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.
