Subsidence of the Valley of the Cam.

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.