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 débris 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 débris), 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 Flam- 
borough, 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 Yorkshirey. 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 be- 
tween 600 and 800 feet. If we reflect what a copious source of 
débris this scarp-shore of chalk, indented with several valleys open- 
ing through it into the great vale beneath, must have been, and 
how such débris 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 
oceurs in the vale of Pickering, which skirts the northern Wold-scarp. 
H2 
