Some interesting thoughts on the
Thames Gravel at Bull’s Lodge quarry at
Boreham, near Chelmsford, Essex kindly sent by Peter Allen.
Edited and incorporated for this website
by Ian Mercer.
The mighty Thames river flowed across Essex and
Suffolk through the Ice Age. Its river beds remain as
thick gravel layers across the area. Over half a million
years ago, the gravel beds around Chelmsford were
deposited by these cold Thames waters. They are
known as the Kesgrave Sands and Gravels. These
gravel beds were deposited in a braided river
environment, typical of cold conditions.
They were deposited during flood episodes as midstream braid bars - just like those seen around the
large ice cap in south-east Iceland today.
If examined closely, the Thames gravels often show
minor breaks, representing different periods of bar
development; sometimes a thin veneer of sand can
be seen, representing a bar-top sand deposited as
the flood waned and had only sufficient power to
move sand. During these quieter periods the water
would be confined to channels in which sand was
deposited.
At Bull’s Lodge, we saw a traverse section across a
channel and elongate sections along the channels.
When the Thames cut down to a lower level, it left
those earlier gravels as a terrace (either the
Waldringfield or Ardleigh Gavel terrace) 650 or 550
thousand years ago. This was followed by at least
one interglacial and glacial before the great Anglian
ice advance around 430 to 450 thousand years ago,
when the ice sheet came south as far as Hornchurch.
Hence the gravels should have both warm and cold
climate indicators.
During this prolonged period, groundwater washed
virtually all of the iron from the sand and gravel,
creating the pale ‘Essex White Ballast’.
The warm indicators are the partial reddening or
rubification of the sediment and, to a lesser degree,
the humic accumulation (typical of temperate
climates, but not unknown from cold environments).
During each cold stage, the groundwater would
supersaturate the gravel as it froze. At depth the
freezing would be permanent - permafrost - but the
uppermost 1 to 3m could thaw during summers and
freeze again during winters, leading to disturbance
structures such as the sand wedge and the
involutions. The sand wedge would start as a narrow
crack which filled with blown sand (possibly it was
surface washed also), but with each winter the crack
would widen and more sand fall in, so the wedge
would have pseudo-bedding parallel with the wedge
sides – that is apparent in one of wedges. The
involutions would form as the permafrost degraded,
so they are end-cold stage features.
You drew attention
to the incipient
ferricrete.
I commented that
such ironenrichment could
be associated with
disturbance. If the
cold stage features
were early Anglian
(quite plausible),
the passage of ice
would be putting a
lateral pressure on
the sediments
below as it
travelled south and there could a slight stress
disturbance at the top of the permafrost (the
permafrost table). It could also be that the ironbearing groundwater could not penetrate the
permafrost so the iron precipitated out at the watertable. As the Kesgrave Gravels typically have very
low iron contents, the iron is an introduction – possibly
from the Anglian glacier and its outwash.
Within the glacial outwash, this photo shows gravel
clusters. Gravel bed rivers and streams seem to
organize the gravel into regularly spaced ribs –
possibly a physical function or possibly due to minor
obstructions. When you look at photos of mountain
streams you often see regularly spaced breaking
water where the water is passing over the ribs. Gravel
cluster (a) seems to have just accumulated, but in
gravel cluster (b) you see the pebbles stacking up
against the downstream part of the cluster.
A note from Ian Mercer –
I would like to thank Peter
Allen for his untiring efforts to
research Ice Age phenomena
and to bring the story to life
wherever we meet and
wonder at the evidence
produced by our everchanging climate.
A story is gradually coming
out of all the digging in Essex
and elsewhere, even if it is often highly complex and
confusing - after all, there is nothing neat and tidy about the
local geography around the edges of an ice sheet and under
a flooding river system, particularly after thousands of years
of erosion, recycling and human farming, development and
urbanization.
It is amazing that there are those who can give any story at
all. That story is ever more important now that we are faced
with a complicated (and often confusing) set of data. We
vitally need data now to help us find our way through the
next big change in climate - whichever way it goes.
There is no substitute for practical field observation and
measurement. Unless you see and ‘get your eye in’ on the
real evidence, then how can you have any opinion on the
nature of climate change? I believe that Essex Rock has as
part of its raison d’être a mission to enlighten those who
want to try to understand the nature of climate change, using
local and wider evidence as a basis. See ESRL for current
evidence. And Essex Rock has evidence beneath its feet.