THE DRIFTS OF SOUTH-WESTERN ESSEX. 157
through alternate freezing and thawing of the ground that
produces a sort of concertina movement, associated with frost
fissures [Paterson, 1940]. We see the effects in the vertical
position of the stones, in festooning, in the filled-in fissures,
and the like. Static deposits, as they were named by Barrow,
also result from soil-creep, rain-wash, and from the activities
of plants and animals.
"Cretaceous Stones."—When the Wealden dome was
first elevated it was covered by a great thickness of chalk.
Much of this had already been removed, and the residual flints
had become Tertiary pebble beds, before the end of the Eocene
period, with the result that the underlying Lower Greensand was
uncovered. The chief durable constituent of the Lower Green-
sand is its chert, which is peculiar to the Wealden area, and
is not found to the N.W. of the Chilterns. There are also
many pebbles of white quartz, and a few of lydian stone, etc.
All these the local rivers gathered into their gravels ; together
with flint nodules which were, at the time, freshly excavated
from the chalk.
Many of the drifts are characterised by the inclusion of the
same constituents, derived from the Wealden area. As a
convention, I shall refer to the suite carrying this implication
of origin, including the chert (primarily), together with flint
nodules as defined above, and lydian stone, etc., as "Cretaceous
stones." Quartz pebbles are excluded as these may not all
carry the same implication.
The apparent origin of very rare erratics may be misleading,
as chert and quartzite occur in Tertiary pebble beds, and igneous
rocks in the Chalk.
The Western Drift [Barrow 1918, Hawkins 1923, Sand-
ford 1929].—I feel that there is much to be said in favour of
an early glacial drift bringing quartz pebbles, Bunter quartzites,
and rocks from Charnwood, Devon, and Wales, into the East
of England. This began before the Pebble Gravel stage, and
appears to have been continued, or revived, after. Some
authorities claim local ice on the Chilterns. This stage is
sometimes correlated with the North Sea Drift of East Anglia,
but for myself I think it is best to leave wider correlations open.
The Pebble Gravel [Prestwich 1890, Salter 1905, Wool-
dridge 1923, 1927].—The oldest fluviatile deposits of the
London Basin are those best seen on the 400-420 ft. plateau of
Hertfordshire, which have been known for many years as "Pebble
Gravel." The constituents are ready-made Tertiary pebbles,
white quartz, "Cretaceous stones" (q.v.), and more rarely
other rocks such as Jurassic Rhaxella-chert or Arngrove stone.