Skeleton lately found at the Tilbury Docks, Essex. 137
deposited is covered by the mud or silt in suspension in the
turbid waters, which fills up the slight hollows and produces
the flatness of surface characteristic of alluvium. This
deposition is accompanied by a corresponding rise in the bed
of the river. The fragments of former alluvial deposits, found
as terraces at various heights on the sides of the valleys of
the Thames and other rivers, mark the levels at which the
stream once flowed, the highest terrace being the oldest.
These terraces are the remains of broad continuous sheets of
ancient alluvium deposited under much the same circumstances
as those in which the lowest flats are now being formed. An
upheaval occurring would increase the fall of the river, and
cause it to deepen its channel. In doing so it would destroy
much of its old alluvial plain, and, sooner or later, cut its
way downward to a point at which it would again cease to
deepen its bed, and would form another flat at a lower level
than the first; and so on.
Tilbury Docks are being made in the newest of the river-
deposits of the Thames Valley, the alluvium of the marshes,
the deposition of which would still be going on, but for the
embankment of the Thames during (or before) the Roman
occupation of Britain. An average section of the beds visible
(May, 1884) would give :—
But though only two peaty beds were visible in the dock-
excavations, we were informed by Mr. J. B. Pinker, the
gentleman connected with the works who explained their
plan during our excursion in May, that a third and still
lower peaty bed had been met with here and there in the
trial-borings made within the dock area. This third bed
is noted in the account of the section of the New Well at