Now we may enquire what are the limits within which we may speculate as
to the age of the Fen Beds.
These Turbiferous deposits all belong to one stage, though it may be
one of long duration. They are sharply separated from the Areniferous
deposits, i.e. the sands and gravels of the terraces and spurs which
always pass under and, in fairly large sections, can always be clearly
distinguished from the resorted layers at the base of the Fen Beds.
There is no definite chronological succession which will hold
throughout the Fens. The variations observed are geographical--clay,
marl, peat, etc., alternating in different order in different
localities and subaerial, fluviatile, estuarine, and marine, having
only a changing topographical significance.
The Fen Beds crept over an area where the underlying formation had been
undergoing vicissitudes due to slow geographical changes--changes
which, being at sea level and near the conflict of tides and upland
water, produced irregular but often important results.
There is not in the Fens any _continuous_ record of what took place
between the age in which the Little Downham Rhinoceros was buried in
the gravel and that in which the Neolithic hunters poleaxed the Urus
in the peat near Burwell.