Geology of Hertfordshire
The geology of Hertfordshire describes the rocks of the
The Cretaceous
On the northern boundary and just inside the county, at the foot of the chalk
The lowest layer of the chalk is the Chalk Marl, which, with the Totternhoe Clunch Stone above it, lies at the base of the Chiltern Hills escarpment. This is visible as a terrace projecting north-westwards, near Whipsnade and Ivinghoe.[1]
Above these beds, the Lower and Middle Chalk, without flints, rise up sharply to form the steepest part of the Dunstable Downs, which are the easterly continuation of the Chiltern Hills.[1][2]
Next comes the Chalk Rock, which, being a hard bed, caps the hilltops by Boxmoor, Apsley End and near Baldock. The Upper Chalk slopes southward towards the Paleogene boundary to the south.[1][2]
All the chalk was deposited between 100 million and 66 million years ago when the area was at the bottom of a shallow sea and some distance from the nearest land.[2]
The chalk is often covered by a clay-with-flints deposit, which is formed of the weathered remnants of Cenozoic rocks and chalk.[1][3]
The Cenozoic
The Palaeocene Reading beds consist of mottled and yellow clays and sands, the latter are frequently hardened into masses made up of pebbles in a siliceous cement, known locally as Hertfordshire puddingstone. Examples of Reading Beds outliers occur in what are otherwise chalky areas at St Albans, Ayot Green, Burnham Green, Micklefield Green, Sarratt, and Bedmond. The Reading Beds were laid down about 60 million years ago when the area was a river estuary receiving river sediment from land to the west.[1][2]
The
The Ice Age
About 478,000 to 424,000 years ago during the
Prior to the ice ages the
At the retreat of the glaciers, wind blown powdered rock known as loess was deposited over the whole county, forming thin layers under a metre thick. This reddish clay is easily formed into bricks at the lowish temperatures attainable in a wood-fired kiln and gained the name brickearth. It gave rise to rural brick-making industries scattered throughout the county. It also makes for fine, easily cultivated and fertile soils.[2][3]
See also
- Hertfordshire for a general description of the county.
- Geology of the United Kingdom
- Geology of England
- Woolwich-and-Reading Beds
Notes
- ^ a b c d e f g h i j public domain: Chisholm, Hugh, ed. (1911). "Hertfordshire". Encyclopædia Britannica. Vol. 13 (11th ed.). Cambridge University Press. p. 398. One or more of the preceding sentences incorporates text from a publication now in the
- ^ a b c d e f g h i j Hertfordshire Geological Society
- ^ a b Dacorum Landscape Character Assessment 2002
References
- Hertfordshire at Natural England. Direct.gov.uk, 2008-11-20.
- Hertfordshire Geological Society - Containing maps and cross section.
- "Dacorum Landscape Character Assessment" (PDF). Countryside Commission / English Nature. 2002. Archived from the original (PDF) on 2011-06-08. Retrieved 2009-03-02. Describing landscape and soil types.
- Asymmetrical Valleys of the Chiltern Hills C. D. Ollier and A. J. Thomasson, The Geographical Journal, Vol. 123, No. 1 (March 1957), pp. 71–80 Published by: Blackwell Publishing on behalf of The Royal Geographical Society (with the Institute of British Geographers).
- Hertfordshire RIGS Group, Herts County Council (2003). "A Geological Conservation Strategy for Hertfordshire" (PDF). Archived from the original (PDF) on 2011-09-27. Retrieved 2011-09-18.
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