Geology Site Account

Site name: Blackmore Post Office Stone

Grid reference: TL 6033 0187

Brief description of site:

Next to the post office in the village of Blackmore is a small boulder of basalt or dolerite 38 cm x 34 cm x at least 20 cm. Erratic boulders of igneous rock such as this are rare in Essex. The stone may be much larger as it is buried in the ground and only part is visible. It was probably transported to Essex from Northern England or Scotland by the Anglian ice sheet about 450,000 years ago.

This stone has clearly been here a very long time and was no doubt found in a local field, or perhaps discovered when the adjacent building was built.

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Details:

Stones such as this are known as glacial erratics and were carried to Essex by the Anglian Ice Sheet that covered almost the whole of Britain during the coldest period of the Ice Age, some 450,000 years ago. At this time Essex was situated at the southern edge of the ice sheet, which was up to 2 kilometres (over a mile) thick in places. 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. A great thickness of Boulder clay exists across north and central Essex except in river valleys where it has been removed by erosion.

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. 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.

The stone outside Blackmore Post Office. Photo: W.H. George