PLEISTOCENE GEOLOGY OF THE THAMES VALLEY. 363
ally descend. The next section to be noticed goes considerably
further than this since it shows that in this process lies the true
secret of the origin of the valleys.
Just to the west of Belmont Castle a small north and south
valley occurs, the floor of which is at an elevation of from ninety
to one hundred feet O.D. Its southern extremity is trancated by
the northern face of the Chalk-quarry of the Grays Portland
Cement Works which thus affords a good transverse section
through it. The surface of the Chalk has been eroded by
carbonated water so as to form a groove into which the Thanet
Sand and High Terrace Gravel have subsided. Here and there
where some portions of the Chalk have offered more resistance
to the solvent action than the main bulk small irregularly
Fig. 3. Section across Valley S.W. of Belmont Castle.
rounded bosses stick up and protrude into the overlying strata.
The axis of this furrow is coincident with that of the valley, both
in direction and in longitudinal position, while the general
contour of the furrow and that of the valley floor as seen in
transverse section are parallel (Fig. 3).
The section afforded by the northern face of the quarry
west of Milwood Lane gives similar evidence of the erosion of
the surface of the Chalk into grooves in this district, but the
relation of the surface contour to that of the groove is not so
apparent here as in the last instance as the ground has been
subsequently modified by other subaerial denuding agents to a
considerable extent.