The Fen Beds belong to the last stage and, notwithstanding their great
local differences, seem all to belong to one continuous series. Seeing
then that their chief characteristic is that they commonly contain
beds of peat it may be convenient to form a word from the late Latin
_turba_, turf or peat, and call them Turbiferous to distinguish them
from the Areniferous series which consists almost entirely of sands
and gravels.
When the land had sunk so far that the velocity of the streams was
checked over the widening estuary and on the other hand the tide and
wind waves had more free access, some outfalls got choked and others
opened; turbid water sometimes spread over the flats and left mud or
was elsewhere filtered through rank plant growth so that it stood clear
in meres and swamps, allowing the formation of peat unmixed with earthy
sediment.
Banks are naturally formed along the margin of rivers by the settling
down of sand and mud when the waters overflow, as seen on a large scale
along the Mississippi, the Po, as well as along the Humber and its
tributaries.
The effect of a break down of the banks is very different. A great hole
is scooped out by the outrush, and the mud, sand and gravel deposited
in a fanshape according to its degree of coarseness and specific
gravity.
A good example of this was seen in the disastrous Mid-Level flood at
Lynn in 1862[1] and the more recent outburst near Denver in the winter
of 1914-15[2], of which accounts were published in contemporary
newspapers. The varied accompanying phenomena can be well studied in
the process of warping in Yorkshire or the colmata in Italy.
[1] _Times_, _Cambridge Chronicle_, May 31, 1862.
[2] _Times_, Jan. 16, 1915.
This was a much commoner catastrophe in old times, before the banks
were artificially raised, and, as the streams could never get back into
their old raised channel, this accounts for the network of ancient
river beds which intersect the Fens.
The bottom of the Turbiferous alluvium is always, as far as my
experience goes, sharply defined. This of course cannot be seen in a
borehole or very small section.
The surface of the older deposits seems to have been often washed clean
either by the encroaching sea or by the upland flood waters.
In saying that there is an absence of sand and gravel in the Fen Beds
we must be careful not to force this description too far. For when the
first encroaching water was washing away any pre-existing superficial
deposits the first material left as the base of the Fen Beds must have
depended upon the character of the underlying strata, the velocity of
the water and other circumstances.
This is well seen in the Whittlesea brickpit where an ancient gravel
with marine shells rests on the Oxford Clay and over the gravel there
creeps the base of the Turbiferous series. It here consists chiefly of
white marl which thins out to the left of the section and above becomes
full of vegetable matter until it passes up into peat, over which there
is a flood-water loam.
About a mile west-north-west of Little Downham near Ely, and within a
couple of hundred yards of Hythe, the Fen Beds were seen in a deep cut
carried close to the gravel hill which here stretches out north into
the Fens.
They consist at the base of material washed down from the spur of
gravel and sand of the Areniferous series against which the Fen Beds
here abut.
This basement bed is succeeded by beds of silt and peat of no great
thickness as they are near the margin of the swamp.
When any considerable thickness of the older Areniferous gravels has
been preserved, the base of the Turbiferous series is smooth or only
gently undulating. But where only small patches or pot-holes of gravel
remain, there the top of the clay has been contorted and over-folded so
as often to contain irregularly curved pipes and even isolated nests of
sand and gravel[3]. The base of the Areniferous gravel must generally
have been thrown down upon clay which had been clean cut to an even
surface by denudation without any soaking of the surface or isolated
heaps of gravel sinking into the clay under alternation of dry and wet
conditions, such as would puddle the surface under the heaps and allow
the masses of heavy gravel to sink in pipes and troughs. These small
outlying patches of gravel are sometimes so little disturbed that we
leave them in the Areniferous, whereas they are sometimes so obviously
rearranged that we must include them in the Turbiferous series, taking
care not to include derivative bones from the older in our list of
fossils from the newer series.
[3] Cf. _Archaeol. Journ._ Vol. LXIX, No. 274 2nd Ser.; Vol.
XIX, No. 2, pp. 205-214.