It is unfortunate that for these calculations so many of the factors are of uncertain value. We may estimate from the present rate of erosion of the coast the amount that has been lost since the sea-level became stationary, or we may take the rate of accumulation of sand-dunes or shingle-spits; or the rate at which our estuaries, harbours, and broads are silting up. It all comes, however, to this—no exact figures can be given; but so many rough calculations lead to approximately the same date, that the date arrived at may be trusted to give some idea of the length of the period which has elapsed since the downward movement ceased.
Working backwards from the present day, step by step, archaeological evidence gives an undoubted period of 2000 years, to the first century B.C., during which no measurable change of sea-level has taken place in the south of England.
To this must be added a few centuries for the growth of the marshes on which Glastonbury and similar lake-dwellings were built, and for the growth of various other marshes at present sea-level known to be earlier than the Roman invasion. Also we must allow for the accumulation of various shingle-spits and sand-dunes then already partly formed.
In general, somewhere about one-third or one-half of this accumulation seems to have taken place before the Roman invasion. This adds another 1500 years; so that about 3500 years ago, we get back to the beginning of the period of unchanging sea-level in which we are still living, and begin to see evidence of earth movements still in progress.
Whether this 3500 years will take us back to the beginning of the Bronze Age in Britain is not yet proved; but so far we seem to discover metals in the whole of the deposits formed whilst the sea-level remained unchanged, and only stone weapons in even the newest of the submerged forests. For the present, we may therefore take it that the two changes nearly coincided. The use of metals began in Britain about the time that the earth-movements ceased—that is to say somewhere about 1600 B.C.
Whether this period of 3500 years will really take us back to the commencement of the Bronze Age is doubtful, for Stonehenge had already been built, and though only stone hammers seem to have been there used, yet one slight streak of bronze or copper has been noticed. Of course, there may have been a similar occasional use of bronze at the time of the last submerged forest; but we have as yet no evidence of this, and the possible correspondence in date between Stonehenge and the last of the submerged forests remains merely a suggestion.
Perhaps we may still find submerged stone-circles or other antiquities of the age of Stonehenge beneath the sea-level; but Stonehenge lies too high above the sea for it in itself to give any clue as to a change of sea-level. We will only make one suggestion. It is probable that when Stonehenge was built, a long arm of the sea extended far up the Avon Valley, so that navigable water was found not far from Stonehenge. There is in Stonehenge an inner circle of smaller stones, not composed of the local greywethers but consisting of large blocks of igneous rock of foreign origin. These blocks, which are sufficiently large to be awkward for land-carriage, have been said to be erratics gathered on Salisbury Plain, just as the greywethers for the main circle were gathered; but there are no erratics on Salisbury Plain. Large erratic blocks of similar character occur, however, abundantly on the lowlands of Selsey Bill, under the lee of the Isle of Wight. Probably a similar erratic-strewn plain once fringed the coast on the west also, though on the exposed side the part above the sea-level has now been entirely swept away by the sea.
I would suggest that the Stonehenge erratics, instead of being brought from any great distance, may have come from a wide plain at the mouth of the Avon, then two or three miles further seaward. From thence they were rafted far up the navigable fjord, not yet silted up, and were only carried a short distance uphill. Igneous rocks such as these, found in a country consisting essentially of chalk and Tertiary strata, would be valuable and probably endowed with magic properties, hence their employment in this inner circle.
Our next enquiry must be into the length of time represented by the series of submerged forests and associated deposits described in the foregoing pages. The newest of them belongs certainly to the age of polished stone, and the earliest also probably comes within the Neolithic Period. Within the period represented by the submerged forests, we have seen that there has been a change of the sea-level to the extent of 80 feet, or perhaps rather more. If we can obtain some measure of the time occupied, this should give us some approximate idea as to the length of the Neolithic period, and of the rate at which changes of the sea-level can take place.
The first point to be considered is the length of time occupied by the growth of the series of submerged forests. On first examining, or reading accounts of, deposits of this sort one obtains a vague impression of long periods, during which mighty oaks flourished. Both the movements of submergence and the intervening periods of vegetable growth seem to require great lapses of time. On closer study, however, the evidence seems scarcely to support this view, for estuarine silts are deposits of exceptionally rapid growth, and one finds that the usual characteristic of a “submerged forest” is that it shows indications of only a single generation of trees. The trees also are usually small, except where the submerged forest rests directly on deposits of much earlier date, or on solid rock.