Geology Site Account

Site name: Ardleigh Gravel Pit SSSI

Grid reference: TM 053 280

Summary of geological interest

Site of Special Scientific Interest designated for the importance of its geology.

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Site description

Martells Quarry at Ardleigh is an important site for reconstructing the past climate and landscape of Essex. The quarry reveals a pale, buff gravel, known as Ardleigh Gravel - part of the Kesgrave Sands and Gravels - which was laid down in cold conditions during the Ice Age by the Thames when it was a very large river flowing across the Tendring peninsula. This is confirmed by beds of organic clay containing fossils of cold-climate plants such as mosses, grasses and sedges. The upper surface of the Ardleigh Gravel contains casts of ice wedges indicating arctic conditions. Also within the Ardleigh Gravel are beds of silty, dark grey organic clay, which had clearly been deposited in temperate (interglacial) conditions as they contained plant fossils including the remains of deciduous trees such as birch, oak, elm and alder and the remains of beetles. The Ardleigh Gravel is therefore a sandwich of sediments with deposits from two cold climate episodes represented here (the Ardleigh Lower Gravel and Ardleigh Upper Gravel), separated by an interglacial period.

The interglacial deposits are of considerable significance and may be unique in Britain. The deposits cannot be dated per se, but by comparison with other sites, and by reference to the geological sequence in the Netherlands, they are thought to belong to the Cromerian Complex. They could be as much as 700,000 years old. It is possible that the interglacial deposits here may justify designating Ardleigh quarry as the type site for a new early Pleistocene interglacial stage.

On top of the Ardleigh Gravel is the Martells Gravel, consisting of brown/orange gravel and sand with a slightly different stone content to the Ardleigh Gravel below (more Rhaxella chert - probably reworked from the Red Crag of the area). Evidence in the gravel of the river current direction shows that the water was flowing towards the west or south west which was a reversal of the direction of the current when the Ardleigh Gravel was deposited. The Martells Gravel was therefore not deposited by the Thames but by a local river flowing from the north-east. In the 1960s a skull fragment of a ziphoid whale was discovered at Ardleigh which was typical of those found in the Red Crag at Walton-on-the-Naze. This bone must have been reworked and transported here from the Walton area by this river.

At the very top of the Martells Gravel is a deformed, iron-rich deposit that is thought to represent a complex ancient soil horizon. This soil horizon has been found elsewhere and is known as the Valley Farm Soil, the redness indicating soil formation under a warm, dry Mediterranean climate. The deformation was caused by later periglacial activity, overprinting it with a cold climate soil known as the Barham Soil.

The Ardleigh interglacial deposits exposed above Ardleigh Lower Gravel in a drainage channel in the floor of Martells Pit, Ardleigh in 1987. Photo © David Bridgland