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The GEOLOGY OF HERTFORDSHIRE describes the rocks of the English county of Hertfordshire
Hertfordshire
which are a northern part of the great shallow syncline known as the London Basin
London Basin
. The beds dip in a south-easterly direction towards the syncline's lowest point roughly under the River Thames . The most important formations are the Cretaceous
Cretaceous
chalks , which are exposed as the high ground in the north and west of the county, and the Cenozoic
Cenozoic
rocks made up of the Paleocene
Paleocene
age Reading beds and Eocene
Eocene
age London Clay
London Clay
that occupies the remaining southern part.

CONTENTS

* 1 The Cretaceous
Cretaceous

* 2 The Cenozoic
Cenozoic

* 2.1 The Ice Age

* 3 See also * 4 Notes * 5 References

THE CRETACEOUS

On the northern boundary and just inside the county, at the foot of the chalk Chiltern Hills
Chiltern Hills
, near Tring
Tring
and Ashwell , there is a small strip of exposed Cretaceous
Cretaceous
Gault Clay
Gault Clay
and Upper Greensand . At 100 million years old, these are the oldest rocks in the county. Rocks get progressively younger as one moves in a south easterly direction through the county.

The lowest layer of the chalk is the Chalk
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
Whipsnade
and Ivinghoe
Ivinghoe
.

Above these beds, the Lower and Middle Chalk, without flints, rise up sharply to form the steepest part of the Dunstable Downs
Dunstable Downs
, which are the easterly continuation of the Chiltern Hills.

Next comes the Chalk
Chalk
Rock, which, being a hard bed, caps the hilltops by Boxmoor , Apsley End and near Baldock
Baldock
. The Upper Chalk
Chalk
slopes southward towards the Paleogene boundary to the south.

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.

The chalk is often covered by a clay-with-flints deposit, which is formed of the weathered remnants of Cenozoic
Cenozoic
rocks and chalk.

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
St Albans
, Ayot Green
Ayot Green
, Burnham Green , Micklefield Green , Sarratt
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.

The London Clay
London Clay
is a stiff, blue clay that weathers to brown and rests nearly everywhere upon the Reading beds . It represents the time 55 to 40 million years ago when Hertfordshire
Hertfordshire
was once again under a deeper sea but was near enough to land to receive fine mud deposits.

THE ICE AGE

Tyttenhanger: Gravel pit workings. Flooded gravel pits to the left with a conveyor belt bringing material into a central batching plant from more distant active workings, the River Colne to the right. The gravel was deposited here by the primordial River Thames
River Thames
as it flowed through Hertfordshire.

About 478,000 to 424,000 years ago during the ice age period known as the Anglian Stage , glaciers approached from the North Sea
North Sea
and reached as far south-west as Bricket Wood
Bricket Wood
. Glacial gravels and boulder clays cover a great deal of the whole area to the north east of the county and the Upper Chalk
Chalk
itself has been disturbed at Reed and Barley by glaciation.

Prior to the ice ages the River Thames
River Thames
followed a path through the southern part of Hertfordshire, running from the area of modern Staines
Staines
up the valley of the Colne to Hatfield and then eastward across Essex originally towards the primeval Rhine
Rhine
but later down the valley of the modern River Lea
River Lea
. This path was blocked by a mass of ice near Hatfield and a lake ponded up to the west of this around St Albans. Waters eventually overflowed near Staines
Staines
to cut the path of the modern Thames through central London
London
. When the ice retreated about 400,000 years ago the river bed along the new route followed the lower path and so the river remained on its present-day course. The flow in the Colne valley reversed, now flowing south as a tributary into the modern Thames. Superficial gravel deposits from the primordial Thames, are found throughout the Vale of St. Albans.

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.

SEE ALSO

* Hertfordshire
Hertfordshire
for a general description of the county. * Geology of the United Kingdom
Geology of the United Kingdom
* Geology of England
England
* Woolwich-and-Reading Beds

NOTES

* ^ A B C D E F G H I J 1911 edition of Encyclopædia Britannica, Hertfordshire
Hertfordshire
entry * ^ A B C D E F G H I J Hertfordshire
Hertfordshire
Geological Society * ^ A B Dacorum Landscape Character Assessment 2002

REFERENCES

* Hertfordshire
Hertfordshire
at Natural England
England
. Direct.gov.uk, 2008-11-20. * Hertfordshire
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
Chiltern Hills
C. D. Ollier and A. J. Thomasson, The Geographical Journal, Vol. 123, No. 1 (Mar., 1957), pp. 71–80 Published by: Blackwell Publishing on behalf of The Royal Geographical Society (with the Institute of British Geographers * Hertfordshire
Hertfordshire
RIGS Group, Herts County Council (2003). "A Geological Conservation Strategy for Hertfordshire" (PDF). * This article incorporates text from a publication now in the public domain : Chisholm, Hugh, ed. (1911). "article name needed". Encyclopædia Britannica (11th ed.). Cambridge University Press.

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