2 THE NEW RAILWAY BETWEEN UPMINSTER AND ROMFORD.
north and south of the Mardyke, consists of old river-gravel, except
where some bare Chalk appears near West Thurrock. This old
river-gravel lies indifferently on the Chalk or Tertiary deposits, and
that on each flank of the Mardyke valley evidently once formed a
continuous sheet. Between a point rather more than a mile south
of the Mardyke, and another between Stubbers and North
Ockendon, this gravel, along the course of the railway, has a surface
level varying between sixty and seventy feet above Ordnance Datum.
In the cutting at and south of the road between Manor Farm and
North Ockendon, brick-earth, clay, and sand appeared, as also in
others northward towards Cranham Hall. Between Cranham Hall
and Upminster the railway is on an embankment. Between the
Mardyke and Upminster the cuttings are shallow, seldom, if ever,
exceeding ten to twelve feet in depth, and in my journeys along the
line I never saw the underlying London Clay in this part of the
course of the railway except for a distance of two or three hundred
yards close to the Mardyke valley. In short, the sections afforded
by the cuttings revealed nothing but the gravel and brick-earth
shown on the map of the Geological Survey.
A glance at the map just referred to makes it obvious that the
Thames in this south-western corner of Essex has once flowed by
Hainault Forest, Romford, Hornchurch, and North Ockendon to
Hangman's Wood, Mucking, and Stanford-le-hope—a course much
more northerly than that of its present channel. And examination of
the tract of river-gravel, which remains as a record of this fact, shows
that it consists of flat ground very gradually increasing in height as
we recede from the present Thames and approach the undulating
London Clay, which lies beyond the gravel plain. Thus we find
that the surface of the gravel of Hainault Forest, of the ground
close to but north of Romford, Hornchurch, and Upminster, and on
the eastern side of North Ockendon, has a height above Ordnance
Datum of 100 feet, or a little more. If we look for gravel
averaging sixty to seventy feet, we find it occupying a belt of ground
roughly parallel with that just indicated, but nearer the Thames, at
Chadwell Heath, south of Hornchurch and Upminster, and west of
South Ockendon. Similarly, areas below fifty feet are those around
Ilford, Dagenham, Barking, and Rainham. These facts all point to
the conclusion that the Thames once flowed at a height of 100
feet or more above its present level, and some four or five
miles northward of its present course ; and that the river has