Submerged forests

by Clement Reid

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.