Local Sites of Special Scientific Interest (SSSIs)
There are two sites of geological importance in our area: Harefield Great Pit and Harrow Weald.

The London Borough of Hillingdon has a high concentration of SSSIs important for biodiversity, underpinned by the local geology:
  • Chalk appears at the surface in various locations - it is a soil conditioner, reducing acidity
  • The area is covered with clay, silt, sand and gravel allowing ancient woodlands to thrive
  • Former gravel pits and chalk quarries provide new wetland habitats
  • Huge flint boulders are found across the region, essential to early human occupation
  • The Colne Valley rivers, canals and lakes form our western flank joining the Thames just south of our area
https://magic.defra.gov.uk/MagicMap.aspx?startTopic=Designations&activelayer=sssiIndex&query=HYPERLINK%3D%271003496%27
Harefield Great Pit SSSI
Little remains to be seen of the Great Pit, a former chalk quarry which closed mid 20th century and was used for landfill in the 1960s. Only a small exposure has been left to preserves the geological record of rock layers directly above the chalk in this area.
Seaford Chalk is seen here with very large flint boulders eroding out at the surface. Interesting in the Chalk at Harefield Pit are the fossil burrows known as Glyphichnus harefieldensis filled with clayey glauconitic sand. These were probably made by crustaceans at the time the Upnor Formation was deposited above the Chalk.
Chalk exposure at Harefield Pit showing trace fossils
This is the best place in London to see the unconformity (time gap) between the Chalk and the much younger Bullhead Bed of the Upnor Formation from the time of the Paleocene-Eocene Thermal Maximum.
A second exposed face about 30m from the Chalk section shows the impressive sands of the Reading Formation. Locally, Lambeth Group sands and clays are interleaved.
Reading Formation sands
The sand was quarried at several pits in Harefield and used in brick making and construction. Here it is golden and surprisingly fine. Badgers choose to build their setts in it because it is easy to dig.

Close to the above conserved faces, in an adjacent gully, is a small exposure ofsandy clays and silty sands of the Harwich Formation (Thames Group). Mudstone nodules are found within the sandy clay layers, some filled with shells.
Harefield Pit Geoconservation Day 2024 Harefield Geoconservation Day 2025
Harrow Weald SSSI
This site provides the most complete exposure of Pleistocene gravel beds resting on the London Clay. This 'pebble gravel' is thought to have been laid down just before the Ice Ages, with similar deposits found capping local hills. Here it is properly called the Stanmore Gravel.
Stanmore Gravel Project Pebble ridge capping hill, Harrow Weald Common
Stanmore gravel consists of a mixture of large rounded pebbles from an ancient sea and some more angular pebbles, all in a matrix of silty, sandy clay. Many questions remain to be answers regarding the deposition of the gravel, so we are conducting a research project led by Prof David Bridgland and Dr Peter Allen, with contributions from local geologists and citizen scientists.
If you are interested, join our Stanmore Gravel Project!
rounded pebbles of Stanmore Gravel Formation Harrow Weald SSSI videos
The components have different origins - but when and how was this gravel layer deposited? Was it laid down by early ice-age meltwaters? By an inland sea? Or by an early tributary of the River Thames?
variety of large pebbles