244 ON A RECENT SUBSIDENCE AT MUCKING, ESSEX.
part of Essex lying south of a line from Purfleet to Stanford-le-
Hope. The lowest formation seen is the chalk, which is visible
over a considerable part of it as far eastward as Grays and
Stifford. Thence to Stanford-le-Hope and East Tilbury it
appears only here and there, at the base of the higher ground
bordering the marshes south of Little Thurrock and West and
East Tilbury. Above the Chalk, with a northerly dip, come the
Tertiary formations ; the Thanet Sand, the lowest of them, being
the only one now existing south of a line ranging from a point
north of Hangman's Wood to Mucking Ford. North of this
line the Woolwich and Reading Beds and the Blackheath Beds
come on above the Thanet Sand. Then the London Clay (which
makes so much of the surface of Essex north of this district)
comes on above the other formations mentioned. But it attains
but little thickness, and covers but little ground within our area,
though prominent at Horndon-on-the-Hill, just beyond it.
Later in date than any of the Tertiary formations mentioned,
and of comparatively little thickness, is the old Thames Gravel,
formed ages ago when that river was flowing at a much higher
level than at present. The deposition of this bed has been a
most important influence in the production of the present
physical geography of the district. Here, as higher up the
stream, the old course of the Thames was usually northward of
that which it now has, the result having been the deposition of
River-gravel over most of the surface between Purfleet and
Stanford-le-Hope and East Tilbury, the Tertiary formations
having been much planed down during the operation. To
illustrate the effects of this planing down I may mention the
following examples. In the great chalk pit west of the road
between Grays and Stifford we find old Thames Gravel with only
8ft. of the Thanet Sand between it and the Chalk. Then at
Hangman's Wood there is about 46ft. of Thanet Sand between
the Chalk below and the Thames Gravel above. At the recent
Mucking subsidence there is in all probability a thickness of
about 150ft. of Tertiary beds between the surface and the top of
the Chalk. And though, owing to its position at the head of a
slight valley, there is no old Thames Gravel at the surface of the
recent subsidence, there is a broad plateau of it a few yards away
both eastward and westward. And in each of these instances
the height of the surface of the plateau above ordnance datum is