Links to our neighbouring Local Geology Groups
Local Geology

The London boroughs of Harrow and Hillingdon are situated in the London Basin, close the Chiltern Hills.
Together they make up the northwestern part of Greater London and are predominantly suburban, with buildings concealing much of the landscape. On this webpage we will explore some of the surface features that offer a window into our local geology: rivers, hills, quarries, sarsens.

2022 Local Geology Series of talks, trips and meetings
Monthly meetings, 2nd Wednesday of each month
February 9th - The geology and Hydrology of Ruislip Woods. Site report by Allan Wheeler followed by discussion of flood management in Park Wood in Pinn Meadows.
March 9th -
The Geology of the Chiltern Chalk and the impact of HS2. Talk given by Haydon Bailey, Herts Geological Society.
April 13th -
The Importance of London Geodiversity Partnership sites around Harrow and Hillingdon. Talk given by Diana Clements (LGP)
June 8th -
Local Rivers Site Meeting at Headstone Manor Park to see the completed restoration works on the Yeading Brook, followed by Zoom meeting with guest speaker Dr Phil Collins of Brunel University
July 13th - The Pleistocene Thames taking in the gravel terraces laid down by the Thames in our area. Zoom talk by Peter Allen.

Sites of Special Scientific Interest (SSSI)
Harefield Pit (GLA 34), on the western margin of Hillingdon was once a chalk quarry but little remains to be seen. In the 1960s it was used as a municipal rubbish dump, filled in then covered over as a green space. A small exposure remains which preserves the geological record of rocks laid down at the time of the Paleocene-Eocene Thermal Maximum. Particularly important are outcrops of the Lambeth Group: Upnor Formation and the Thames Group: Harwich Formation
Harrow Weald (GLA 18) has been designated a Site of Special Scientific Interest because it provides the most complete exposure of Pleistocene gravel beds above the Claygate Beds, the youngest layer of London Clay.

Local Rivers
Harrow and Hillingdon lie within the Thames basin and our local rivers ultimately drain into the Thames. To the west the Colne Valley Regional Park holds an important water system that includes 60 lakes and 200km of rivers and canals. The River Pinn is a tributary of the Colne; it rises in Harrow Weald and flows through Pinner, Eastcote, Ruislip, Ickenham, Uxbridge and Cowley. To the east of the Pinn, the Yeading Brook rises near Headstone Manor in North Harrow and is joined by another headstream near RAF Northolt. It becomes the River Crane from Cranford and enters the Thames at Isleworth.
Colne Valley
River Pinn
Yeading Brook (HA2 6PX)
The Importance of our local Geodiversity Sites
Diana Clements Lecture: 13 April 22
What is Geodiversity?
Rocks, minerals, soils, landforms and topography, mountains, reivers and lakes - the non-living elements of nature that underpin every ecosystem and enable biodiversity on our planet.

Why is local geodiversity important?
This part of north-west London provides essential resources for building our city, such as brickearth and gravels for use in the construction of buildings and roads.

Major engineering projects in London require a detailed understanding of each site's geology, especially when constructing tall buildings and tunnels.

Geodiversity is the basis for the landscapes we love - rivers, woods, hills, wetlands.
The Chilterns and Local Chalk
Meeting - 9 March 2022
The Geology of the Chiltern Chalk and the impact of HS2.
Talk by
Dr Haydon Bailey, Geological Advisor, The Chiltern Society and Chairman, Hertfordshire Geological Society.

March talk by Haydon Bailey Local Chalk: Find out more Chalk Streams (Hertfordshire GS)
Flood Management in Ruislip Woods and Pinn Meadows
Site Visits To Park Wood and Pinn Meadows, 2021-22
Site Report by Allan Wheeler, available 9 Feb 2022
Clay, sand, gravel - Find out more. What is the Lambeth Group? Find out about Iron-oxidising Bacteria
Photos taken during site visit, 29 Jan 2022
flint pebble in clay leaky dams gravel in ditch Grub Ground Pond drainage Pinn Meadows ditch
In August 2021 we were contacted by John Scrivens of North Ruislip Flood Action Group and Ruislip Woods Management Advisory Group, asking for help to identify a geological feature in Ruislip Woods. Both groups had been collaborating with Hillingdon Council and the Environment Agency for several years to develop flood prevention measures in Park Wood. Large volumes of water flow out of the wood during heavy downpours of rain, putting houses that back onto the woods or the nearby stretch of the River Pinn at risk of flooding.

The feature of interest was a layer of compacted gravel in Park Wood, thought to belong to the Lambeth Group of sediment layers made up of clay, sand and gravel. A cemented layer of gravel from the Lambeth Group exists locally in Pinner, seen underground in the disused Pinner Chalk Mine.

In Park Wood, the compacted gravel, in a sandy clay matrix, can be seen along paths and watercourses where surface layers have been washed away by rainwater. It is known that Lambeth Group sedimentary layers are exposed in Ruislip Woods where the stream that flows into Ruislip Lido (a former reservoir built in 1811 to feed the Grand Junction, now Grand Union Canal) has cut down through the younger London Clay. What may be Lambeth Group gravels can also be seen nearby in the bed of the River Pinn. However, cemented layers like that in Pinner chalk mine have not been previously recorded in Ruislip Woods.

Why does it matter?
The Flood Action Group are trying to find out whether the hard, impermeable gravel layers might occur just beneath the surface in Park Wood. And if they do, could that explain why such large volumes of water flow out of the wood during heavy downpours of rain?
Thanks to John Scrivens and Stephen Heneker for the photos below:
Bacterial film Red colouration Geological map Stephen in ditch John, Allan, Grub Ground Pond
Downloadable resources
Local Flood Action Groups are interested in the geology of Ruislip Woods. Find out more:
Flood Management in Ruislip Woods and Pinn Meadows
Climate Change in our region:
To coincide with COP26 in November 2021, we took a closer look at the rocks of Harrow and Hillingdon for the GA Festival of Geology. Download the PDF here:
Evidence of past climates from local rocks
London's Geodiversity:
What makes the geology of Harrow and Hillingdon special?
Coming soon:
Sites of Special Scientific Interest / Regional Importance
Urban Geology:
Take a walk in our area - download a PDF below for some easy to follow geological highlights to be seen as you walk along:
Uxbridge Building Stones Walk Pinner Building Stones Walk Pinner - A Geological Trail
Geological legacy in Harrow and Hillingdon
Local Features:
River Pinn (Follow the Celandine Route)
Yeading Brook (follow the Hillingdon Trail)
Gravel Pits
Pinner Chalk Mine (not open to the public)
Ruislip Woods (National Nature Reserve)
Fossils of Harefield (by Jon Noad)

Photo location: Yeading Brook, near Ruislip Gardens
Have you noticed amazing fossils of ancient sea creatures in the floor of Uxbridge Shopping Centre?

Rock Stories - Coming Soon!

Pinner Chalk Mine
Chalk from Harefield Pit


Harrow Weald SSSI (GLA18)
photo by Dudley Miles - Own work, CC BY-SA 3.0,
Harefield Pit SSSI (GLA34)
Stanmore gravel at Harrow Weald Common

Yeading Brook, Ickenham Marsh
Clay in Yeading Brook, Ickenham

Hertfordshire Puddingstone (Uxbridge Rock Show Exhibit)
Sarsen stone at Monor Farm, Ruislip
HHGS at Pretty Corner, Eastcote