"Marl," as commonly used, is Clay or Carbonate of Lime of a clayey
texture or any mixture of these.
Beds of shell marl tell the same tale as the peat. Shells do not
accumulate to any extent in the bed of a river. They are pounded up and
decomposed or rolled along and buried where mud or gravel finds a
resting place. Only sometimes, where things of small specific gravity
are gathered in holes and embayed corners, a layer of freshwater shells
may be seen.
But to produce a bed of pure shell marl the quantity of dead shells
must be very large and the amount of sediment carried over the area
very small, while the margin of the pond or mere in which the formation
of such a bed is possible must have an abundant growth of confervoid
algae and other water plants to furnish sustenance for the molluscs.
Shell marl therefore suggests ponds and meres. Of course it must be
borne in mind that in a region of hard water, such as is yielded in
springs all along the outcrop of the chalk, there is often a
considerable precipitation of carbonate of lime, especially where such
plants as Chara help to collect it, as the Callothrix and Leptothrix
help to throw down the Geyserite.
These beds of white marls, whether due to shells or to precipitation,
are thus of great importance for our present enquiry as they throw
light on the history of the Fens.
We should have few opportunities of examining the marl were it not for
its value to the agriculturist. As it consists of clay and lime, it is
not only a useful fertiliser but also helps to retain the dusty peat,
which when dry and pulverised is easily blown away. Moreover, as the
marl occurs at a small depth and often over large areas, it can
commonly be obtained by trenching on the ground where it is most
wanted.