58
CONTRIBUTIONS TO PLEISTOCENE GEOLOGY
mere granules. This boss affords one of the finest illustrations of the solvent
processes of acidic water that we have as yet seen in the district. It forms the
western boundary of a groove in the chalk, which is coincident in direction with
the small valley or gap through which Milwood Lane here passes. Crossing the
lane and the valley, we enter the Lion Pit, and here the surface of the chalk is.
seen rising up again to the east, forming the eastern boundary of the groove. The
floor of this valley is formed here by the Thanet Sand, and to the north by the
High Terrace gravel as well, which has subsided into the groove. As to the-
precise depth of the furrow or the thickness of the Thanet Sand let down into it
we are ignorant, but examination of the sections on either side shows that both
must be considerable.
The section in the Lion pit is of an imposing character, being quarried in one
working so as to give a face more than 100 feet in height. At the top are a few
feet of bedded gravel, then very many feet of Thanet Sand with a bed of green-
coated flints resting upon the chalk. The thicknesses, owing to the method of
working, are rather difficult to estimate.
II. THE PHYSICAL HISTORY OF THE LOWER-
THAMES VALLEY.
Our labours have led us to infer that the key to the physical
history of the Lower Thames Valley and its contained deposits-
lies really in ascertaining whether the unaided processes of river
action would be capable of producing the structures therein
developed, or whether they are the result of river action operating
under varying influences of environment, e.g., the relation of the
land to the sea. It is necessary first, before endeavouring
to answer these questions, to inquire into the nature and power
of river action.
Every river has an inherent power to sweep along detritus..
The degree to which this power is developed depends upon the
two factors to which the power owes its origin, viz., the volume
of water composing the river and the declivity of the channel
over which the water flows, for the declivity and the volume
govern the velocity of the stream. Therefore the capacity for
transporting detritus in a river of given volume depends upon
its velocity, which, in turn, is dependent upon the declivity of its
channel. The burden to be carried is supplied at the outset to-
the river by rain and frost. The material thus supplied may be
termed the initial load of the river, and forms the tool with which
the latter commences its work.
Now it usually happens that this initial load of sediment does-
not satisfy to the full the capacity for transportation which the
stream possesses. Consequently it is in a position to add to its