submerged forest can be seen rooted into hard rock, or into firm undisturbed strata of ancient date, there can, however, be no question that their position below sea-level is due to subsidence of the land or to a rise of the sea, and not to compression. But in certain cases it is found that our submerged land-surface rests on a considerable thickness of soft alluvial strata, consisting of alternate beds of silt and vegetable matter. Here it is perfectly obvious that in course of time the vegetable matter will decay, and the silt will pack more closely, thus causing the land-surface above slowly to sink. Subsidence of this character is well known in the Fenland and in Holland, and we must be careful not to be misled by it into thinking that a change of sea-level has happened within the last few centuries. The sinking of the Fenland due to this cause amounts to several feet.
The third cause of uncertainty above mentioned, destruction of some bank which formerly protected the forest, needs a few words. It is a real difficulty in some cases, and is very liable to mislead the archaeologist. We shall see, however, that it can apply only to a very limited range of level.
Extensive areas of marsh or meadow, protected by a high shingle-beach or chain of sand-dunes, are not uncommon, especially along our eastern coast. These marshes may be quite fresh, and even have trees growing on them, below the level of high tide,
