Submerged forests

by Clement Reid

It is necessary to make this digression as to the effects of tidal scour, for we are sometimes told that the various basins, troughs, and channels shown on the charts represent submerged land-valleys, and thus prove enormous changes of sea-level in modern times. How a submerged valley in a narrow sea with sandy bottom, like the English Channel, could remain long without silting up is not clear; the sand-banks on either side should tend to wash into and fill up the hollows. The troughs, however, all coincide with lines of tidal scour; they do not continue the lines of existing valleys, unless these valleys are so large as to produce a great scour, and unless this scour is aided by the oscillation of the waves. A glance at the Admiralty chart will show that no submerged channel crosses the direction of the tidal scour or of the Atlantic swell; the channels are scoured where tide and swell act together.

We conclude therefore that Plymouth Sound probably represents a basin once filled with soft Tertiary and Secondary deposits, and that these soft deposits were cleared out by the sea, leaving the rocky floor of the basin bare at a considerable depth below sea-level. In part the basin has now silted up again; but we may fairly consider that at the time of greatest elevation, when the submerged valleys were being eroded, the depth of water in the Sound was much the same as it is now. Then as now the rivers seem to have discharged into a wide open gulf occupied by the sea.

However this may be, we see now a series of deeply trenched valleys, partly submerged and all opening into a wide and deep bay. These valleys do not now show rocky bottoms gradually sloping into the open harbour. The rock floor ceases several miles up and gives place first to an alluvial flat and then to an arm of the harbour. Like all the other valleys with which we have been dealing they cut to a definite base-level, approximately that of the sea, and the parts below that level are rapidly silting up.

Fortunately a large series of bridge-foundations has shown well the character of these valleys, where the rocky floor passes beneath the sea-level, and the late R. H. Worth gave an excellent series of sections across them. He took their contours to be evidence of glaciations. In this I cannot agree with him; but think rather that the extraordinary flatness of the valley-bottoms, and especially the uniform depth to which they were excavated, point to the attainment of a definite base-level.

Commencing with the most easterly of the rivers which enter the Sound, we find that the Laira Railway Viaduct, across the Cattewater, proved a breadth of 212 feet at the centre of the channel, with the rock-floor practically level at 87 feet below low water; no V-shaped valley or gorge was met with. At Saltash the foundations of the bridge show the depth to the rock-bottom to be 75 feet; but the viaduct across the Hamoaze is about three miles higher up the river than the Laira Viaduct. The Tavy Viaduct, nearly two miles further from the sea, shows a width of 240 feet of practically level rocky floor at 67 feet below sea-level. Thus all this evidence is consistent with the existence of a series of wide open flat-bottomed valleys, now partly submerged, with a fall of about five feet in the mile. This is about the fall necessary for even a rapid river flowing through a flat so full of boulders and coarse gravel as this must have been. It must not be forgotten also that this five feet in the mile is the general fall of the valley-bottom, not of the water, and that a river winding from side to side would have about half or one-third of this fall. The slope was probably just sufficient to keep the channel clear and let the water escape.

We may take it, therefore, that the ancient valleys opening into Plymouth Harbour cut to about 100 feet below mean tide, as do the Thames and Humber, and that this was the measure of the greatest elevation of the land in Pleistocene times, for these valleys opened suddenly into a sea of considerably greater depth. A word of explanation is still required as to the meaning of the extremely flat rock-bottom, for one might have expected more of a U-shaped or V-shaped valley, unless the period of stationary sea-level were very long.

Owing to the great rush of water from Dartmoor during floods, and the enormous amount of coarse gravel swept down, the erosive power of these streams is very great. This was greatly exaggerated during the Glacial Epoch, to which the formation of the tin-ground and of the flat bottom belong. The melting of the snow in spring must have caused far more severe floods than we now see, and these floods must have brought down large quantities of river-ice heavily charged with boulders of hard and angular metamorphic rocks, such as would erode and trench in a way that does not now happen. Thus as the river changed its course or swung from side to side according to the varying amount of water, the ice-laden water must have had an erosive power more like that of a Canadian river in spring than like anything we now see in Britain. The wide and deep flat-bottomed trench need not have taken any enormous length of time to form, for river-ice and anchor-ice were constantly at work removing the loose material and laying bare the rock-face so that it could be again attacked.

The period of exceptionally rapid erosion and of low sea-level above postulated must be our starting point in Devonshire and Cornwall as elsewhere, for it fixed the shape and depth of the submerged valleys over wide areas. This erosion came somewhat earlier than the growth of the submerged forests; but it is impossible to treat of any particular period of history without some mention of what has gone before and led up to it. I may say also that I doubt whether there is any such great gap as is commonly supposed between the Glacial period and later times.

Unfortunately the succession of the newer deposits in the submerged valleys near Plymouth appears never to have been worked out, attention having been concentrated on the contour of the rocky floor. The recently completed Devonport dock excavations, which I examined, showed only very modern alluvium and silted-up channels with logs of wood cut by metal tools. Submerged forests do not appear to have been met with.

Though Plymouth Harbour has not yielded much information concerning the particular period with which we are dealing, it is important as fixing the maximum amount of elevation to which the land was subjected in Pleistocene or more recent times. We will now turn to the Cornish stream-tin works, which give more detail as to the later changes; we regret however that these most interesting excavations were closed so long ago, for various points were noted about which we should like further information, and this is not now obtainable. The old diluvian hypothesis has much to answer for in the long neglect of those modern strata which help to tie on geology to archaeology and history.