Geology Site Account

Site name: Holdens Wood Gravel Pits (disused)

Grid reference: TQ 591913

Brief description of site:

The disused pits in Holdens Wood are scientifically important because the origin of this gravel, known as Warley Gravel or Stanmore Gravel, has not yet been resolved. The gravel in the pits here has been studied for over a century and are therefore also important in the history of geology. The wood is privately owned but the pits can be viewed from the road and the footpath.

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Summary of the geological interest

Holdens Wood is a classic site that has been mentioned in scientific papers since at least the 1880s. The disused gravel pits in the wood are the type-site for the Warley Gravel, a subdivision of what was formerly called Pebble Gravel (now renamed Stanmore Gravel). Similar gravel is found on hilltops elsewhere in the London Basin and its origin is still debated today. Warley has been pivotal in this debate for over a century.

Warley Gravel caps the highest ground at Warley and Brentwood and its high altitude (over 100 metres above sea level) indicates that it dates from the early Ice Age, probably over one million years ago. One theory is that the gravel was laid down by a northward-flowing tributary of the Thames, at a time when the Thames flowed across what is now North Essex and Suffolk.

Warley Gravel consists mostly of extremely well-rounded flint pebbles in a sandy matrix. These pebbles were originally formed on an ancient beach and are probably derived from marine deposits of Eocene age in Kent. In the topmost layers the pebbles are densely packed together with little sand between them. These pebbles are visible in the paths, in the pit sides, and in the roots of fallen trees. There is also evidence of repeated freezing and thawing (permafrost), which must have occurred during the most recent glacial stage.

Two Edwardian brick cottages in Warley Hill, nearby, are faced with rounded pebbles which most likely came from these pits.

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Scientific interest and site importance

Warley Gravel outcrops on the high ground at Warley and Brentwood and similar gravels exist at lower altitudes around Doddinghurst and Kelvedon Hatch. All these deposits have now been reclassified as Stanmore Gravel based on a description of similar outcrops at Harrow Weald (Ellison 2004). The origin of the Warley Gravel/Stanmore Gravel is discussed by Gibbard (1994) who favoured the idea of a northward-flowing Thames tributary; and Ellison (2004) who resurrected an old established theory that it may be an early Ice Age marine deposit.

At Holdens Wood the Warley Gravel appears to be about 3 metres thick which was the average depth of the pits. The gravel lies on the bedrock of Bagshot Sands, which is exposed in the bottom of the pits. Dines and Edmunds (1925) provide a descriptive section through the gravel at Holdens Wood. They also provide the following summary:

œIt consists of pebbles of flint set densely in a matrix of coarse-grained red or yellow micaceous sand, often clayey. Small quartz pebbles are common and slightly worn flints, some of them seven or eight inches in greatest length are to be found. Lower Greensand chert is present, but not common. The flint pebbles are often stained deep red or zoned with red and white banding. Some of them are desilicified, when they are white, soft and friable. The characteristic feature of the gravel is the presence of elongated, roughly cigar-shaped, flint pebbles of all sizes up to six inches or more in length. Between 20 and 30 per cent of the flint pebbles are of this form. In the uppermost 3 or 4 feet of gravel the pebbles and stones lie with their longer axes vertical, a phenomenon usually attributed to repeated freezing of the soil.€

The pits are historically important having been studied for over a century. They were first visited by the Geologists Association and the Essex Field Club in 1889 (Herries et.al. 1889). They remain scientifically important as the origin of the gravels is still unresolved.

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Other information

The area of scientific importance is restricted to the well-known pits at the northern edge of Holdens Wood. The site of the lesser known pit in the centre of the wood is now occupied by Brentwood Leisure Park. The wood is privately owned. At the time of writing part of the wood is used for outdoor gaming.

A working gravel pit in Holdens Wood in 1923. Photo © British Geological Survey (P202458).