(c) Above the vegetable matter and leaves (b) was found a “stratum of sludge or silt” 10 feet in thickness. It showed little variation except from a brownish to a lead colour. “The whole is sprinkled with recent shells, together with wood, hazel nuts, and sometimes the bones and horns of deer, oxen, etc. The shells, particularly the flat ones, are frequently found in rows or layers; they are often double or closed, with their opening part upwards.” From Colenso’s account it seems probable that this bed was a marine silt with Scrobicularia and cockles in the position of life. He goes on to say that “There has been recently found imbedded in the silt, about two feet from the top, a piece of oak, that had been brought into form by the hand of man; it is about six feet long, one inch and a half broad, and less than half an inch thick; this is the greatest depth at which I have ever seen any converted substance. It appears to have floated in the sea, as at one end, which is much decayed, a small barnacle has fixed its habitation.”
(d) A stratum of sea-sand, about four inches in thickness; this is easily distinguished from the river-sand, being much finer, and having always more or less shells mixed with it.
(e) Silt two feet, with concretions containing wood and bones.
(f) Another stratum of sea-sand, 20 feet in thickness. In all parts of this sand there are timber trees, chiefly oaks, lying in all directions; also remains of animals such as red deer, “heads of oxen of a different description from any now known in Britain, the horns of which all turn downwards.” Human skulls were also found near the bottom of the sand, and one of these with other fossils was presented by Colenso to the Royal Geological Society of Cornwall. In the upper part of this sand nearer the mouth of the harbour, the bones of a large whale were found. The sea at this time seems to have extended about a mile up the valley.
(g) A bed of rough river-sand and gravel, here and there mixed with sea-sand and silt. About 20 feet in thickness. In this sand was found “the remains of a row of wooden piles, sharpened for the purpose of driving, which appear to have been used for forming a wooden bridge for foot passengers: they crossed the valley, and were about six feet long; their tops being about 24 feet from the present surface—just on a level with the present low water at spring tides. Had the sea-level been then as now, such a bridge would have been nearly useless.”
At Wheal Virgin, which was the upward extension of the Happy Union works, about a mile higher up the valley than the bridge just mentioned, the tin ground was only 32 feet from the surface. Here Colenso mentions seeing “on the surface of the tin-ground two small pieces of oak, with artificial holes in them: and there were near them several oak stakes, sharpened and driven into the ground, and supported by large stones. Near the same spot has been found a substance resembling the ashes of charcoal.” This account suggests a fish or otter trap of some sort and the charcoal below the sea-level suggests that it must date back to at least as early a period as the submerged bridge. It is a great pity that antiquaries were not at that period more alive to the great interest of these finds.
Carnon stream-works, on a navigable branch of the Fal, showed a very similar section, for below about 54 feet of alternating sand and silt was found, according to Henwood, a bed one and a half feet thick of wood, moss, leaves, nuts, etc., a few oyster shells, remains of deer and other mammals, and some human skulls. Below this came the tin ground varying in thickness from a few inches to 12 feet. Here also no organic remains were found in the tin ground itself.
The above records may be accepted as giving fair samples of the deposits which now fill the lower parts of the submerged valleys of Devon and Cornwall. These valleys were all at one time long creeks or arms of the sea, navigable for a considerable distance inland and affording a fine series of sheltered harbours at short distances apart. A few of these harbours were so deep and large that they have not yet been obliterated, as is seen in the case of the Dart, the branches of Plymouth Harbour, the Fal, the Gannel, etc. A rapid silting-up is, however, now going on, greatly aided by the refuse from the mines and china-clay works. In the days whilst the subsidence was in progress Cornwall was essentially a country of fjords, though now good harbours are few or blocked with sand-banks.
The abundance of sheltered creeks must have had considerable influence on the manner of living of the inhabitants; but it is noticeable that though many acres of the silts have been removed in tinning, and a good many human remains have been found, there is no mention of boats. This absence of any record of boats in any of the marine silts associated with or below submerged forests cannot be an accident; for old boats and dug-out canoes are constantly being discovered in later alluvial and fen deposits. It looks as if in those early days man had either no boats, or only used coracles of skin and wicker, such as would entirely decay and leave no trace.
It may be remarked that the higher submerged forest, that lying just about low-water level, is not recorded in the deep excavations at Pentuan and Carnon, though these old land-surfaces are so conspicuous on the foreshore opposite every smaller creek, when the sea happens to scour away the sand and beach. A little consideration will show the reason of this difference. The extensive stream-works of Pentuan and Carnon happen to lie at the mouths of two of the larger and deeper creeks, in which silting-up could not keep pace with the subsidence. Thus the seaward ends were continuously occupied by sea, from the time when the oak-forest sank right on into historic times, and over this deeply buried oak-forest we only find alternate layers of silt and sea-sand. Evidence of the later submerged forest, however, is not entirely wanting, for the submerged wooden bridge or causeway of Pentuan must belong to the period when the trees seen on the foreshore elsewhere were flourishing well above high-water mark.