98 SUBSIDENCE OF EASTERN ENGLAND.
shores may be the deepening trough between these flanking
masses in their relentless, but fortunately slow, approach. Such
differential movement has been intermittently in process through-
out the Pliocene and subsequent epochs, but without definite
gauges to register the rate of submergence or emergence. It has
been shown 1that Holland has a vastly greater thickness of beds
of the Pliocene and Glacial periods than are known on either
side or southward, and it has long been known that southern
Sweden is in course of subsidence. On the latter rocky coast,
such secular changes of level are comparatively easy to register
and gauge; whilst, for our softer beds, under stress of erosion,
such record is less easy to secure, It is evident, however, that
the rapid depression found at Lille (.78 m., or 2.56 feet in
27 years: that is 9'6" per century) does not persist into our
Essex area, or the alluvial lands round our coasts would be
deeply submerged. Fowlness, existing and inhabited before the
Roman invasion, is still at half-tide level, and has no trace of
freshwater deposits in its area, patches of sea-shells occurring in
several places. Apart from the evidence of ancient occupation,
its position as an island, separated by navigable channels from
the mainland and from other islands, enables us to be certain
that historical records do not refer to higher land in the
vicinity.
As in France, the line of maximum depression may be
narrow and sinuous, passing from Harwich and Colchester,
along the course of the high-level Thames gravel to Southend
and Tilbury, whilst imperceptible on Fowlness, and with little
or no confirmatory evidence in central and western Essex.
That the inner parts of the Southminster and Burnham marshes
are slightly lower than the outer, may be attributed in part to
the more recent enclosure of the latter, and in part to the greater
porosity of the deposits there. Clay land shrinks by drying more
than shelly sands. There may, nevertheless, be some slight
subsidence on the western side of these broad levels. Is the
loss of land at Fambridge attributable in any degree to such
movement?
Further west, it has been suggested that the position of the
Lea and Roding rivers, on the eastern side of their respective
valleys, with a sharp rise to the eastward, and a gentle gravel-
1 Harmer, Quart. Journ. Geol. Soc., vol. lii, p. 748, 1896.