174
THE ESSEX NATURALIST
In January 1904 a widening scheme for the Hockley Road,
400 yards (365 m) to the north-north-east of Rayleigh Church
(809911) exposed a very irregular contact between the Pebble
Gravels and the underlying Bagshot Sands (Fig. 4A). The ex-
posure has now gone, but about 4 feet (1.2 m) of gravel, analogous
in all respects to that at Hambro Hill, could be seen occupying
a series of asymmetrical trenches and grooves, some with steep,
overturned walls, cut into fine laminated, microfaulted sand. As
at Hambro Hill, the axial trend of the trenches could not be
satisfactorily determined.
The origin of the Pebble Gravel and contact structures seen at
Rayleigh is a problem unalleviated in any way by the presence
of similar deposits and structures at a higher altitude in the
Billericay area. If the deposits are approximately of the same
age in the two areas there is a possible implication of post-Pebble
Gravel tilting towards the North Sea basin. Alternatively the
difference in height might reflect the progressive lowering of water-
level in a lake or tongue of the sea occupying the Crouch Valley.
In which case the gravels, even if formed by analogous marine
processes to those envisaged at Billericay, are not strictly of the
same age, being younger at Rayleigh.
In discussing the origin of the basal structures of the Pebble
Gravel it has to be borne in mind that the Rayleigh area was
proximal to the front of the eastern ice sheets and the ground
subject to intensive and recurrent freeze-thaw action. Under the
circumstances the water saturated gravels may well have been
prone to solifluction and allied effects leading to vertical sagging
and creep of materials down slope. Some of the bottom struc-
tures could be ascribed to scour activated by loading in combina-
tion with solifluction. This mode of genesis may only apply to the
Rayleigh structures; it does not satisfactorily account for all the
features of analogous but larger-scale structures in the Billericay
area.
Census of Essex Rookeries, 1966
By M. T. Parker
In 1965 members of the Essex Field Club were asked to
co-operate in a Rook census of the County. The following
information was called for:—map reference of the rookery,
number of nests, kind of tree, and any other useful information.
The results of this preliminary census were quite satisfactory,
but it was felt that not enough recording had been done to be
of real value.
In 1966 the Census was continued, and a proforma was circu-
lated to members for completion and return to the organiser. A
summary of these reports is outlined in the following short
account.