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BOULDEE-CLATS OF LINCOLNSHIRE. 129
east of the Lincolnshire Wolds do not form parallel series. If the
Chalky Boulder-clay and the Purple Boulder- clay were correla-
tives, we should be confronted with the anomaly of a continuous
succession existing on one side of the Wolds, while there is a
discontinuous succession on the other.
Assuming that they are not parallel series, and also supposing
for the moment that the brown clays on either side of the dividing
ridge in Lincolnshire are complete correlatives, then it is the
Chalky Clay which is unrepresented on the eastern side. Now in
East Lincolnshire the base cannot be seen ; but in Yorkshire there is
a grey and chalky Basement-clay (beneath the Purple-clay) which
was originally regarded by Messrs. Wood and Rome as the equivalent
of the Upper Glacial-clay of East Anglia. Moreover we have Mr.
Lamplugh's testimony as to the appearance of a break with more
or less unconformity between this Basement-clay and the overlying
Purple-clay.
I have little hesitation therefore in considering the Basement-
clay of Holderness the sole representative of the grey and white
Boulder-clays of Lincolnshire, and in regarding the overlying beds
of Brown Boulder-clay, with their associated loams, sands, and
gravels, as newer and higher members of the great Glacial series.
This conclusion differs from that recently enunciated* by Mr.
Searles Wood, but it is not the only point on which his earlier views
appear to me to be sounder that those he has subsequently
elaborated.
The position of the chalky " Basement-clay " at and below the
sea-level in Holderness, while on the Wolds of Lincolnshire it rises
to a great height above the sea, offers no difficulty to this correlation ;
because the same clay occurs at nearly as low a level on the west
side of those Wolds. I apprehend, indeed, that the Chalk Wolds were
at one time completely mantled by the " Chalky Clay" from side to
side ; just as the Jurassic ridge was clearly so covered in Rutland
and South Lincolnshire.
Fig. 4. — Diagram to shoiu the Erosion of the Chalk Wolds,
o
O, older Boulder-clay. N, newer Boulder-clay.
The available evidence seems to indicate that the eastern slope of
the Chalk W^olds was invaded and destroyed by marine erosion
during a subsequent submergence, at or immediately before the time
when the newer (brown) clays were deposited, and that these were
laid down upon the plane of erosion so formed. This idea is
expressed in fig. 4, which is not a section along any particular line,
but only a diagrammatic representation of the relations above
suggested, the plane of erosion being partly across the Basemen t-

• Quart. Journ. Geol. Soc. vol. xxxvi. p. 527, and xxxviii. p. 712.