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. TURBIFEROUS AND ARENIFEROUS SERIES. The Fen Beds belong to the last stage and, notwithstanding their great local differences, seem all to belong to one continuous series. Seeing then that their chief characteristic is that they commonly contain beds of peat it may be convenient to form a word from the late Latin _turba_, turf or peat, and call them Turbiferous to distinguish them from the Areniferous series which consists almost entirely of sands and gravels. When the land had sunk so far that the velocity of the streams was checked over the widening estuary and on the other hand the tide and wind waves had more free access, some outfalls got choked and others opened; turbid water sometimes spread over the flats and left mud or was elsewhere filtered through rank plant growth so that it stood clear in meres and swamps, allowing the formation of peat unmixed with earthy sediment. Banks are naturally formed along the margin of rivers by the settling down of sand and mud when the waters overflow, as seen on a large scale along the Mississippi, the Po, as well as along the Humber and its tributaries. The effect of a break down of the banks is very different. A great hole is scooped out by the outrush, and the mud, sand and gravel deposited in a fanshape according to its degree of coarseness and specific gravity.