beds, extending over great part of the Fenland. In fact the whole Fenland and Wash was once a slightly undulating plain, cut into by numerous shallow open valleys. The effect of the submergence of this area has been to cause the greater part of it to silt up to a uniform level, through the accumulation of warp and growth of peat; so that now the Fenland has become a dead level, out of which a few low hills rise abruptly. The islands of the Fenland, such as those on which Ely and March are built, are merely almost submerged hill-tops; they were not isolated by marine action.
It is obvious that a wide sheltered bay of this sort forms an ideal area in which to study the gradual filling up and obliteration of the valleys, as the land sank; and it may enable us to learn the maximum amount of the change of sea-level. The Fenland unfortunately does not contain very deep dock excavations, and we have only various shallower engineering works to depend on, though numerous borings reach the old floor.
A preliminary difficulty, however, meets us in the study of the Fen deposits; it is the same difficulty that we have already referred to when describing Clacton and Grays, and we shall meet with it again. In certain parts of the Fenland, particularly about March and Chatteris, a sheet of shoal-water marine gravelly sand caps some of the low hills, which rise a
