The Geology of Central Croydon

[Written for Croydon Local Studies Library by Ken Maggs]

The rocks exposed at the surface in the centre of Croydon very much reflect what has happened to the area within the last 300,000 years. Unlike many parts of the British Isles, the surface rocks in Croydon are relatively recent and almost entirely mask the much older strata laid down over many hundreds of millions of years beneath. However, both the underlying rocks (the solid geology) and the overlying materials (the superficial deposits) have had a part to play in the initial siting and development of Croydon.

The solid geology consists of rocks belonging to two periods: the Cretaceous, represented here by the chalk laid down between 65 and 100 million years ago, and the Tertiary period, with rocks of the Eocene epoch deposited from about 55 million years ago for a period of some 16 million years.

The chalk, which here is probably over 600 feet thick, is present over the whole area but only appears at the surface south of a line from Waddon Ponds to Coombe Road. As this part of Croydon is largely built up there are few exposures and the only one to be seen is in the old quarry (now a children's playground) in Lloyd Park.

The Eocene rocks are represented by four main rock formations: the older and lowest deposit, the Thanet Sand; the Woolwich and Reading Beds - a mixture of clay and loams; the Blackheath pebble Beds; and the London Clay.

The Thanet Sand runs as an almost continuous belt from Waddon to Lloyd Park narrowing to the east where, with the overlying Woolwich and Reading Beds and Blackheath Beds, it forms a marked escarpment rising up over 50 feet. It varies considerably in thickness but is probably about 50 feet thick in this area. Like the chalk, it is largely covered with houses but it could be seen in the diggings of the badger setts in Hammond Wood in Lloyd Park. It was the presence of this rock (which is a valuable water-bearing stratum) that gave rise to a whole number of springs and so determined the early site of Croydon.

The Woolwich and Reading Beds average about 70 feet in thickness and mostly occur to the east of the London-Brighton railway line, but there is an isolated patch at the top of Borough Hill and Harrison's Rise north of Duppas Hill.

The Blackheath Beds consist of between 30 - 40 feet of sands and rounded grey pebbles of flint and can be found over most of the Park Hill area except for the zone between Park Hill, Addiscombe and Chepstow Roads as well as on the top of the ridge running to the Water Tower where part of the original capping of London Clay still remains.

All the superficial deposits of Central Croydon are river gravels - deposits of sand and flint laid down over a period of hundreds of thousands of years and belonging to the Pleistocene Period. They appear in a number of paired river terraces of different ages marking earlier courses of the River Wandle. Each time world sea-levels dropped as a result of the building up of ice sheets and the consequent shortage of water emptying into the seas, rivers had to adjust and lower their beds. This resulted in them leaving their former valley floors lined with gravel high and dry as river terraces.

Three such terraces have been traced in Central Croydon: the first, and oldest, being the Fairfield terrace at the height of between 170 and 200 feet stretching over and area from just to the east of North End and London road to Selhurst and the edge of Addiscombe, taking in Fairfield, East Croydon Station and Thornton Heath. Only a fragment of this terrace survives on the west side of the Wandle valley - at the top of the hill in Warrington Road, north of Duppas Hill.

The second terrace, known as the Haling Park terrace, lies between 170 and 210 feet and extends from Whitgift School to Duppas Hill. Its eastern counterpart forms a band on either side of the London Road and North End, from the old Town Hall in Katharine Street to Broad Green.

The third, and most important, terrace in Central Croydon is the Mitcham terrace which covers the bottom of the Wandle Valley from South Croydon through Old Town to Wandle Park and Beddington Sewage Farm. The River Wandle is of course no longer to be seen as all of its route through Central Croydon was culverted in the late 1840s.

On the eastern side of the valley between the second and third terraces there is a marked former river cliff, now somewhat eroded to a steep slope, but still rising as much as 20 feet. The roads up Scarbrook Hill and Crown Hill show this feature to the full.

Detailed geological maps are held in the Local Studies Library.

Simplified geological maps:

• Northern section
• Central section
• Southern section

Warning: these are all large files

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Friday, 25 July, 2008

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