NOTES ON THE ANCIENT PHYSIOGRAPHY OF SOUTH ESSEX. 197
If we consider the evidence as to the date at which the Mardyke
began to flow, we find that it must have begun to exist at a very late
period in the history of the Thames Valley ; for, high above the
western flank of the Mardyke Valley, at North and South Ockendon,
we have a plateau capped by gravel belonging to the Thames Valley
system, which must have been deposited there when there was
higher ground eastward, where the Mardyke now flows 60 or 70 feet
below. And that stream, in the lower part of its course, cuts through
the gravel, not only of the higher, but also of the lower terraces of
the Thames Valley system.
And when we turn to the valley of the Crouch we meet with
similar evidence pointing to the lateness of its origin. Assuming,
with the Geological Surveyors, that the deposits of gravel and loam
which are found about Southend, Rochford, and Canewdon, south of
the Crouch, and at Burnham, Southminster, and Bradwell, north of
it, are all one series, and were formed on the western flank of the
ancient valley of the Thames, the following conclusions are sug-
gested : The maps show that these beds are deposited only on the
eastern flank of the high ground of what may be called the
Laindon, Rayleigh, and Althorne range, the Crouch Valley being
almost wholly destitute of river deposits, with the exception of
modern alluvium (just as is that of the Mardyke), though the gravels,
etc., of the Thames Valley system are cut through in the lower part
of the Crouch Valley as they are in that of the Mardyke. This
tends to show that when the gravels, etc., of the Thames Valley system
were deposited hereabouts the high ground westward was unbroken
by the present outfall of the Crouch. While the existing state of
things would naturally arise if the Crouch began to flow only after
the destruction, in the fashion shown at Romford, of an earlier
stream which flowed through the lowland tract between Cold
Norton and Althorne into the Blackwater below Maldon. For the
existence of the Crouch would then date from a time when this
earlier stream had eroded away everything in its valley above the
London Clay, and after the deposition of the Thames Valley
gravel and loam between Southend and Bradwell.
The water-parting now existing between the basin of the Crouch
and that of the Blackwater is extremely low and narrow between
Cold Norton on the west and St. Michael's Church, west of
Althorne, on the east. Between Norton Hall and St. Michael's
Church, a distance of about two miles, the highest ground now