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Parsons Spring Gravel Pits, HIGHWOOD, Chelmsford District, TL62390284, Notified Local Geological Site
Site category: Pebble gravel (Stanmore Gravel)
Site name: Parsons Spring Gravel Pits, near Highwood
Grid reference: TL 6239 0284
Brief description of site:
The woodland of Parsons Spring contains a number of disused gravel pits. The origin of this gravel, known as Stanmore Gravel (formerly called 'pebble gravel'), is unclear. It dates from the early part of the Ice Age and may have been deposited by northward-flowing tributaries of the pre-diversion Thames, or it may be of marine origin. Exposures of this gravel therefore have the potential for future research. The geology of this area is interesting with Anglian boulder clay, or till, outcropping to the north and west of the wood and Claygate Beds and Bagshot Sand outcropping in the woods to the south.
The wood is private property but the pits are clearly visible as hollows either side of Old Barns Lane, a public footpath running through the wood north from Blackmore Road.
Parsons Spring (also known as Parsons Wood) has numerous shallow but steep-sided pits where sand and gravel was once exploited, probably for hoggin for use on the paths of Writtle Park estate. This sand and gravel is referred to on geological maps as ‘Stanmore Gravel’ (formerly called 'pebble gravel') and it occurs elsewhere on the high ground of the adjoining Brentwood district. How it was formed has been debated by geologists since the 19th century. Its origin is still unclear today although one theory is that it was laid down by a river, probably over one million years ago, during the early part of the Ice Age. This was at a time when the Thames flowed across what is now North Essex and Suffolk. The river in question may therefore have been a northward-flowing tributary of the Thames.
The deposits are very variable both vertically and horizontally, and in places very colourful. It is mostly silty sand and sandy clay - sometimes mottled grey and bright orange - with layers of flint pebbles. There are also minor amounts of other rock types. The flint pebbles are mostly well-rounded - originally formed on an ancient beach - and probably derived from marine deposits in south Essex and Kent. There is also evidence in the gravel of repeated freezing and thawing of the ground (permafrost) during the Ice Age.
The pits are clearly visible as overgrown hollows either side of the public footpath that runs through the wood. Some sand and gravel can be seen in the sides of the pits and especially in the roots of fallen trees where the well-rounded pebbles are conspicuous.
Scientific interest and site importance
The outcrop of sand and gravel at Parsons Spring is about 300 metres in diameter and lies on top of bedrock of Claygate Beds. A 3.3 metre section through these deposits was revealed in a trial pit dug by the British Geological Survey and described in the 1987 geological survey memoir (Millward et al. 1987). The deposits are described in the memoir as 'older Head’ and their origin is discussed. More recent geological mapping by the BGS has reclassified these deposits as Stanmore Gravel. The origin of Stanmore Gravel is discussed by Ellison (2004) who resurrects an old established theory that it may be an early Ice Age marine deposit.
Similar deposits occur on high ground elsewhere in this part of Essex (generally above 90 metres OD) but the outcrop at Parsons Spring is one of the best described. It is also one of the thickest, up to 7.2 metres having been recorded here.
The great variation of the deposits is clear from the BGS trial pit which was dug at TL 6229 0280. It generally revealed silt, sand and clay, often mottled and with great colour variation. There are pockets and layers of gravel, both of angular and well-rounded, mainly white, patinated flints. Lateral variation may be due to cryoturbation, which has also resulted in flints with their long axes vertical. No study has yet been made of the different rock types in the deposit in Parsons Spring but in College Wood to the south, the gravel was found to contain vein quartz, quartzite and rare Lower Greensand pebbles. The gravel outcrops in Parsons Spring and College Wood are similar and are characterized by rather different pebble assemblages compared to occurrences elsewhere in the area (Millward et al 1987). Future research here may therefore be important in helping to establish an origin for this enigmatic deposit.
Well rounded pebbles in tree roots at Parsons Spring Gravel Pits
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Reference: Ellison 2004 (P. 52), Milward et al 1987 (p. 25, 26, & 28).
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