Edited and Incorporated for This Website by Ian Mercer

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 mid- stream 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 iron- enrichment 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 iron- bearing groundwater could not penetrate the permafrost so the iron precipitated out at the water- table. 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 ever- changing 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.