+---------- High water (12' 6" above O.D.)
|
12' 6" |
12' 6" +---------- Ordnance Datum
4' 0" | Silt and clay
16' 6" +----------
{|
{+---------- Low water (6' 0" below O.D.)
{|
{|
19' 6"{|
{|
{|
{+---------- Bed of river (17' 6" below O.D.)
{|
36' 0" +----------
|
|
|
9' 0" | Sand with shells
|
|
|
45' 0" +----------
3' 6" | Loam and sand
48' 6" +----------
|
5' 6" | Ballast with shells
|
54' 0" +----------
3' 6" | Loam with Peat
57' 6" +----------
3' 6" | Fine red ballast
| mixed with clay
61' 0" +----------
5' 0" | Blue and grey clay
| mixed with sand
66' 0" +----------
1' 0" | Ballast
67' 0" +----------
|
4' 6" | Silty Sand
71' 6" +----------
| Ballast with flint
1' 6" | and stone
73' 0" +----------
|
|
| Stiff grey clay
|
|
[Illustration: Fig. 5. Section seen at Sutton Bridge.]
Here again we see that the only peat is a bed between three and four
feet in thickness of mixed loam and peat more than 40 feet below mean
sea level.
From these sections it is clear that along the direct and more
permanent outfall from Cambridge to the north, peat forms but a small
part of the Fen Beds.
Peat is a substance of so much value as fuel, of such importance to the
agriculturist, of such commercial value in what we may call its
by-products, and of such scientific interest in the history of its
formation and the remains which its antiseptic properties have
preserved, that it has, as might be expected, a large literature of its
own.
I have before me a list of more than 150 references to peat or to the
Fens.
PEAT; TREES AND OTHER PLANTS; TARN PEAT AND HILL PEAT; BOG-OAK AND
BOG-IRON.
When we turn aside into the areas cut off by spurs of gravel and
islands of Jurassic rock, we find wide and deep masses of peat which
has grown and been preserved from denudation in these embayed and
isolated areas. Burwell Fen, for instance, protected on the north and
west by the Cretaceous ridge of Wicken and the Jurassic ridge of
Upware, furnishes most of the peat used in the surrounding district. If
we travel about two miles to the north-west from the pit dug near the
railway station (see Fig. 4, p. 11) over the hill on which Ely stands,
we shall come to West Fen, where there is a great mass of peat which
has grown in a basin now almost quite surrounded by Kimmeridge Clay.
In this there is a great quantity of timber at a small depth from the
surface. The tree trunks almost all lie with their root-end to the
south-west, but some are broken off, some are uprooted, telling
clearly a story of growth on the peat which had increased and swelled
till the surface was lifted above the level of floods. Then some
change--perhaps more rapid subsidence, perhaps changes in the
outfalls--let in flood water, the roots rotted and a storm from the
south-west, which was the most exposed side and the direction of the
prevalent winds, laid them low. The frequent occurrence of large
funguses, _Hypoxylon_, _Polyporus_, etc., points to conditions at
times unfavourable to the healthy growth of timber.
It is worth noting when trying to read the story of the Fens as
recorded by their fallen trees that in all forests we find now and then
a few trees blown down together though the surrounding trees are left.
This may be the result of a fierce eddy in the cycloidal path of the
storm, but more commonly it seems to be due to the fact that every tree
has its "play," like a fishing rod, and recurring gusts, not coinciding
with its rhythm, sometimes catch it at a disadvantage and break or blow
it down.
The story told by the West Fen trees is quite different from that told
by the water-borne and water-worn trunks in the section by Ely station.
The same variable conditions prevailed also in the more westerly tracts
of the Fen Basin, but the above examples are sufficient for our present
purpose.
From the large numbers of trees found in some localities and from
records referring to parts of the Fens as _forest_ it has sometimes
been supposed that the Fens were well wooded, but forest did not
generally and does not now always mean a wood, as for example in the
case of the deer forests of Scotland.
When Ingulph[5] says that portions of the Fenland were disafforested by
Henry I, Stephen, Henry II, and Richard, who gave permission to build
upon the marshes, this probably meant that they no longer preserved
them so strictly, but allowed people to build on the gravel banks and
islands in them.
[5] _History of Croyland_, Bohn's edition, p. 282.
Dugdale, recording a stricter enforcement of game-laws, quotes
proceedings against certain persons in Whittlesea, Thorney and Ramsey
for having "wasted all the fen of Kynges-delfe of the alders, hassacks
and rushes so that the King's deer could not harbour there." He does
not mention forest trees.
In the growth and accidents of vegetation in a swamp there are some
circumstances which are of importance to note with a view to the
interpretation of the results observed in the Fens.
For instance in fine weather there is a constant lifting and floating
of the confervoid algae which grow on the muddy bed of the stream. This
is brought about by the development of gas under the sun's influence in
the thick fibrous growth of the alga. The little bubbles give it a
silvery gleam and by and by produce sufficient buoyancy in the mass to
tear it out and make it rise to the surface dropping fine mud as it
goes and thus making the water turbid. Other plants, such as
Utricularia, Duckweed, etc., have their period of flotation, and in the
"Breaking of the Mere" in Shropshire we have a similar phenomenon. In
the "Floating Island" on Derwentwater the same sort of thing is seen
with coarser plants. All these processes are going on in the meres and
in the streams which meander through the Fens and did so more freely
before their reclamation. But besides this, when the top of the spongy
peat is raised above the water level and dries by evaporation, then
heath, ferns and other plants and at last trees grow on it, until
accident submerges it all again.
This at once shows why we often find an upper peat with a different
group of plant remains resting upon a lower peat with plants that grow
under water.
The most conspicuous examples of these various kinds of peat we see in
the mountainous regions of the North and West, where the highest hills
are often capped with peat from eight to ten feet in thickness,
creeping over the brow and hanging on the steep mountain sides.
Sometimes, close by, we see the gradual growth of peat from the margin
of a tarn where only water-weeds can flourish.
The "Hill Peat" is made up of Sphagnum and other mosses and of ferns
and heather.
The "Tarn Peat" of conferva, potamogeton, reeds, etc.
As Hill Peat now grows on the heights and steeps where no water can
stand and Tarn Peat in lakes and ponds lying in the hollows of the
mountains and moors, so the changes in the outfalls and the swelling
and sinking of the peat have given us in the Fens, here the results of
a dry surface with its heather and ferns and trees, and there products
of water-weeds only, and, from the nature of the case, the subaerial
growth is apt to be above the subaqueous.
One explanation of the growth of peat under both of these two very
different geographical conditions is probably the absence of
earthworms. The work of the earthworm is to drag down and destroy
decaying vegetable matter and to cast the mineral soil on to the
surface, but earthworms cannot live in water or in waterlogged land,
and where there are no earthworms the decaying vegetation accumulates
in layer after layer upon the surface, modified only by newer growths.
Some years ago a great flood kept the land along the Bin Brook under
water for several days and the earthworms were all killed, covering the
paddock in front of St John's New Buildings in such numbers that when
they began to decompose it was quite disagreeable to walk that way. It
reminded me of the effects of storm on the cocklebeds at the mouth of
the Medway, where the shells were washed out of the mud, the animals
died on the shore and the empty shells were in time washed round the
coast of Sheppey to the sheltered corner at Shellness. Here they lie
some ten feet deep and are dug to furnish the material for London
pathways.
In those cases when the storm had passed the earthworms and the cockles
came again, but the Hill Peat is always full of water retained by the
spongy Sphagnum and similar plants, and the Fens are or were
continually, and in some places continuously, submerged and no
earthworms could live under such conditions.
The blackness of peat and of bog-oak may be largely but certainly not
wholly due to carbonaceous matter. Iron must play an important part.
There is in the Sedgwick Museum part of the trunk of a Sussex oak which
had grown over some iron railings and extended some eight inches or
more beyond the outside of the part which was originally driven in to
hold the rails. Mr Kett came upon the buried iron when sawing up the
tree in his works and kindly gave it to me. From the iron a deep black
stain has travelled with the sap along the grain, as if the iron of the
rail and the tannin of the oak had combined to produce an ink. The
well-known occurrence of bog-iron in peat strengthens this suggestion.
An opportunity of observing this enveloping growth of wood round iron
railings is offered in front of No. 1, Benet Place, Lensfield Road.
The trees in the Fens often lie at a small depth and when exposed to
surface changes perish by splitting along the medullary rays.
It is not clear how long it takes to impart a peaty stain to bone, but
we do find a difference between those which are undoubtedly very old
and others which we have reason to believe may be more recent. Compare
the almost black bones of the beaver, for instance, with the light
brown bones of the otter in the two mounted skeletons in the Sedgwick
Museum.