Submerged forests

by Clement Reid

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.

It is fortunate that the channels do not coincide, for this enables us to distinguish the more ancient deposits. A glance at a geological map shows, however, that they must coincide elsewhere, and where the Thames has re-occupied its old channel it is clear that the destruction of the earlier deposits may have led to a mixture of fossils and implements belonging to three different dates. Mammoth teeth and Palaeolithic implements, Irish elk and polished stone implements, may all be dredged up in the modern river gravel, associated with bits of iron chain, old shoes, and pottery. Such a mixture does actually occur in the Thames estuary, and it makes us hesitate to accept the teeth of mammoth which were dredged in the Thames as really belonging to so late a period as that of the submerged forests.

At Clacton a similar difficulty is met with, for there again an ancient channel contains alternating estuarine and freshwater deposits with layers of peat, and is full of bones belonging to rhinoceros, hippopotamus, elephant and other extinct mammalia. Of course the peat-beds in this channel are just as much entitled to the name “submerged forest” as the more modern deposits to which recent usage restricts it. They belong, however, to another and more ancient chapter of the geological record than that with which we are now dealing. I do not say a less interesting one, for they are of the greatest importance when we study the times when Palaeolithic man flourished; but at present we have as much as we can do to understand the later deposits and to realize the great changes to which they point. We must not turn aside for everything of interest that we come across in this study; these earlier strata are worthy of a book to themselves.

As we travel northward along the coast, again and again we meet with evidence of a submerged nearly level platform, “basal plane,” or ancient “plane of marine denudation,” lying about 50 feet below the sea. We find it at Langer Fort, which lies opposite to Harwich on a spit of sand and shingle stretching across Harwich Harbour. Here the floor of London Clay was met with in a boring at 54 feet below the surface.

The Suffolk coast north of Southwold yields yet another complication, for between Southwold and Sherringham in Norfolk there appears at the sea-level a land-surface considerably more ancient than anything we have yet been dealing with. This is the so-called “Cromer Forest-bed,” which consists of alternating freshwater and estuarine beds, with ancient land-surfaces and masses of peat. It contains numerous extinct mammals, mainly of species older than and different from those of Clacton and Grays.

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.