Geology Site Account

Site name: Wrabness London Clay Cliffs (part of Stour Estuary SSSI)

Grid reference: TM 172323

Brief description of site:

Site of Special Scientific Interest designated for the importance of its geology. The cliffs at Wrabness provide the best onshore exposure of the Harwich Formation. They are the highest vertical cliffs in Essex.

-----------------------------------------------------------------

Site description

The London Clay cliffs on the River Stour at Wrabness are the highest vertical cliffs in Essex and consist of the upper part of the Eocene Harwich Formation and the lower few metres of the Walton Member of the London Clay. They provide the best onshore exposure of the Harwich Formation.

The Wrabness cliffs are of particular interest because they contain a complete sequence of bands of volcanic ash, which probably originated from volcanoes in Scotland. These ash bands are present from the Harwich Stone Band to the top of the formation. Over 30 separate ash layers occur throughout some 10 metres of clay and silty clay, which was deposited in a subtropical sea about 50 million years ago. The succession has been dated partly by analysing the iron-rich minerals in the ash, which have preserved the direction of the Earths magnetic field when the ash was laid down. This indicates that the succession is the same age as the Oldhaven Beds of Kent. The exposure of the Harwich Formation at Wrabness is of national importance as it provides the most complete succession of volcanic ashes attesting to the influence of early Eocene volcanism in southern England.

The same Eocene sediments contain an important fossil flora. Although the fossil fruit and seed flora from Wrabness is limited by comparison with some other sites, the importance of the Wrabness flora is in its mode of preservation. Here, some of the fruit and seeds are preserved in concretions as opposed to the carbonaceous preservation or preservation in iron pyrite that occurs elsewhere in the London Clay. This form of preservation is particularly important, as much detail has been retained which would otherwise have been obscured by the growth of pyrite. The preservation of these plant fossils in concretions has also protected them from the distortion that would have occurred as a result of shrinkage or through compression on burial. Wrabness has a great potential to provide new and significantly different insights into the structure and anatomy of the fossil floras of the London Clay.

The Wrabness cliffs in some places show faults in the London Clay, which are records of prehistoric earthquakes. A fault was visible in the 19th century and illustrated in the 1885 geological survey memoir. It was obscured by a cliff fall but is said to be still visible at TM 170 322.

Part of the cliffs at Wrabness showing bands of volcanic ash from Scottish volcanoes. Photo © G. Lucy.