, I think, perceive the impossibility of a sea-drift in any direction whatever preventing the introduction of chalk debris into it.

In the accompanying sketch map (see Plate) I have delineated, by a strong line, the exact trend of the Wold-scarp, and indicated by shading the respective positions where the clay without and that with chalk occur; and, to render the position of the clay without chalk relatively to the Wold more clear, I have added a small section (No. 2, see Plate) that will answer for the direction A to B, or A to C, indifferently.

If the Wold was uncovered by the sea (which it must have been to have supplied chalk debris), it is apparent that it must have formed a shore to any sea extending where this chalkless clay occurs, and must have arrested any drift, causing this to go off in the direction of the arrows — that is, either south-east in the direction of Flamborough, or south-west in the direction of York. Nevertheless in both these directions the clay is destitute of chalk. In the former it is so, both at high and low levels, for nearly 15 miles south-east of the northern apex of the Wolds near A, and in the latter for a much greater distance, viz. beyond York, even to the southern part of central Yorkshire†. The northern apex of the Wold rises to elevations of between 400 and 575 feet, the very highest summits (which are towards the north-west angle of the Wold), ranging between 600 and 800 feet. If we reflect what a copious source of debris this scarp-shore of chalk, indented with several valleys opening through it into the great vale beneath, must have been, and how such debris must have been swept into a sea occupying this great vale, it seems to me to be repugnant to the operation of natural causes to suppose that clay wholly destitute of chalk could be deposited in this great valley, while clay teeming with chalk was being deposited in Holderness. So obvious does this appear to me, that it is unnecessary to add to the case by appealing to the fact that the same clay without chalk envelopes both the high and low parts of the chalk Wold down nearly to Flamborough.

When we come to consider the volume and origin of the chalk


 * The upper representation of the triple section (Pl. VII.) shows this.

† The clay in the vale of York is, in some parts, overlain by Postglacial sands, containing flint derived from the Wold. Postglacial gravel, with flint, also occurs in the vale of Pickering, which skirts the northern Wold-scarp.

h2