THE EAST COAST
It is not our purpose to describe in detail the many exposures of
submerged land-surfaces which have been seen on the shores of the
North Sea. This would serve no useful purpose and would be merely
tedious. We need only say that the floor of Eocene or Cretaceous
strata on which these ancient subaerial deposits rest is constantly
found at depths of 50 or 60 feet below the level of the existing
salt-marsh. But where, as in the estuary of the Thames and Humber, an
older channel underlies a modern channel, the floor sinks about 30
feet lower. From present marsh-level to ancient marsh-level is about
60 feet; from present river-bottom to old river-bottom is also about
60 feet. This, therefore, is the extent of the former elevation,
unless we can prove that the sea was then so far away that the river
once had many miles to flow before reaching it. This is the point we
have now to consider as we trace the submerged forests northward and
towards the deeper seas.
Before we leave the southern part of the North Sea basin it will be
well to draw attention to a few of the half-tide exposures which for
one reason or another may tend to mislead the observer. The mere
occurrence of roots below tide marks is not sufficient to prove that
the land-surfaces seen are all of one date.
Not far from Tilbury is found the well-known geological hunting
ground of Grays, where the brick-yards have yielded numerous extinct
mammalia and several land and freshwater shells now extinct in
Britain. These deposits lie in an old channel of the Thames, cut to
below mean-tide level, but here not coinciding exactly in position
either with the channel of the existing river, or with the channel in
which the submerged forests lie.
It is fortunate that the channels do not coincide, for this enables
us to distinguish the more ancient deposits. A glance at a geological
map shows, however, that they must coincide elsewhere, and where
the Thames has re-occupied its old channel it is clear that the
destruction of the earlier deposits may have led to a mixture of
fossils and implements belonging to three different dates. Mammoth
teeth and Palaeolithic implements, Irish elk and polished stone
implements, may all be dredged up in the modern river gravel,
associated with bits of iron chain, old shoes, and pottery. Such
a mixture does actually occur in the Thames estuary, and it makes
us hesitate to accept the teeth of mammoth which were dredged in
the Thames as really belonging to so late a period as that of the
submerged forests.
At Clacton a similar difficulty is met with, for there again an
ancient channel contains alternating estuarine and freshwater
deposits with layers of peat, and is full of bones belonging to
rhinoceros, hippopotamus, elephant and other extinct mammalia. Of
course the peat-beds in this channel are just as much entitled to
the name “submerged forest” as the more modern deposits to which
recent usage restricts it. They belong, however, to another and more
ancient chapter of the geological record than that with which we are
now dealing. I do not say a less interesting one, for they are of the
greatest importance when we study the times when Palaeolithic man
flourished; but at present we have as much as we can do to understand
the later deposits and to realize the great changes to which they
point. We must not turn aside for everything of interest that we come
across in this study; these earlier strata are worthy of a book to
themselves.
As we travel northward along the coast, again and again we meet with
evidence of a submerged nearly level platform, “basal plane,” or
ancient “plane of marine denudation,” lying about 50 feet below the
sea. We find it at Langer Fort, which lies opposite to Harwich on a
spit of sand and shingle stretching across Harwich Harbour. Here the
floor of London Clay was met with in a boring at 54 feet below the
surface.
The Suffolk coast north of Southwold yields yet another complication,
for between Southwold and Sherringham in Norfolk there appears at
the sea-level a land-surface considerably more ancient than anything
we have yet been dealing with. This is the so-called “Cromer
Forest-bed,” which consists of alternating freshwater and estuarine
beds, with ancient land-surfaces and masses of peat. It contains
numerous extinct mammals, mainly of species older than and different
from those of Clacton and Grays.
The mammalian remains differentiate these deposits at once; but if
no determinable mammals are found, the crushing of the bones and
the greater compression and alteration of the peaty beds serves to
distinguish them, for this Forest-bed dates back to Pliocene times,
passes under a considerable thickness of glacial beds, and has been
over-ridden by the ice-sheet during the Glacial epoch.
The Cromer Forest-bed has been exposed particularly well of late
years at Kessingland, near Lowestoft, where the sea has encroached
greatly. It is well worth while to make a comparative study of this
deposit, of the Grays and Clacton _Cyrena_-bed, of the submerged
forests of the Thames docks, and of the strata now being formed in
and around the Norfolk Broads. By such a comparison we can trace the
effects of similar conditions occurring again and again. The fauna
and flora slowly change, species come and go, man appears and races
change: though the same physical conditions may recur life ever
changes.
The Norfolk Broads, just referred to, deserve study from another
point of view: their origin is directly connected with the
submergence which forms the theme of this book. These broads are
shallow lakes, always occupying part of the widest alluvial flats
which border the rivers; but they are usually out of the direct
course of the present river; they therefore receive little of the
sediment brought down in flood-time. On the other hand they are
steadily being filled up with growing vegetation and turned into peat
mosses.
The origin of these shallow freshwater lakes, which form a
characteristic feature in the scenery of East Anglia, has been
much debated; but with the knowledge obtained from a study of the
submerged forests the explanation is perfectly simple. During this
period of slow submergence each of the shallow valleys in which the
broads now lie was turned into a wide and deep navigable estuary,
which extended inland for many miles. When the subsidence stopped the
sea and tides soon formed bars and sand-banks at the mouths of the
estuaries, and lateral tributaries pushed their deltas across. The
Norfolk rivers, being small and sluggish, were driven to one side,
and could neither cut away the sand-banks nor fill up with sediment
such wide expanses. These estuaries therefore were silted up with
tidal mud and turned into irregular chains of lakes, separated by
irregular bars and sand-banks. The lakes, instead of becoming rapidly
obliterated and filled up by deltas which crept gradually seaward,
remained as freshwater broads; for as soon as a bank became high
enough for the growth of reeds and sedges the river mud was strained
out and only nearly clean water reached the lagoon behind. Thus a
depression once left, provided it was out of the direct course of the
river, tended to remain as a freshwater lake until vegetable growth
could fill it, and the river mud was spread out over the salt-marshes
or went to raise the sand-banks till they became alluvial flats, and
thus still more thoroughly isolated the broad.
A few centuries will see the disappearance of the last of the broads,
which have silted up to an enormous extent within historic times;
but the fact that so many of these broads still exist may be taken
as clear evidence of the recent date of the depression which led to
their formation.
When we look at ancient records, and notice the rapidity with which
the broads and navigable estuaries are becoming obliterated, we
cannot help wondering whether the measure of this silting up may not
give us the date of the last change of sea-level. It should do so
if we could obtain accurate measurements of the amount of sediment
deposited annually, of the rate at which the sea is now washing
it in, and of the rate at which the rivers are bringing it down.
All these factors, however, are uncertain, and it is particularly
difficult to ascertain the part played by the muddy tidal stream
which flows in after storms and spreads far and wide over the marsh.
Though all the factors are so uncertain, we can form some idea of
the date of the submergence. Many years ago I made a series of
calculations, founded on the silting up of our east coast estuaries,
the growth of the shingle-spits, and the accumulation of sand-dunes.
The results were only roughly concordant, but they seemed to show
that the subsidence stopped about 2500 years ago and was probably
still in progress at a date 500 years earlier. This question of dates
will be again referred to in a later chapter.
Before leaving the Broad district we must refer to a boring made at
Yarmouth, which, according to Prof. Prestwich, showed that the recent
estuarine deposits are there 120 feet thick, and consequently that
the ancient valley was far deeper than any recorded in the foregoing
pages. There is no doubt, however, that this interpretation is
founded on a mistake, for other borings at Yarmouth, Lowestoft, and
Beccles came to muddy sands and clays belonging to the upper part of
the Crag, now known to thicken greatly eastward. The recent deposits
descend only to a depth of about 50 feet at Yarmouth, and consist
of sand and shingle; the beds below contain Pliocene mollusca. This
emendation is also borne out by the entirely different character
of the recent estuarine deposits at Potter Heigham, where we again
find a submerged forest at about 56 feet below the marsh-level. The
section recorded by Mr Blake is as follows:—
feet
Bluish-grey loam 24
Grey silty sand ½ to 2
Stiff bluish-grey loam, clay, and silt
full of cockles, &c. 13
Black peat, hard, and much compressed 17
White and buff sand 2
—-
58
It will be noticed that here only one peat bed was found, and was at
the usual depth of the lowest submerged forest. Possibly the white
sand below was the bleached top of the Crag; but this point was not
cleared up.
If we resume our journey northward along the Norfolk coast we come
to the well-known locality of Eccles, where the old church tower
described and figured by Lyell in his _Principles of Geology_ long
stood on the foreshore, washed by every spring tide. The position of
this church formed a striking illustration of the protection afforded
by a chain of sand-dunes. The church was originally built on the
marshes inside these dunes, at a level just below that of high-water
spring tides. But as the dunes were driven inland they gradually
overwhelmed the church, till only the top of its tower appeared above
the sand. In this state it was pictured by Lyell in the year 1839.
Later on (in 1862) it was again sketched by the Rev. S. W. King, and
stood on the seaward side of the dune and almost free from sand. For
a series of years, from 1877 onward, I watched the advance of the
sea, and as the church tower was more and more often reached by the
tides, its foundations were laid bare and attacked by the waves, till
at last the tower fell.
Not only were the foundations of Eccles church exposed on the
foreshore, but an old road across the marshes also appeared on
the seaward side of the dunes, giving a still more exact idea of
the former great influence of the chain of dunes in damping the
oscillations of the tidal wave. The tide outside now rises and falls
some 12 or 15 feet; on the marsh within its influence is only felt
under exceptional circumstances. A road across the marsh at a level
four or five feet below high-water, as this one stood, would still be
passable, except during unusual floods.
Eccles Church is an excellent example of the way in which an ancient
land-surface may now be found below the level of high-water, and yet
no subsidence of the land has taken place. But this coast can give
even more curious examples. It does not need a sand-dune to deaden
the rise and fall of the tides; even a submerged bank will have much
the same effect. Extensive submerged sand-banks extend parallel with
the coast, protecting the anchorage known as Yarmouth Roads. These
banks rise so nearly to the surface of the sea that not only do they
protect the town and anchorage against the waves, they deaden the
tidal oscillation to such an extent that its range is much greater
outside the bank than within.
If these submerged outer banks were to be swept away by some change
in the set of the currents, large areas now cultivated and inhabited
would be flooded by salt water at every spring tide, and the turf
of the meadows would be covered by a layer of marine silt, such
as we see alternating with the submerged forests in the docks of
the Thames. Such alternations, if thin, do not necessarily prove a
change in the level of the sea; they may only point to the alternate
accumulation and removal of sand-banks in a distant part of the
estuary.
The Norfolk coast trends westward soon after leaving Cromer, and
where the cliff seems to pass inland at Weybourn we enter an ancient
valley, one side of which has been entirely cut away by the sea,
except for a few relics of the further bank, now included in the
shingle beach which runs out to sea nearly parallel with the coast
and protects Blakeney Harbour. Here again we find that in the bottom
of the valley there must be a submerged forest, for slabs of peat are
often thrown up at Weybourn, and by the use of a grapnel the peat was
found in place off Weybourn at a depth of several fathoms.
When the coast turns southward again, and the wide bay of the Wash is
entered, we find an extensive development of submerged land-surfaces
and peat beds, extending over great part of the Fenland. In fact the
whole Fenland and Wash was once a slightly undulating plain, cut into
by numerous shallow open valleys. The effect of the submergence of
this area has been to cause the greater part of it to silt up to a
uniform level, through the accumulation of warp and growth of peat;
so that now the Fenland has become a dead level, out of which a few
low hills rise abruptly. The islands of the Fenland, such as those on
which Ely and March are built, are merely almost submerged hill-tops;
they were not isolated by marine action.
It is obvious that a wide sheltered bay of this sort forms an ideal
area in which to study the gradual filling up and obliteration of the
valleys, as the land sank; and it may enable us to learn the maximum
amount of the change of sea-level. The Fenland unfortunately does
not contain very deep dock excavations, and we have only various
shallower engineering works to depend on, though numerous borings
reach the old floor.
A preliminary difficulty, however, meets us in the study of the Fen
deposits; it is the same difficulty that we have already referred
to when describing Clacton and Grays, and we shall meet with it
again. In certain parts of the Fenland, particularly about March and
Chatteris, a sheet of shoal-water marine gravelly sand caps some of
the low hills, which rise a few feet above the fen-level. The gravel
for long was taken to be the same bed that passes under the marshes.
Later work showed however that these gravels, with their sub-arctic
marine fauna and containing also _Corbicula fluminalis_, were of
much earlier date than the true fen-deposits. Just as we saw happen
in the Thames Valley, a wide plain and estuary existed long before
the deeper channels containing the submerged forests were cut; and
the deposits of this older estuary and its tributaries are still to
be found in patches here and there. Sometimes, as at March, they cap
hills a few feet above the fen-level; but as often they fill channels
not quite coinciding with the later channels; just as they do at
Grays. Or two deposits of quite different date may lie side by side,
as they do in the Nar Valley, or at Clacton, or on the Sussex coast.
The true fen-deposits were carefully examined by Messrs. Marshall,
Fisher, and Skertchly, as far as the shallow sections would allow,
and the following account is mainly condensed from that given by Mr
Skertchly in his _Geology of the Fenland_.
During the excavation of certain deep dykes for the purpose of
draining the fens there was discovered at a depth of about 10 feet
below the surface a forest of oaks, with their roots imbedded in the
underlying Kimmeridge Clay. The trunks were broken off at a height of
about three feet. Some of the fallen trees were of fine proportion,
measuring three feet in diameter, quite straight and seldom forked.
At an average height of two feet above this “forest No. 1” the
remains of another were found (in the peat) consisting of oaks and
yews. Three feet above “forest No. 2” lay the remains of another, in
which the trees are all Scotch firs, some of which were three feet in
diameter. Above this and near to the surface was seen a still newer
forest of small firs. The peat close to the surface contains remains
of sallow and alder, and was formed with the sea at its present level.
It will be noticed that the greatest depth at which these rooted
trees were found was only about ten feet below the sea-level. At
this high level we must expect to find that the growth of the peat
was practically continuous, and that the different submerged forests
run together. In adjoining depressions the different forests would
occur at lower levels and would be separated by beds of marine silt.
It does not follow from the position that a low-level submerged
land-surface is older than one at a higher elevation, for above the
present sea-level all these stages are represented by a few inches
of soil, on which forest after forest has grown and decayed. Anyone
who has collected antiquities on fields knows what a curious jumble
of Palaeolithic, Neolithic, bronze age, Roman, mediaeval and recent
things may be found mixed in these few inches of soil, or may be
thrown up by an uprooted tree. The great advantage of studying the
deeply submerged forests is that in them the successive stages are
separated and isolated, instead of being mingled in so confusing a
fashion.
For further information as to the more deeply submerged land-surfaces
we may turn to the numerous records of borings made in the Fenland
and collected by the Geological Survey. These show that the thickness
of the fen-deposits varies considerably from place to place, that
the floor below undulates and is by no means so flat as the surface
of the fen above. Most of these borings, however, were not continued
through the gravels which lie at the base of the deposit, and thus
we can only be certain of the total depth to the Jurassic clay or
boulder clay in a few places. The maximum thickness of the fen-beds
yet penetrated is less than 60 feet, and a submerged forest was found
at Eaubrink at about 40 feet. It is possible however that none of
these scattered borings has happened to hit upon one of the buried
river-channels, which formerly wandered through this clayey lowland;
if one were found it would probably show that the alluvial deposits
are somewhat thicker than these measurements, and that they descend
to a depth about equal to that reached in the valleys of the Thames
or Humber.
It is useless to discuss in more detail the lower submerged forests
of the Fenland, for we cannot get at them to examine them properly.
They have been as effectually overwhelmed and hidden as the remains
of King John’s baggage train, which has never been seen again
since it wandered off the flooded causeway during the disastrous
spring-tide of October 11, 1216, and sank into the soft clay and
quicksands.
The higher submerged forests of the Fenland are however of great
interest, and as already pointed out they have been exposed to view
in cutting the fen dykes, especially near Ely. Perhaps a closer
study of these might enable us to arrive at some idea of the time
taken for the growth of a series of forests of this sort, and for
the accompanying mass of peat. The variations in the flora also
need more exact analysis before we can say what they betoken. The
oak-forest at the bottom is what we should expect on a clay soil; but
the reason for the succession of trees above is not obvious. It need
not necessarily point to climatic change, though it may do so; but
it certainly looks as if the peaty bogs were alternately wetter and
drier, so that sometimes moss grew, and sometimes fir-trees. Neither
need this change imply an up-and-down movement of the land, though it
may be due to such a cause.
Subsidence would destroy the oaks and allow a peat-moss to form;
but if the subsidence were intermittent the moss would increase in
thickness, become more compact, and its surface rise, till it was
dry enough for pines. Another subsidence would cause spongy peat
again to spread and kill the pine, and so on. Intermittent subsidence
seems sufficient to account for all the changes of vegetation we have
yet noticed in connexion with these submerged land-surfaces.
Of the fauna of the fen-silts and peats it is very difficult to give
any satisfactory account. If we put aside the March and Chatteris
marine gravels with _Corbicula fluminalis_, and the Nar Valley Clay
with its northern marine mollusca as being of older date; and if
we also reject the marginal gravels with hippopotamus and mammoth
as being more ancient, there only remain a few mammals such as the
beaver, wolf, wild boar, and certain cetacea, which we can be sure
came out of the true fen-deposits. Implements made by man have only
been found in the higher layers, and there seems to be no record in
this area of a stone implement found below a submerged forest.
Submerged forests of the ordinary type are often to be seen between
tide-marks on the flat shores of Lincolnshire; but as they still
await proper study they need not here detain us, and we will pass on
to the next large indentation of the coast-line, the estuary of the
Humber.
Here, owing to the excavation of extensive docks, and to a series of
trial borings for a tunnel beneath the Humber, the structure of the
valley has been clearly laid open. It is much the same as that of the
Thames; but as we are in a glaciated area we find, as in the Fenland,
that much of the erosion had taken place before or during the Glacial
Epoch, for boulder clay occupies part of the valley.
[Illustration: Fig. 3.]
Boulder clay or till not only occupies part of the valley, it
descends far below the present river bottom and even below the
lowest submerged forest. This we find always to be the case in the
glaciated parts of Britain; but whether the deep trenching is due
to the ploughing out of a trough by a tongue of the ice-sheet,
to sub-glacial streams below sea-level, or to erosion by a true
subaerial river is still a doubtful point. However, this question
must not detain us; we are not now dealing with elevations and
depressions of so ancient a date, and must confine our attention to
post-glacial movements.
The section shown in fig. 3 will explain better than any words the
structure of the Humber Valley. It is drawn to scale from the
engineer’s section, and shows at a glance the three channels. The
deepest and widest channel is that occupied by glacial deposits;
an intermediate channel (shown in black) is occupied by silt and
submerged forests; and a shallower channel is occupied by the present
Humber and its alluvium. One interesting point, however, this section
does not happen to illustrate. Somewhat lower down the Humber we
come to gravels and silts full of sub-arctic marine mollusca and
_Corbicula fluminalis_, exactly as in regions further south, and
presumably of the same age as the deposits we have already mentioned
as found at March in the Fenland and at Grays in Essex. The exact
relation of these _Corbicula_-beds to the deep channel filled with
glacial drift, below the marshes of the present Humber, is still
somewhat uncertain, but the marine beds clearly rest on boulder clay,
and seem also to be overlain by another glacial deposit.
The section leaves no doubt that in post-glacial times the Humber
cut a channel about 60 feet below its present bed, or to just the
same depth as did the Thames. This may possibly be an accidental
coincidence; but it is very suggestive that both these rivers should
have cut their beds to the same depth. Such coincidences suggest that
we are dealing with a period when each of our great rivers was able
to cut to a definite base-level, below which it could not go. This
base-level must either have been the sea, or some vast alluvial plain
then occupying the bed of the North Sea. In either case the plain
must then have been fully 60 feet lower than the present sea-level.
Not only did the ancient Humber cut to the same depth as the ancient
Thames, but in each area the ancient river was flanked by a wide
alluvial flat which now lies from 40 to 60 feet below the modern
marsh level.
The flat coast of Holderness, which stretches from the Humber
northward to Flamborough Head, shows also occasional submerged
forests; but the want of excavations beneath the sea-level makes
it impossible to say much about them. North of Flamborough Head it
seems as though depression gave place to elevation, and when we
pass into Scotland the Neolithic deposits seem to be raised beaches
instead of submerged forests. We need not therefore devote more
time to a consideration of the details connected with the submerged
land-surfaces which border the lands facing the North Sea. They
evidently once formed part of a wide alluvial flat stretching seaward
and running up all our larger valleys. We must now consider how far
seaward this plain formerly extended.
Here, fortunately, we meet with a most surprising piece of evidence,
which adds enormously to the importance of this plain, and shows that
the submergence is no local phenomenon, but a widespread movement
of depression which must greatly have altered the physical geography
of north-western Europe during times within the memory of man. This
evidence deserves a separate chapter.