Ashdown Formation
Ashdown Formation | |
---|---|
Stratigraphic range: | |
An exposure of the Ashdown Formation on the East Sussex coast. | |
Type | Geological formation |
Unit of | Wealden Group |
Sub-units | Fairlight Clays |
Underlies | Wadhurst Clay Formation |
Overlies | Durlston Formation |
Thickness | typically in the range 200-230 m |
Lithology | |
Primary | Siltstone, Sandstone |
Other | Mudstone, Clay |
Location | |
Country | ![]() |
Extent | Weald Basin |
Type section | |
Named for | Ashdown Forest |
The Ashdown Formation is a geological unit, which forms part of the
The other component formations of the
Lithology
The Ashdown Formation typically comprises sandstones, siltstones and mudstones. In the east of the county, the formation tends to be more argillaceous in its lowermost part and fines up to a sandier division in the uppermost 30 to 50m. The clays are identified by their characteristic purple and brick-red mottled nature. In early references, these variations give rise to the division of the formation into the ‘Fairlight Clays’ and the ‘Ashdown Sands’.[2] However, it is now considered as one due to the impersistence of the clays across the Weald. Despite this the variations of clays and sands in the formation are usually marked separately on the maps and records of the British Geological Survey. In its entirety the formation is usually found to be between 180 and 215m thick [3]
The base of the
Hastings to Cliff End
The Ashdown Beds are best exposed in the 8 km cliff section between Hastings and Pett Level. Part of this section has been designated a Site of Special Scientific Interest, cited by Natural England because of its geological importance.[6] The cliffs between Hastings and Pett Level are difficult to get to safely because of the tidal range of the English Channel and the unstable cliffs.
At this location the formation can be followed from the axis of the Wealden Anticline at Lee Ness Ledge through the well distinguished marker beds and horizons to its juncture with the
The Lee Ness Ledge is known for the prominent Lee Ness Sandstone and its many well preserved fossilised dinosaur footprints, particularly Iguanodon.[7]
Engineering geology
![](http://upload.wikimedia.org/wikipedia/commons/thumb/2/2e/Fairlight_landslide.jpg/220px-Fairlight_landslide.jpg)
The Hastings to Pett Level section of the coast has suffered a number of significant recent and historic landslips, dating back to the 18th century. These can be seen in and around Covehurst Wood and the Fairlight, Ecclesbourne and Warren Glens.[8] This has been attributed to lithological variations of the Ashdown Beds and also to the effects of wave action at the base of the cliffs. At high tide, waves cut into the lowermost, argillaceous part of the cliffs and undercut the overlying sandstones and siltstones resulting in toppling and rock falls. Where sandstones are interbedded with silts and clays, this may result in single and multiple rotational type landslides as well as block slides and mud flows. In theory, the remains of previous landslips should then protect the toe of the cliff from further weathering. However, these blocks and sediments are transported East with shingle and other beach deposits by long-shore drift, leaving the cliffs vulnerable to wave action and susceptible to further landslides.[5]
See also
References
- ^ Hopson, P.M., Wilkinson, I.P. and Woods, M.A. (2010) A stratigraphical framework for the Lower Cretaceous of England. Research Report RR/08/03. British Geological Survey, Keyworth.
- ^ Ruffell, A., Ross, A. & Taylor (1996) Early Cretaceous Environments of the Weald. Geologists’s Association Guide No. 55, Geologists’s Association, London.
- ^ a b Lake, R.D. & Shepard-Thorn, E.R. (1987) Geology of the country around Hastings and Dungeness: Memoir for 1:50,000 geological sheets 320 and 321. British Geological Survey, London.
- ^ Allen, P. (1948) Wealden Petrology: The Top Ashdown Pebble Bed and the Top Ashdown Sandstone. Quarterly Journal of the Geological Society, 104, 257-321.
- ^ a b Codd, J.W. (2007) Analysis of the distribution and characteristics of landslips in the Weald of East Sussex. MSc dissertation, University of Brighton.
- ^ "Pett Level (SSSI)". Natural England. Archived from the original on 3 November 2012. Retrieved 2 March 2012.
- ^ Fitch, Rachel (26 September 2006). "Jamies monster find sheds light on history". The Argus. Retrieved 2 March 2012.
- ^ Robinson, D.A. & Williams, R.B.G. (1984) Classic Landforms of the Weald. The Geographical Association, Sheffield.