During the slow denudation which resulted in the formation of this
basin many things happened. There were intermittent and probably
irregular movements of elevation and depression. Glacial conditions
supervened and passed away.
The proof of this may be seen in the Sections, Figs. 1, 2 and 3, pp. 8,
9 and 10.
At Sutton Bridge the alluvium has been proved to a depth of 73 feet
resting on Boulder Clay. At Impington the Boulder Clay runs down to a
depth of 86 feet below the surface level of the alluvium. That means
that this part of the valley was scooped out before the glacial
deposits were dropped in it, and that the bottom of the ancient valley
is now far below sea level.
In front of Jesus College, gravel with _Elephas primigenius_ was
excavated down to a depth of 30 feet below the street, while in the
Paddocks behind Trinity College the still more recent alluvium was
proved to a depth of 45 feet, i.e. 16 feet below O.D. These facts
indicate a comparatively recent subsidence along the valley, as no
river could scoop out its bed below sea level.
We need not for our present purpose stop to enquire whether this
depression was confined to the line of the valley or was part of more
widespread East Anglian movements which are not so easy to detect on
the higher ground. From the above-mentioned sections it is clear that
the denudation, which resulted in the formation of the basin in the
lowest hollow of which the Fen Beds lie, was a slow process begun and
carried on long before glacial conditions prevailed and before the
gravel terraces were formed.
As soon as the sea began to ebb and flow through the opening in the
barrier, the conditions were greatly altered and we see the results of
the conflict between the mud-carrying upland waters and the
beach-forming sea.