PEOPLE OF THE SUBMERGED FOREST 29 
to a compression or condensation of the various layers of 
peat — at least two in number (see fig. 12) — which lie 
between the " Roman " and "submerged-forest" strata. 
When, however, we find in the neighbouring valley of the 
Medway, evidence of a similar degree of subsidence, one 
is led to the conclusion that we have to deal with a 
simultaneous process affecting the whole country. The 
ford by which the Romans crossed the Medway at 
Rochester, and the road which led to the ford, are now 
about 8 feet below the level of mean tides. The borough 
surveyor of Rochester is of opinion that there has been 
a subsidence of about 8 feet since the Romans used thef- 
ford. The evidence, then, points to a subsidence in the 
lower parts of the adjoining valleys of the Thames and 
Medway of about 4 feet in a thousand years. If the rate 
of subsidence has been nearly uniform, then it would 
take between seven thousand and eight thousand years to 
give a submergence of 30 feet at Tilbury. In that 
case the Tilbury man would have been a contemporary 
of the predynastic Egyptians. 
In producing such evidence, we are trying to obtain 
some conception of the duration of the Neolit-hic period. 
It is clear that if the process of subsidence is not 
continuous, but takes place at irregular intervals, with 
upward as well as downward movements, our calculations 
may be seriously upset. Besides the deepest old land 
surface represented by the submerged forest zone, there 
may occur, in the submerged land deposits, three other 
zones of peat or vegetable matter, which seem to indicate 
three later stationary periods — intervals at which subsidence 
did not occur, or took place very slowly. The antiquity 
of the Tilbury man and the duration of the Neolithic 
period may be longer than the estimate just given. 
Indeed, there are two lines of evidence which lead us to 
suspect that such may be the case. The submerged forest 
sprang up on a soil deposited over the wide and deep 
stratum of ballast gravel. At some places the ballast gravel 
is 50 or 70 feet deep, and stretches across wide tracts of 
the bottom of the Thames valley. The gravel represents 
