The interval of time may have been very great. But
we have no scale on which to measure it.'
Let us suppose, then, the era of 'gravels' past; the valleys which
open into the fen sawn out by rivers to about their present depth.
What was the special cause of the fen itself? why did not the great
lowland become a fertile 'carse' of firm alluvial soil, like that of
Stirling?
One reason is, that the carse of Stirling has been upheaved some
twenty feet, and thereby more or less drained, since the time of the
Romans. A fact patent and provable from Cramond (the old Roman port
of Alaterna) up to Blair Drummond above Stirling, where whales'
skeletons, and bone tools by them, have been found in loam and peat,
twenty feet above high-water mark. The alluvium of the fens, on the
other hand, has very probably suffered a slight depression.
But the main reason is, that the silt brought down by the fen rivers
cannot, like that of the Forth and its neighbouring streams, get safe
away to sea. From Flamborough Head, in Yorkshire, all down the
Lincolnshire coast, the land is falling, falling for ever into the
waves; and swept southward by tide and current, the debris turns into
the Wash between Lincolnshire and Norfolk, there to repose, as in a
quiet haven.
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