The Fenland is a buried basin behind a breached barrier. It is the
"drowned" lower end of a valley system in which glacial, marine,
estuarine, fluviatile, and subaerial deposits have gradually
accumulated, while the area has been intermittently depressed until
much of the Fenland is now many feet below high water in the adjoining
seas.
The history of the denudation which produced the large geographical
features upon which the character of the Fenland depends needs no long
discussion, as there are numerous other districts where different
stages of the same action can be observed.
In the Weald for instance where the Darent and the Medway once ran off
higher ground over the chalk to the north, cutting down their channels
through what became the North Downs, as the more rapidly denuded beds
on the south of the barrier were being lowered. The character of the
basin is less clear in this case because it is cut off by the sea on
the east, but the cutting down of the gorges _pari passu_ with the
denudation of the hinterland can be well seen.
The Thames near Oxford began to run in its present course when the land
was high enough to let the river flow eastward over the outcrops of
Oolitic limestones which, by the denudation of the clay lands on the
west, by and by stood out as ridges through which the river still holds
its course to the sea--the lowering of the clay lands on the west
having to wait for the deepening of the gorges through the limestone
ridges. A submergence which would allow the sea to ebb and flow through
these widening gaps would produce conditions there similar to those of
our fenlands. So also the Witham and the Till kept on lowering their
basin in the Lias and Trias, while their united waters cut down the
gorge near Lincoln through a barrier now 250 feet high.
The basin of the Humber gives us an example of a more advanced stage in
the process. The river once found its way to the sea at a much higher
level over the outcrops of Jurassic and Cretaceous rocks west of Hull,
cutting down and widening the opening, while the Yorkshire Ouse, with
the Aire, the Calder and other tributaries, were levelling the New Red
Sandstone plain and valleys west of the barrier and tapping more and
more of the water from the uplands beyond. The equivalent of the Wash
is not seen behind the barrier in the estuary of the Humber, but the
tidal water runs far up the river and produces the fertile estuarine
silt known as the Warp.
The Fenland is only an example of a still further stage in this
process. The Great Ouse and its tributaries kept on levelling the Gault
and Kimmeridge and Oxford Clays at the back of the chalk barrier which
once crossed the Wash between Hunstanton and Skegness.
The lowlands thus formed lie in the basin of the Great Ouse which
includes the Fenland, while the Fenland includes more than the Fens
properly defined, so that things recorded as found in the Fenland may
be much older than the Fen deposits.