For each flood deposits its silt upon the immediate
bank of the river, raising it year by year; till--as in the case of
the 'Levee' of the Mississippi, and probably of every one of the old
fen rivers--the stream runs at last between two natural dykes, at a
level considerably higher than that of the now swamped and
undrainable lands right and left of it.
If we add to this, a slope in the fen rivers so extraordinarily
slight, that the river at Cambridge is only thirteen and a half feet
above the mean sea level, five-and-thirty miles away, and that if the
great sea-sluice of Denver, the key of all the eastern fen, were
washed away, the tide would back up the Cam to within ten miles of
Cambridge; if we add again the rainfall upon that vast flat area,
utterly unable to escape through rivers which have enough to do to
drain the hills around; it is easy to understand how peat, the
certain product of standing water, has slowly overwhelmed the rich
alluvium, fattened by the washing of those phosphatic greensand beds,
which (discovered by the science of the lamented Professor Henslow)
are now yielding round Cambridge supplies of manure seemingly
inexhaustible.
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