234
THE ESSEX NATURALIST
Within the area of the present section mapped by Clayton
(1964) as far north as Great Hallingbury (124), the main boulder
clay is recognised as Springfield Till, while the older Hanningfield
Till is thought to cap the ridges of Epping Forest and Epping Long
Green. The Chelmsford Gravels underlie the Springfield Till, but
both Maldon Till and Danbury Gravels are unrepresented. The
Hanningfield—Springfield dichotomy, in particular, does not cor-
respond in detail to the borehole descriptions, notably those
between Theydon Bois (50) and Pincey Brook (100). Differences
are apparent in three respects:
(a) The boulder clay sections differentiated by Clayton are all
lithologically and pedologically identical.
(b) There is no stratigraphic distinction at the junction of the
Foster Street (Hanningfield) and Hobbs Cross (Springfield)
sections which are, indeed, morphologically continuous.
(c) The boulder clay mapped on Coopersale Common (as already
described) does not pass downward into calcareous till and,
consequently, cannot be regarded as Hanningfield Till. How-
ever, that mapped as Hanningfield Till at the southern
extremity of the Cripsey Brook section (69) is calcareous but
really forms a natural continuation of the Springfield Till
(70-771/2).
These conclusions are corroborated by an analysis of textural
variability as indicated by soil mechanics data. Table 2 sum-
marizes the mean values of Atterberg limits and derived indices
for grouped samples along the section. Of these data, the liquid
limit and liquidity index are the most useful, diagnostically. The
liquid limit is sensitive to changes in particle size distribution and
type of minerals present in a clay (Skempton, 1944) and thus
reflects the composition of material as it was deposited plus
changes in mineralogy since deposition. The liquidity index, on
the other hand, being a measure of the relation of natural water
content to the Atterberg limits, strongly reflects the loading
history of a soil (Rominger and Rutledge, 1952). It must be
admitted, however, that the genetic significance of the LI cannot
be realized in this case since, unlike typical clays deposited sub-
aqueously, glacial clays were not deposited at a natural water
content equal to their liquid limit. Furthermore, they are not
"normally-consolidated" since they have suffered greater over-
burden pressure in the past (i.e. beneath a considerable ice load)
than exists at the present time. For boulder clay therefore, the
liquidity index has only limited interpretative value, but is none-
theless useful as a descriptive measure of consolidation since soft
clays yield an Ll of 0.5 to 10 and firm to stiff clays 0.0 to 0.5
while very stiff clays and compacted shales yield a negative value
(Skempton, 1944).
Applying these considerations to the present data (Table 2),
southwards from Pincey Brook (100) there is a continuum of