It must not, however, be imagined that the Fen Beds consist wholly or
even chiefly of peat. As we travel north from Cambridge the surface of
the alluvium is brown earth for miles and only here and there shows the
black surface of peat. The numerous ditches for draining the land
confirm this observation, and when we have the opportunity of examining
excavations carried down to great depths into the alluvium we usually
find only a little peat on the surface or in thin beds alternating with
silt and clay and marl. Sometimes, but only sometimes, we have evidence
of the growth of peat for a long time, then of the incoming of turbid
water leaving beds of clay, then again of the tranquil growth of peat.
All this points to changes of local conditions and shifting channels
during a gradual sinking of the area, for some of the peat is below sea
level.
I believe that the volume of clay is much greater than that of peat,
although from the common occurrence of peat on the surface and clay in
the depth the area over which peat is seen is greater. We have not,
however, the data for estimating the proportion of each.
In embayed corners along the river even above Cambridge we find little
patches of peat, while on the other hand in deep excavations near the
middle of the valley we find only thin streaks of peat or peaty silt.
In the trial boreholes at the Backs of the Colleges there was only this
kind of record of former swamp vegetation.