The mammalian remains differentiate these deposits at once; but if no determinable mammals are found, the crushing of the bones and the greater compression and alteration of the peaty beds serves to distinguish them, for this Forest-bed dates back to Pliocene times, passes under a considerable thickness of glacial beds, and has been over-ridden by the ice-sheet during the Glacial epoch. The Cromer Forest-bed has been exposed particularly well of late years at Kessingland, near Lowestoft, where the sea has encroached greatly. It is well worth while to make a comparative study of this deposit, of the Grays and Clacton _Cyrena_-bed, of the submerged forests of the Thames docks, and of the strata now being formed in and around the Norfolk Broads. By such a comparison we can trace the effects of similar conditions occurring again and again. The fauna and flora slowly change, species come and go, man appears and races change: though the same physical conditions may recur life ever changes. The Norfolk Broads, just referred to, deserve study from another point of view: their origin is directly connected with the submergence which forms the theme of this book. These broads are shallow lakes, always occupying part of the widest alluvial flats which border the rivers; but they are usually out of the direct course of the present river; they therefore receive little of the sediment brought down in flood-time. On the other hand they are steadily being filled up with growing vegetation and turned into peat mosses.