228
THE ESSEX NATURALIST
Figure 2. Transect diagram of the area examined, showing surface
deposits as revealed. Prepared from the O.S. 1/25000 map.
hill on the opposite side of this valley which also lies at about
200 feet is capped by the chalky boulder clay.
The two river terraces observed are not immediately apparent
on the ground, but the section exposed showed them incised
into the hillside and made up of dark alluvial clay, the whole
being masked by a thin deposit of hillwash derived from the
chalky boulder clay. The northern slope (south facing) of
the valley side showed no traces of alluvium or river terrace
deposits and is formed on London Clay. The hill top, as noted
above, is capped by chalky boulder clay. The succeeding small
valley is a flat marshy area filled with gleyed alluvium. Above
the level of the marsh, the hill, as far as the railway line, was seen
to consist of London Clay which here contains Septarian nodules.
No section was cut across the railway and when picked up again
on the northern side London Clay was still present which at
about the 225 foot contour gave way to Claygate beds. The
Claygate beds were distinguished from the London Clay by
the absence of stones in the clay. Despite using this criterion
no precise boundary could be observed. The Claygate beds form
the surface feature until just below the road at about 290 feet
where they are overlain by a stratum of orange clay capped by
clay and pebble beds of the Bagshot series. These give way at
about 325 feet to Pebble gravel which caps the ridge.
No further observations could be made beyond this point.
Discussion
The traverse is comparable to the circumstances noted to the
south of Theydon Bois on sheet 240 N.S. of the geological map
(Romford) and the solid geology covers the typical central and
western Essex range of the London Clay succeeded by the upper
London Tertiary series (see Dines and Edmunds, 1925).
The division of glacial clays on Priory Hill, into a lower
containing chalk and an upper without chalk may be compared to
Clayton's definition of the Maldon, Springfield and Hanningfield