GLA 36: Pinner Chalk Mines

GLA 36: Pinner Chalk Mines, Proposed RIGS
London Borough of Harrow, TQ 11538 90483
Ownership: Local Authority. Access has formerly been available but is no longer possible due to health and safety concerns.

Pinner Chalk Mines

Pinner Chalk Mines extend over a large area, with mixed extraction methods recorded from the 14th century. Access is not currently possible. When accessible, it is one of the few locations still existing in London where the Chalk can be examined without being masked by vegetation. The properties of mined chalk were thought to be superior to those of surface material, which led to a number of shallow mines, and the more primitive dene holes, being dug within the London Basin. Throughout the history of the mines chalk was probably quarried primarily for lime but from 1830 Dingles mine was owned by a local brick maker, and chalk was taken and added to clays and sands of the local Reading Beds and London Clay in order to stop the finished products cracking. Dates found in the mines, burnt into the chalk with candles, suggest that the working year usually began in the spring. Much of the excavated area is now built over but the area around No. 3 shaft remains as open ground with a sports field, allotments and Dingles Wood. It is securely locked & gated.

The Chalk

The chalk exposed is relatively uniform soft white chalk with several horizons of flints. Comparison with the large chalk quarries along the Colne suggests that it is likely to be part of the Seaford Chalk Formation. Since, however, no research has been undertaken no fossils have been recorded to precisely date the mines. Workings in the Pinner mines were by the ‘pillar and stall’ method exposing a height of about 6 m.

Hertfordshire Puddingstone

Pinner Chalk Mines are one of the very few places where Hertfordshire Puddingstone can be found in situ, making this an important site. Hertfordshire Puddingstone is a strongly-cemented conglomerate of flint pebbles that forms a layer about 1.5 m above the top of the chalk. It allowed miners to excavate the chalk to within 50 cm of the junction with the overlying sands and clays without major fear of collapse. The Hertfordshire Puddingstone can be seen from the rope ladder about 17 m down the shaft. For most of its length the shaft is brick-lined but, because of its competency, the Hertfordshire Puddingstone itself is not encased. Occasional roof falls allow further exposure along with the sands and clays beneath. The conglomerate lies at the junction of the Upnor and Reading Formations (both are part of the Lambeth Group). The Thanet Sand Formation does not reach as far east as Pinner but the typical large, green-coated, unweathered flints usually found overlying the chalk elsewhere are also found here.

At the moment there is no public access to the chalk mines but a virtual tour can be made via Ken Kirkman’s website ( The geology of the mines is described in more detail in GA Guide 68, pp. 31-38 and in the 1992 Pinner Chalk Mines publication below.

Kirkman, K., 1992. Pinner Chalk Mines. Harrow, Pinner Local History Society, Vol 13, 67pp.

Hertfordshire Puddingstone from a roof collapse in the Pinner Chalk Mines Length of clast is c. 10 cm
Source: Diana Clements (collection J. Pester)


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