PLEISTOCENE GEOLOGY OF THE THAMES VALLEY.
365
acidic water upon the Chalk of the district. It is evident,
however, that a process which in other districts has produced,
but comparatively infinitesimal results must have been favoured
in this neighbourhood by conditions which have greatly accele-
rated and regulated its working. Sir Joseph Prestwich in his
masterly essay upon the origin of the Sand and Gravel-pipes in
the Chalk (9) was the first to attack the subject of the conditions
which must necessarily govern their formation. He maintained
that pipes were not only the result of the chemical dissolution of
the Chalk, but, moreover, under existing conditions in the great
majority of instances they could not now be formed. He argued
that the existence of a higher level of saturation was essential
to the formation of pipes, and this increase in height was of a
secondary nature, depending not so much upon the elevation
of the land above the sea as upon the relative permeability of
the Chalk and its superincumbent strata together with the
unbroken continuity of the latter across broad stretches of
country. Further he held that the pipes did not necessarily
imply the existence of fissures or joints in the Chalk, but that
such occurrences would retard rather than accelerate their
formation. In the case before us, however, Prestwich's con-
clusions cannot be applied in their totality, but rather require
some little modification.
It is clear that there is one important distinction between
the dissolution of the Chalk into pipes on the one hand and the
formation of valleys by the same means on the other. In the
one case the conditions rendering the Chalk liable to the attacks
of solvents are extremely local and may be but slight, but in
the case of the valleys the conditions are not local, in the sense
of the term applied to the pipes, but on the contrary extend
over a considerable horizontal distance, giving rise to definite
lines or planes of weakness. The nature of these lines of weak-
ness, which constitute the valley axes, is shown by the geotec-
tonic geology of the district.
The Chalk of the Grays Thurrock Area occurs as an inlier
brought up by a small anticlinal fold, later in date than the great
Wealden and London Basin movements as is indicated by the
following data. At Stifford it in common with the Lower
London Tertiaries dips to the north at an angle of about
10°, and thus is soon lost beneath the London Clay. But in