76 THE ANTIQUITY OF MAN 
surface on which Palaeolithic man made his hearths. 
Eventually that land surface became covered with 5 feet 
of deposit, and then the formation of the terrace ceased. 
We know why the formation ceased. Under the valley 
of the Medway, recalling the condition seen in the adjoin- 
ing valley of the Thames, is the old buried channel of the 
river. Its bottom lies about 60 feet below the level of 
the Hailing terrace. We have seen that it was about 
the commencement of the Neolithic period that the land 
reached its highest point of elevation ; it was then that 
the Medway occupied its buried channel. It is apparent, 
then, that the formation of the Hailing terrace must 
have ceased when the process of elevation set in — the 
process which culminated in the Medway carving out the 
HIQHER TEFIFtACB 
1 BfUCyfEAf^TH 
y^ / ALLUVIUM 
HIGHER TeRRACE 
^^4^^^s^^ ^_4^j\^ 
R.MEDH^AY 
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ghavel. 
HALLINGMAN 
f &URIED 
CHANNEL 
TIUBURY MAN 
Fig. 30. —Diagrammatic section across the valley of the Medway (W. H. Cook). 
valley to the depth of the buried channel. If Neolithic 
man appeared when the land had reached its highest point 
of elevation, and when the Medway had reached its lowest 
bed, he would have found the Hailing terrace, not as we 
see it to-day, only a few feet above the level of the tide, 
but on the sides of the valley, 40 feet or more beyond 
the reach of the greatest floods. Tilbury man did not 
appear at the point of greatest elevation ; submergence 
was well under way — the river valley was being submerged 
and filled up when he was living. Since his day, sub- 
mergence had proceeded, bringing the Hailing terrace 
almost back to its original level as regards the river bed. 
Now, it is plain that if we allow eight thousand or ten 
thousand years for the antiquity of the Tilbury man, we 
must, if we count by the rate of elevation or submergence 
of the land, allow much more than that period of time 
to cover the centuries which must have elapsed between 
