Stanmore Gravel Project

Across this area to the northwest of London where there are hills, we find quite distinctive gravel beds capping the higher ground. The gravel is unusual because it is made up of mostly rounded pebbles in a sandy-clay matrix. The majority of other gravels in London have been broken down by rivers into more angular pieces.
Our local "Pebble Gravel" gives its name to the Stanmore Gravel Formation for which Harrow Weald Common SSSI is the type locality.
At HHGS we’ve been looking into the published data linking the gravels to an ancient tributary of the River Thames, and we invited Dr Peter Allen to talk about this in July 2022. Peter set us a challenge - to carry out some research into the origins of the Stanmore Gravel at our local SSSI in order to add to what is currently known about it. This was the start of the Stanmore Gravel Project. Prof David Bridgland (Durham University) and Dr Phil Collins (Brunel University) agreed to guide us, and we set up a project which anyone with an interest in the topic could join.

The aim of this research is to update knowledge of the geological SSSI at Harrow Weald Common. The methods of testing and any results we achieve will be made available on this website. If they are considered of value to science we would publish a paper in the Proceedings of the Geologists Association (PGA).

Citizen Science
This project gives us the opportunity to advance public appreciation of geology by observation, discussion, field studies, research, illustrated talks and exhibitions. Most of the participants are Citizen Scientists: enthusiasts who wish to understand geology better. The Stanmore Gravel Project will record the steps taken to conduct a piece of locally relevant research. Look out for us on social media #peoplelovepebbles Get involved! email:

View from Harrow Weald Common looking south across the London Basin to the North Downs
[For accessible slides click to open online PPT]
Why are we investigating the Stanmore Gravel?

The Stanmore Gravel at Harrow Weald SSSIl is thought to have been deposited some time before any glaciation occurred, about 2 million years ago.

There is general agreement that the early River Thames took a more northerly course before the Ice Ages and migrated south over time. One school of thought says that early Thames tributaries flowed north through our area and joined the ancestral Thames near Ware in Hertfordshire.

Could the Harrow Weald deposit tell us something about the early development of the River Thames?
"Uncertain age and origin"

There is much in the description of the Stanmore Gravel Formation that is unknown and remains open for discussion. According to the British Geological Survey (BGS), the Type Area for the Stanmore Gravel Formation is Harrow Weald Common; the Formation is said to extend across a wide area as far as Essex and Kent and is categorised as part of the Crag Group. However, the gravel to be found at Harrow Weald Common may tell a different story from some of the other gravels that have been included in the same Formation.

BGS Lithological Description: Gravel and sand, clayey near base. Gravel mostly composed of flints, up to 150mm in diameter, with a little quartz, quartzite and Lower Greensand chert in the fine fractions. Matrix of orange-brown, pale grey, red mottled clay and sandy clay, with pockets of coarse sand. Locally with layers of silt, clay or peat. Interpreted as offshore or beach gravels (Ellison et al 2004), or possibly fluvial (Bridgland 1994).

BGS Geographical Lmits of the Stanmore Gravel Formation:
Thames Valley and Colne and Lea valleys region; plateau cappings from Stanmore, Middlesex to Billericay, Essex, and Shooter's Hill, Kent.
Digging into the Gravel video
exposed bank showing pebble distribution gravel found in a tree root
sample of clayey matrix taken from Stanmore Gravel clay 'sausage' from tree root within the SSSI
large pebble / cobble from tree root in SSSI both angular and rounded pebbles found together
pebbles and sandy matrix from tree root outside SSSI gravel sorted by size including matrix
pebbles sorted by colour: black/grey/white pebbles sorted by colour: red/brown/yellow
You can follow the progress of the Stanmore Gravel Project through video summaries recorded at the end of our monthly discussion meetings.

Project Playlist on YouTube

If you would like to take part, please email:
New pond on Harrow Weald Common
Dr Phil Collins points to bank at Location X
Deep pit at Location Y, flat surface at top
Planning the new Geotrail on Harrow Weald Common
April 2024 update
  • Our lead researchers and 10 volunteers excavated the bank adjoining the SSSI at Location X on Harrow Weald Common. See Progress Report above and video: Digging into the Gravel.
  • Samples of pebbles have been taken to Durham University for analysis. Bulk samples and fine fractions have been collected for analysis at Brunel University.
  • Natural England will push the SSSI landowners to respond to us with permission to conduct research.
Next steps:
  • Visit other sites nearby to look for potentially undisturbed gravel.
  • Produce a map of the Stanmore Gravels and also a longitudinal study.
  • Look for satellite images and LIDAR for the area.
Video summary for April on YouTube:
Monthly summaries on YouTube
Uplift and Folding (in prep)
Sampling and Analysis (in prep)