5
Crag banks stood up as islands well above the water, and then
sank again, but not quite uniformly, till deep water covered their
highest parts. At Sutton we find that a low cliff was cut by the
sea into the consolidated Coralline Crag during a stoppage of the
downward movement. Then this cliff sank out of reach of the
waves, and during another pause a second cliff was formed a few
feet back from the first. Ultimately this was also submerged,
and the Red Crag was banked up against the faces of both, with
great blocks of the Coralline Crag, some of them more than a ton
in weight, fallen into it from the cliff, and indenting the beds that
had been formed previously to the fall. On the face and brow of
the lower cliff are found the burrows of Pholas, often with the
shells in the position of life.
Although the Red Crag reaches the thickness of 120 feet in
places, the lower part only of the deposit generally contains
shells, as percolating water has usuall}' dissolved away all
calcareous matter from the upper part, leaving unfossiliferous red
sand, sometimes cemented by iron-oxide into a hard rock. Cases
are known in which a cap of clay, by stopping percolation, retains
shelly Crag under it, rising as a pillar in unfossiliferous, because
decalcified, sand. In other cases, a bed of pebbles may be seen
passing continuously from the shelly into the barren sand.
The most interesting part of the Red Crag is, perhaps, the
thin, but valuable, layer at the bottom, extensively worked for
the phosphatic nodules of which it largely consists, and which are
generally miscalled " coprolites." They are not fossil excrement,
(as that term implies), but merely lumps of London-clay and
Septaria, saturated with phosphate of lime. Mingled with these
are many bones and teeth of marine and terrestrial animals, also
highly phosphatised, and stones of various sorts, flint being the
most common. All these are much rolled and waterworn, a
collection, in fact, of the heavier matters which have worked
their way down to the original sea-floor in the repeated shiftings
of the shelly sands.
But a still more important element (at least from a scientific
point of view), in the composition of the nodule-bed, is the
presence in considerable quantity of irregular lumps, generally
rounded, of a tough brown sandstone, often containing the cast
of a shell, and therefore called " box-stones " by the workmen.
Their fossils show them to belong to the Diestian, or lowermost
division of the Pliocene, represented in England only by some