Archaeological Horizons in the North Kent Marshes

ARCHAEOLOGICAL HORIZONS IN THE NORTH KENT MARSHES confined to glacial and sub-glacial areas, and its effects would naturally decline towards the south; it is asserted that its influence did not extend south of Latitude 52° N., thus excluding the southern part of the North Sea. These alleged limits of isostacy must be borne in mind when comparing archaeological horizons within and outside its zone of influence. It is known, too, that the basin of the North Sea itself is gradually sinking, a movement which is related to the instabUity of the earth's crust, whUe the unstable character of many coastal regions has long been noted. A more local cause of land sinking is that due to compaction or the consolidation of soft and wet deposits beneath the surface of the land. In Holland the draining of the land results in local and limited subsidence due to the drying out and compression of the thick peat layers. Finally, returning to the British scene, we come to the theory of tilt. It has been thought that during recent times Britain has been slowly tilting so that the northern and north-western parts of the island are rising whUe the south-eastern region is correspondingly sinking. The observed depression of the land in the eastern and south eastern counties has been ascribed to this tilting process, but the theory has been much criticized, and it may be that the observed phenomena is, in part, due to local and irregular movements caused by the coastal instability mentioned above. We thus have one explanation for a fluctuating sea level and several for movements of the land mass, and their combined operations will result in an apparent rise or fall in the land as compared with sea level, or a condition of stabUity or very slight movement. Clearly the rate of movement will be more rapid when both factors are operating in unison, that is, when a falling sea level coincides with land elevation ; while when both land and sea levels are rising or faUing a condition of stabUity or only slight movement wiU result. The effects of land elevation (or regression of the sea) are erosion in the river valleys with deposition of heavy detritus (gravel) in the lower reaches, and the spread of vegetation whose remains will be sealed up later as peat deposits under conditions of aggradation ; while conversely the effects of land submergence (or sea aggression) are drowned estuaries, a much reduced erosive capacity of the river streams, permitting only the deposition of lighter materials in the process of marsh building. THE MEDWAY MAESHES The lower Medway valley presents an impressive example of a typical drowned estuary similar to many associated with the North Sea basin. The appearance of this estuary at high Spring tide is that of a great lagoon filling a broad valley, for the only land which is 105