Of the fauna of the fen-silts and peats it is very difficult to give any satisfactory account. If we put aside the March and Chatteris marine gravels with _Corbicula fluminalis_, and the Nar Valley Clay with its northern marine mollusca as being of older date; and if we also reject the marginal gravels with hippopotamus and mammoth as being more ancient, there only remain a few mammals such as the beaver, wolf, wild boar, and certain cetacea, which we can be sure came out of the true fen-deposits. Implements made by man have only been found in the higher layers, and there seems to be no record in this area of a stone implement found below a submerged forest. Submerged forests of the ordinary type are often to be seen between tide-marks on the flat shores of Lincolnshire; but as they still await proper study they need not here detain us, and we will pass on to the next large indentation of the coast-line, the estuary of the Humber. Here, owing to the excavation of extensive docks, and to a series of trial borings for a tunnel beneath the Humber, the structure of the valley has been clearly laid open. It is much the same as that of the Thames; but as we are in a glaciated area we find, as in the Fenland, that much of the erosion had taken place before or during the Glacial Epoch, for boulder clay occupies part of the valley. [Illustration: Fig. 3.] Boulder clay or till not only occupies part of the valley, it descends far below the present river bottom and even below the lowest submerged forest. This we find always to be the case in the glaciated parts of Britain; but whether the deep trenching is due to the ploughing out of a trough by a tongue of the ice-sheet, to sub-glacial streams below sea-level, or to erosion by a true subaerial river is still a doubtful point. However, this question must not detain us; we are not now dealing with elevations and depressions of so ancient a date, and must confine our attention to post-glacial movements.