18 The Essex Naturalist
of channels, now buried, in eastern Essex between Southend and Clacton. The
Black Park Gravel is probably represented by the lower part of the infills of these
channels. In Black Park and Orsett Heath times, the Thames flowed across Fairlop
to Corbets Tey whence it looped to Aveley and Purfleet before continuing to the
Southend area where it joined the Medway and flowed northwards to the Clacton
area. In Corbets Tey times, the Thames followed the same route but on a more
southerly alignment and crossed the coast where the River Crouch now exits. By
Mucking times, the river had shifted significantly to the south, taking a more
direct route to Purfleet and eventually crossed the coast in the Crouch area. The
Kempton Park Gravels closely follow the line of the present Thames, though they
are now represented by terrace gravels beneath the sea off Southend.
This sequence of terrace gravels is characterised by a significant change in the
lithological content from the earlier gravels :
8-32mm Black Park Gravel
Angular flint 70-80%
Rounded flint 5-11
Quartz 5-10
Quartzite 1- 6
Greensand chert 0- 5
Carboniferous chert 0- 0.5
The different size range counted accounts for some of the differences between
these gravel counts and the earlier ones, but there does appear to be a significant
increase in the amount of angular flint, probably due to an input by the Anglian
ice. The decrease in the amount of quartz, quartzite and Carboniferous chert is
thought to reflect a change in the headwaters of the Thames, that the
connection with the Midlands was lost, probably due to glacial interference in
the Anglian. Thus the catchment changed to resemble the present one.
Dating
There is growing agreement that the Anglian cold stage accords with Oxygen
Isotope Stage (OIS) 12. Thus the Thames was diverted in OIS 12 and the
terrace sequence of the Lower Thames post-dates that Stage.
Orsett Heath Formation
These gravels were formerly mapped to include the Hoxnian interglacial
sediments at Swanscombe, famous for its human skull, as well as mollusca,
vertebrates and pollen. However, more careful examination of the gravels shows
that the Boyn Hill Gravels correlate with the Middle Gravels at Swanscombe
and so overlie the interglacial sediments. This stratigraphic relationship also
applies for Hoxnian channel infills at Cudmore Grove on Mersea Island and at
Clacton in Essex. The Boyn Hill Gravels, therefore, post-date the Hoxnian.