Geology and Soils
Dr Peter Allen and M.W.Hanson
Recorders: Dr Peter Allen, M.W.Hanson, Martin Heywood and GR.Ward
Geology
In 2002 a shallow trench was excavated across Hylands Park, about a metre deep and several
hundred metres in length, to accommodate a new high voltage electricity supply cable to the house.
In November the trench (and spoil) was investigated for its geology.
The trench ran from approximately TL 688043 to TL 686044. The trench, nearest the formal gardens,
exposed a pebbly clay. Downslope this was seen to overlie till (chalky boulder clay) and nearer to
the eastern end of the trench, till was exposed immediately beneath the soil and was seen to overlie
weathered London Clay. Pebbly clay was again exposed near the perimeter wall.
The stratigraphic sequence is interpreted as:
Pebbly clay Head Devensian }
} Quaternary
Chalky boulder clay Great Waltham Till Anglian }
Brown clay London Clay Eocene Tertiary
The till was considered to be typical of the Great Waltham variant of the Lowestoft Till. The
exposure is rather further south than would be expected, but may have been deposited by a finger
of ice that extended along what was to become the valley of the River Wid. This deposit is thought
to date from the Anglian stage (about 450,000 years ago).
The pebbly clay was interpreted as head on the basis of its texture, position in the landscape and
distribution. It contained a mixture of rounded, chatter-marked (Tertian) and sub-angular flints of
relatively local origin. There were also significant amounts of white/colourless vein quartz,
occasionally with red staining, and brown and pink quartzites, originally from the Midlands. Rare
pieces of grey-green igneous rock (rhyolite/tuff) from the Welsh borderlands and veined tourmaline
(Lydian stone) probably originally from south-west England (eg Dartmoor) were also noted. This
assemblage is typical of the Kesgrave (sand and gravel) Formation, deposited by an early Thames
which crossed central Essex.
Local glacial outwash can have a similar assemblage, generally with more sub-angular flint but,
importantly, also with Rhaxella chert, which is diagnostic of the glacial deposits. None of Illis chert
was identified, but a sample has been collected for a laborator}' stone count, to resolve this issue. The
gravel, whatever its origin, subsequent to its deposition on higher ground over London Clay, solifluctcd
downslope, mixing with the clay to create the head. Some parts of the head extended beyond the
general cover, moving down minor valley heads, cutting through the thin outcrop of till. The pebbly
clay is thought to derive from the last glacial phase of the Pleistocene (25,000+ years ago).
The prevailing clay types of the underlying geology at Hylands explain why Hylands can be very
muddy in wet winters and at other periods of heavy rainfall. It is not uncommon for vehicles driven
off-road at these times to get set in the park. Heavy work, because of the potential (and actual)
damage it can cause, should be scheduled for dry periods when the ground can support heavy
vehicles, such as I.C.Bs and H.GVs. The prevalence of clay also probably indicates that drainage
Essex Parks: Section 2 - Hylands Park
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