Geology Site Account

Site name: Channels Till Section

Grid reference: TL 7217 1100

Brief description of site:

In Lower Wimbush Road in the new Channels development a section through the Anglian till (boulder clay) has been preserved and provided with a signboard. It is all that remains of a cliff of till that was the edge of the former Broomfield Gravel Pit.

The exposed rock was laid down by the Anglian Ice Sheet 450,000 years ago. The section is of educational and scientific interest containing rocks transported by the ice from the north.

Summary of the geological interest:

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 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 Hornchurch and Hanningfield and so north of this line can be found deposits of boulder clay except where it has been removed by erosion.

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The origin and constituents of till (boulder clay)

As the ice moved it ground up and carried along pieces of the rocks over which it passed, just as glaciers and ice sheets do today, and when the ice melted an unsorted clayey residue called boulder clay, or till, was left behind. Most boulder clay 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. Boulder clay contains rocks transported long distances by the ice and 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.

The composition of boulder clay varies according to the rocks over which the ice passed. In Essex the boulder clay usually contains a lot of chalk picked up as the ice passed over the chalk hills of Cambridgeshire, Suffolk and north-west Essex. Typical fossils found include belemnites from the Oxford Clay and the bivalve Gryphaea (also known as the Devils toenail) from the Lower Lias. The chalk fragments are usually well-rounded and often have flat surfaces with striations caused when they were lodged at the base of the ice sheet and the ice, containing other rocks, passed over them.