The Ashdown Formation is a

geological unit A stratigraphic unit is a volume of rock of identifiable origin and relative age range that is defined by the distinctive and dominant, easily mapped and recognizable petrographic, lithologic or paleontologic features ( facies) that characteriz ...

, which forms part of the

Wealden Group The Wealden Group, occasionally also referred to as the Wealden Supergroup, is a group (stratigraphy), group (a sequence of rock strata) in the lithostratigraphy of southern England. The Wealden group consists of wiktionary:paralic, paralic to c ...

and the lowermost and oldest part of the now unofficial

Hastings Beds The Wealden Group, occasionally also referred to as the Wealden Supergroup, is a group (a sequence of rock strata) in the lithostratigraphy of southern England. The Wealden group consists of paralic to continental (freshwater) facies sedimentar ...

. These geological units make up the core of the

Weald The Weald () is an area of South East England between the parallel chalk escarpments of the North and the South Downs. It crosses the counties of Hampshire, Surrey, West Sussex, East Sussex, and Kent. It has three parts, the sandstone "High W ...

in the English counties of

East Sussex East Sussex is a Ceremonial counties of England, ceremonial county in South East England. It is bordered by Kent to the north-east, West Sussex to the west, Surrey to the north-west, and the English Channel to the south. The largest settlement ...

and

Kent Kent is a Ceremonial counties of England, ceremonial county in South East England. It is bordered by Essex across the Thames Estuary to the north, the Strait of Dover to the south-east, East Sussex to the south-west, Surrey to the west, and Gr ...

. The other component formations of the

are the overlying

Wadhurst Clay Formation The Wadhurst Clay Formation is a geological unit which forms part of the Wealden Group and the middle part of the now unofficial Hastings Beds. These geological units make up the core of the geology of the High Weald in the English counties of West ...

and the

Tunbridge Wells Sand Formation The Tunbridge Wells Sand Formation is a geological unit which forms part of the Wealden Group and the uppermost and youngest part of the unofficial Hastings Beds. These geological units make up the core of the geology of the Weald in the English ...

. The

in turn forms part of the

Wealden Supergroup The Wealden Group, occasionally also referred to as the Wealden Supergroup, is a group (a sequence of rock strata) in the lithostratigraphy of southern England. The Wealden group consists of paralic to continental (freshwater) facies sedimentar ...

which underlies much of

South East England South East England is one of the nine official regions of England, regions of England that are in the ITL 1 statistical regions of England, top level category for Statistics, statistical purposes. It consists of the nine counties of england, ...

. The sediments of the

, including the Ashdown Formation, were deposited during the

Early Cretaceous The Early Cretaceous (geochronology, geochronological name) or the Lower Cretaceous (chronostratigraphy, chronostratigraphic name) is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 143.1 ...

Period, which lasted for approximately 40 million years from 140 to 100 million years ago. The Ashdown Formation is of Late

Berriasian In the geological timescale, the Berriasian is an age/ stage of the Early/Lower Cretaceous. It is the oldest subdivision in the entire Cretaceous. It has been taken to span the time between 143.1 ±0.6 Ma and 137.05 ± 0.2 (million years ago) ...

to Early

Valanginian In the geologic timescale, the Valanginian is an age or stage of the Early or Lower Cretaceous. It spans between 137.05 ± 0.2 Ma and 132.6 ± 0.2 Ma (million years ago). The Valanginian Stage succeeds the Berriasian Stage of the Lower Cretac ...

to age.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. The formation takes its name from the

Ashdown Forest Ashdown Forest is an ancient area of open heathland occupying the highest sandy ridge-top of the High Weald National Landscape. It is situated south of London in the county East Sussex, England. Rising to an elevation of above sea level, its ...

in the

High Weald High may refer to: Science and technology * Height * High (atmospheric), a high-pressure area * High (computability), a quality of a Turing degree, in computability theory * High (tectonics), in geology an area where relative tectonic uplift to ...

Sussex Sussex (Help:IPA/English, /ˈsʌsɪks/; from the Old English ''Sūþseaxe''; lit. 'South Saxons'; 'Sussex') is an area within South East England that was historically a kingdom of Sussex, kingdom and, later, a Historic counties of England, ...

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 . 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’. However, it is now considered as one due to the of the clays across the

. 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 The British Geological Survey (BGS) is a partly publicly funded body which aims to advance Earth science, geoscientific knowledge of the United Kingdom landmass and its continental shelf by means of systematic surveying, monitoring and research. ...

. In its entirety the formation is usually found to be between 180 and 215m thick 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. The base of the

and the Ashdown Formation is taken at the top of the Greys Limestones Member of the

Purbeck Group The Purbeck Group is an Upper Jurassic to Lower Cretaceous lithostratigraphy, lithostratigraphic group (stratigraphy), group (a sequence of rock strata) in south-east England. The name is derived from the district known as the Isle of Purbeck in ...

, although this boundary is not currently exposed anywhere in the

. The top of the Ashdown Formation is marked as the top of a massive sandstone bed known as the Top Ashdown Sandstone. This is overlain by an easily identified very coarse grained sandstone known as the Top Ashdown Pebble Bed, which is considered to be part of the overlying

. The Top Ashdown Sandstone is the most persistent marker horizon in

strata throughout the region. It comprises a fine to medium grained sandstone ranging in thickness from 1.2m to 8m. Other marker beds can be found throughout the formation including the Cliff End Sandstone and the Lee Ness Sandstone.Codd, J.W. (2007) ''Analysis of the distribution and characteristics of landslips in the Weald of East Sussex''. MSc dissertation, University of Brighton.

Hastings to Cliff End

The Ashdown Beds are best exposed in the 8 km cliff section between

Hastings Hastings ( ) is a seaside town and Borough status in the United Kingdom, borough in East Sussex on the south coast of England, east of Lewes and south east of London. The town gives its name to the Battle of Hastings, which took place to th ...

and

Pett Pett is a village and civil parish in the Rother district of East Sussex, England. The village is located north-east of Hastings on the edge of Pett Level, the one-time marshes stretching along the coast of Rye Bay. The road through the vill ...

Level. Part of this section has been designated a

Site of Special Scientific Interest A Site of Special Scientific Interest (SSSI) in Great Britain, or an Area of Special Scientific Interest (ASSI) in the Isle of Man and Northern Ireland, is a conservation designation denoting a protected area in the United Kingdom and Isle ...

, cited by

Natural England Natural England is a non-departmental public body in the United Kingdom sponsored by the Department for Environment, Food and Rural Affairs. It is responsible for ensuring that England's natural environment, including its land, flora and fauna, ...

because of its geological importance. The cliffs between

and

Level are difficult to get to safely because of the tidal range of the

English Channel The English Channel, also known as the Channel, is an arm of the Atlantic Ocean that separates Southern England from northern France. It links to the southern part of the North Sea by the Strait of Dover at its northeastern end. It is the busi ...

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

Wadhurst Clay The Wadhurst Clay Formation is a geological unit which forms part of the Wealden Group and the middle part of the now unofficial Hastings Beds. These geological units make up the core of the geology of the High Weald in the English counties of We ...

Hastings Castle Hastings Castle is a keep and bailey castle ruin situated in the town of Hastings, East Sussex. It overlooks the English Channel, into which large parts of the castle have fallen over the years. History Immediately after landing in England i ...

to the west and Cliff End to the east. The mottled heavily degraded silty clays of the former ‘Fairlight Clays’ can be easily distinguished against the well bedded sandstones and interbedded siltstones of the ‘Ashdown Sands’. The Lee Ness Ledge is known for the prominent Lee Ness Sandstone and its many well preserved fossilised dinosaur footprints, particularly

Iguanodon ''Iguanodon'' ( ; meaning 'iguana-tooth'), named in 1825, is a genus of iguanodontian dinosaur. While many species found worldwide have been classified in the genus ''Iguanodon'', dating from the Late Jurassic to Early Cretaceous, Taxonomy (bi ...

Engineering geology

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.Robinson, D.A. & Williams, R.B.G. (1984) Classic Landforms of the Weald. The Geographical Association, Sheffield. 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.

References

{{Reflist

External links

British Geological Survey lexicon
Lithostratigraphy of England Lower Cretaceous Series of Europe Valanginian Stage Berriasian Stage Ashdown Forest

Lithology

Hastings to Cliff End

Engineering geology

See also

References

External links