few feet above the fen-level. The gravel for long was taken to be the same bed that passes under the marshes. Later work showed however that these gravels, with their sub-arctic marine fauna and containing also Corbicula fluminalis, were of much earlier date than the true fen-deposits. Just as we saw happen in the Thames Valley, a wide plain and estuary existed long before the deeper channels containing the submerged forests were cut; and the deposits of this older estuary and its tributaries are still to be found in patches here and there. Sometimes, as at March, they cap hills a few feet above the fen-level; but as often they fill channels not quite coinciding with the later channels; just as they do at Grays. Or two deposits of quite different date may lie side by side, as they do in the Nar Valley, or at Clacton, or on the Sussex coast.
The true fen-deposits were carefully examined by Messrs. Marshall, Fisher, and Skertchly, as far as the shallow sections would allow, and the following account is mainly condensed from that given by Mr Skertchly in his Geology of the Fenland.
During the excavation of certain deep dykes for the purpose of draining the fens there was discovered at a depth of about 10 feet below the surface a forest of oaks, with their roots imbedded in the underlying Kimmeridge Clay. The trunks were broken off at a height of about three feet. Some of the fallen trees
