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
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. Of great importance are two layers from the time of the Paleocene-Eocene Thermal Maximum:
Lambeth Group: Upnor Formation
Thames Group: Harwich Formation
Chalk exposure at Harefield Pit showing trace fossils
Sands within the Lambeth Group
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.
Harefield Pit Geoconservation Day 2024
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.
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.
rounded pebbles of Stanmore Gravel Formation Harrow Weald SSSI videos Stanmore Gravel Project
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