For further information as to the more deeply submerged land-surfaces we may turn to the numerous records of borings made in the Fenland and collected by the Geological Survey. These show that the thickness of the fen-deposits varies considerably from place to place, that the floor below undulates and is by no means so flat as the surface of the fen above. Most of these borings, however, were not continued through the gravels which lie at the base of the deposit, and thus we can only be certain of the total depth to the Jurassic clay or boulder clay in a few places. The maximum thickness of the fen-beds yet penetrated is less than 60 feet, and a submerged forest was found at Eaubrink at about 40 feet. It is possible however that none of these scattered borings has happened to hit upon one of the buried river-channels, which formerly wandered through this clayey lowland; if one were found it would probably show that the alluvial deposits are somewhat thicker than these measurements, and that they descend to a depth about equal to that reached in the valleys of the Thames or Humber. It is useless to discuss in more detail the lower submerged forests of the Fenland, for we cannot get at them to examine them properly. They have been as effectually overwhelmed and hidden as the remains of King John’s baggage train, which has never been seen again since it wandered off the flooded causeway during the disastrous spring-tide of October 11, 1216, and sank into the soft clay and quicksands. The higher submerged forests of the Fenland are however of great interest, and as already pointed out they have been exposed to view in cutting the fen dykes, especially near Ely. Perhaps a closer study of these might enable us to arrive at some idea of the time taken for the growth of a series of forests of this sort, and for the accompanying mass of peat. The variations in the flora also need more exact analysis before we can say what they betoken. The oak-forest at the bottom is what we should expect on a clay soil; but the reason for the succession of trees above is not obvious. It need not necessarily point to climatic change, though it may do so; but it certainly looks as if the peaty bogs were alternately wetter and drier, so that sometimes moss grew, and sometimes fir-trees. Neither need this change imply an up-and-down movement of the land, though it may be due to such a cause. Subsidence would destroy the oaks and allow a peat-moss to form; but if the subsidence were intermittent the moss would increase in thickness, become more compact, and its surface rise, till it was dry enough for pines. Another subsidence would cause spongy peat again to spread and kill the pine, and so on. Intermittent subsidence seems sufficient to account for all the changes of vegetation we have yet noticed in connexion with these submerged land-surfaces.