As far as the Tilbury evidence goes it suggests a maximum elevation of the land of about 80 feet above its present level; but we will return to this question when we have dealt with the other rivers flowing more directly into deep sea. The animals and plants found at Tilbury were all living species. It is unnecessary here to discuss more fully the submerged forests seen in dock and other excavations in the Thames flats, for they occupy a good many pages in the _Geology of London_ published by the Geological Survey. Even 250 years ago, the hazel trees were noticed by the inquisitive Pepys during one of his official visits to the dockyards, and later writers are full of remarks on the ancient yew trees and oaks found well below the sea-level. Most of these early accounts are, however, of little scientific value. CHAPTER III THE EAST COAST It is not our purpose to describe in detail the many exposures of submerged land-surfaces which have been seen on the shores of the North Sea. This would serve no useful purpose and would be merely tedious. We need only say that the floor of Eocene or Cretaceous strata on which these ancient subaerial deposits rest is constantly found at depths of 50 or 60 feet below the level of the existing salt-marsh. But where, as in the estuary of the Thames and Humber, an older channel underlies a modern channel, the floor sinks about 30 feet lower. From present marsh-level to ancient marsh-level is about 60 feet; from present river-bottom to old river-bottom is also about 60 feet. This, therefore, is the extent of the former elevation, unless we can prove that the sea was then so far away that the river once had many miles to flow before reaching it. This is the point we have now to consider as we trace the submerged forests northward and towards the deeper seas. Before we leave the southern part of the North Sea basin it will be well to draw attention to a few of the half-tide exposures which for one reason or another may tend to mislead the observer. The mere occurrence of roots below tide marks is not sufficient to prove that the land-surfaces seen are all of one date. Not far from Tilbury is found the well-known geological hunting ground of Grays, where the brick-yards have yielded numerous extinct mammalia and several land and freshwater shells now extinct in Britain. These deposits lie in an old channel of the Thames, cut to below mean-tide level, but here not coinciding exactly in position either with the channel of the existing river, or with the channel in which the submerged forests lie.