Froghall Brickworks SSSI

Bucks County Council administration area: Chiltern District Council

Grid reference: SU 977 941

Area of site: 0.26 ha; 0.65 acres

Access, location and parking: northwest of Chalfont St Giles via minor roads. The site is fenced with a padlocked gate but keys can be obtained from the owners. Inside the fence there is ample parking for several vehicles. Outside the gates there is space for only 2 cars.

Site description: Intermittently used brick pit and brick kilns (in disrepair).

Geological interpretation: There are a number of excavations on site. These expose the mottled clays (a palaeosol) of the Reading Beds which formed the resource for the brick industry, the sandy facies of the Reading Beds and overlying glacial gravels. This pit is of crucial interest for the gravels which form part of the proto-Thames deposits in the area, but are unique for being the only exposure of the Westland Green Gravels in the Middle Thames area. The SSSI notification is therefore based on the stratigraphic importance of these gravels for understanding the history of the ancient Thames and for correlation between the Upper and Lower Thames . The gravel represents the earliest recognised stage of the Thames and has been traced from the Goring Gap to Norwich . It is evidence for the former course of the Thames (before the Anglian glaciation of  450,000 years ago) when the Thames flowed north of Goring through the Vale of St Albans to East Anglia . The current, more southerly, course of the Thames, through London , was only cut after the Anglian Ice sheet began to retreat. The height, distribution, and pebble content of the gravels are all evidence for this early history of the Thames .

The Reading Beds are also of interest for Buckinghamshire as there are very few exposures of these sediments in the county. These show red-purple-brown mottled clays which are typical of fossil soils. The sandy sediment was subsequently deposited over the clay soils from rivers that criss-crossed this tropical landscape of the Tertiary period.

Current and potential educational  use (geology, ecology, archaeology, industrial archaeology): The site is intermittently worked and so exposure and condition of the geological exposure can vary. It has great importance for geological (Quaternary) specialists. However, the site has great potential for training of school, college and higher levels of geological study. Geological societies would also show a keen interest if they are aware of how to obtain permission. The woodland could offer an opportunity for nature studies of various types (botanical, entomological, ornithological). There is also industrial archaeology in the form of the brick-kilns, which the owner has expressed an interest for renovating and allowing public access possibly on regulated ‘open days’. This would be well worth considering if funding could be obtained.

Conservation interest: To maintain the integrity of the gravels (within the SSSI notification) and also the two facies of the Reading Beds present in one or more of the 3 different locations on site. To assist, if possible, in the rebuilding of the kilns as part of our industrial archaeology heritage.

Threats to site (present and future): The major threats are from overgrowth of vegetation and gradual slumping and deterioration of the face. Vegetation causes problems in two ways: restricting access to the faces, but in addition the sedimentary structure of the gravels and the Reading Beds could be disturbed by penetrating roots. This is of greater importance to the gravel horizons as pebble imbrication is crucial to an understanding of current flow of the ancient Thames . Also, cryoturbation caused by ice movement or freeze-thaw activity would be destroyed or damaged by roots. This would significantly reduce the scientific interest. Infill would also constitute a loss of the geological exposure (not a threat at present).

Constraints (basic risk assessment): slopes in the gravel area are not steep or deep (only about 2 to 2.5 m high). The slopes into the Reading Beds clay area are currently too steep to allow easy access for viewing. If general access were to be encouraged (via group visits with an experienced leader obtaining the key and permissions) then a safer access point to the Reading Beds should be arranged. The kilns are in a dangerous condition with loose and collapsing brickwork. A warning sign should be erected if access to the site is encouraged.

Reference list/bibliography:

Barrow, G. 1919. Some future work for the Geologists’ Association. Proceedings of   the Geologists’ Association 30, 1-48.

Bowen, D. Q. et al. 1986. Correlation of Quaternary glaciations in England and Ireland , Scotland and Wales . Quaternary Science Reviews. 5, 299-340.

Bridgland, D. R. (ed.) 1993. The Pleistocene Quaternary of the Thames . Geological Conservation Review Series, Chapman and Hall.

Cheshire , D. A. 1986. The lithology and stratigraphy of the Anglian deposits of the Lea Basin. Unpublished PhD, Hatfield Polytechnic.

Green, C. P. et al. 1980. Volcanic pebbles in Pleistocene gravels of the Thames in Buckinghamshire and Hertfordshire. Geological Magazine, 117, 59-64.

Green, C. P. and McGregor, D. F. 1978. Pleistocene gravel trains of the River Thames . Proceedings of the Geologists’ Association, 89, 143-56.

Hey, R. W. 1965. Highly quartzose pebble gravels in the London Basin . Proceedings of the Geologists’ Association, 76, 403-20.

Hey, R. W. 1980. Equivalents of the Westland Green Gravels in Essex and East Anglia . Proceedings of the Geologists’ Association, 91, 279-90.

Hey, R. W. 1982. Composition of pre-Anglian gravels in Norfolk . Bulletin of the Geological Society of Norfolk , 32, 51-59.

Hey, R. W. 1983. Furneux Pelham. In: Diversion of the Thames (ed. J. Rose). Field guide, Quaternary Research Association, Cambridge , 94-95.

Moffat, A. J. 1980. The Plio-Pleistocene transgression in the northern part of the London Basin - a re-examination. Unpublished PhD thesis, University of   London .

Moffat, A. J. 1986. Quartz signatures in Plio-Pleistocene gravels in the northern part of the London Basin . In: Clast lithological analysis. (ed. D. R. Bridgland) Technical Guide 3. Quaternary Research Association, Cambridge , 117-28.

Moffat, A. J. and Catt, J. A. 1986. A re-examination of the evidence for Plio-Pleistocene marine transgression on the Chiltern Hills , III. Deposits. Earth Surface Processes and Landforms, 11, 233-47.

Whiteman, C. A. 1990. Early and Middle Pleistocene stratigraphy and soils in central Essex , England . Unpublished PhD thesis, University of London .

Recommendations: Liaise with Natural England to find out if the site could be opened up occasionally for organised group visits.