It is not easy to realise what the conditions were during the formation
of the later Terrace Gravels (Barnwell type), and, if it is a fact, why
there was not then, as in later times, a marshy peat-bearing area here
and there between the torrential deposits of the upper streams near the
foot of the hills and the region where the tide met the upland waters.
A few plants have been found in the Barnwell gravel but they are very
rare in this series. The older Terrace Gravel (Barrington type) might
be expected to furnish evidence of the existence of abundant vegetation
if we are right in assigning it to about the age of the peaty deposits
overlying the Weybourn Crag. But at present we have no evidence of any
such deposit in the Cambridge gravels.
Although there are great masses of vegetable matter formed in the
swamps of tropical regions, peat is essentially a product of northern
climes. Pliny[4] evidently refers to peat as used in Friesland but not
as a thing with which he was familiar.
[4] Lib. XVI, cap. 1.