80 CONTRIBUTIONS TO PLEISTOCENE GEOLOGY
The later history of the Mardyke from the time of its
diversion to the end of the Pleistocene period is of considerable
interest. If we take our stand at the Trigonometrical station
situated on the summit of the High Terrace of the Thames to
the south of Chalk Pit Farm, and which is at an elevation of
108 feet O.D., we have between us and Stifford a valley whose
floor lies between 50 and 60 feet above Ordnance Datum, and
which attains a lower level to the west. To the north, between
this valley and that much deeper one occupied to-day by the
Mardyke, we have gravel-capped ground, which at Stifford
attains an elevation of 70 feet O.D., but which, when followed
westwards, is found to slope down rapidly. Thus we have
before us, from our standpoint, three parallel east and west lines ;
firstly, the valley; secondly, the gravel terrace or spread at a
slightly higher elevation ; thirdly, the present lower valley of
the Mardyke. Here and there side valleys cut through the
gravel spread, having their heads in the higher or first valley and
their mouths opening on to the alluvium floor of the Mardyke
valley of to-day.
It is clear that the gravel spread is the older of the three
features just enumerated ; therefore we will deal with it first.
The following patches of gravel enter into its composition,,
enumerated from east to west :—
1. Stifford main mass.
2. Combe Wood patch.
3. Mitchell's Wood patch.
4. Watts' Wood patches.
In traversing the district it becomes obvious that these out-
lying masses of gravel were once quite continuous, and the
length of ground occupied by them is about four miles. Taking
them collectively, they show a pretty continuous fall from 70
down to about 30 feet O.D., or rather less, as far as we have
traced them to the west. Sections in these deposits are rare, and
such as do occur are now, unfortunately, very much overgrown
or else very small, and, therefore, not of much utility, except as
serving to check some details in mapping. A roadside section,
just west of Stifford, showed some four or five feet of evenly
bedded ochreous gravel resting on about 10 feet of Thanet Sand,
and in the fields between Milwood Lane and Clockhouse Lane
near the Stifford and Aveley Road, some foundation excavations