About 450,000 years ago, during the coldest part of the Ice Age, Essex was situated at the southern edge of the Anglian Ice Sheet. The ice sheet was up to 2 kilometres (over a mile) thick in places, and covered most of Britain. Its extent is fairly well known because it has left behind evidence of its existence in the form of a rock called boulder clay, or till (often referred to as Anglian till). The ice sheet extended as far south as Chigwell and Hornchurch and so north of this line can be found deposits of boulder clay except where it has been removed by erosion.
In the southern part of the Phase 6 housing development area at Channels is a cliff of glacial till in an interesting preserved edge of part of the former Broomfield Quarry. The quarry originally worked the underlying Kesgrave (Thames) sands and gravels which involved removal of the overburden of till prior to gravel extraction. It is rare for such a cliff of till to have survived.
It is one of the only places where you can see direct evidence for the former existence of glaciers in Essex and the only such site that is publicly viewable.
This till was formed as the ice moved south grounding up and carrying along pieces of the rocks over which it passed, just as glaciers and ice sheets do today. When the ice melted an unsorted clayey residue called boulder clay, or till, was left behind. Most till was probably laid down or ‘lodged’ at the base of the moving ice sheet as the immense pressure caused the ice to melt; it is therefore sometimes referred to as ‘lodgement till’. Till contains rocks transported long distances by the ice, known as glacial erratics. By matching rock types with known outcrops in other parts of Britain geologists are able to establish the direction of ice movement across the country from its origins in Scotland or Scandinavia. Many erratics show scratches that were received when the rocks passed over each other at the base of the ice sheet.
In Essex the till usually contains a lot of chalk picked up as the ice passed over the chalk hills of Cambridgeshire, Suffolk and north-west Essex and further north. This exposure shows typical Essex ‘chalky till’, which consists of up to 30% chalk fragments and flints in a matrix of grey clay which has weathered to a brown colour.
The clay content of the till is mostly derived from the Jurassic clays of Cambridgeshire and the East Midlands and it often contains Jurassic fossils, also brought south by the ice. The chalk content makes for fertile soil.