- Geology of Hertfordshire
The rocks of the English county of Hertfordshire belong to the great shallow
synclineknown as the 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 chalks, which are exposed as the high ground in the north and west of the county, and the Tertiaryrocks made up of the Paleoceneage Reading Beds and Eoceneage London Claythat occupies the remaining southern part.
On the northern boundary and just inside the county, at the foot of the chalk
Chiltern Hills, near Tringand Ashwell, there is a small strip of exposed Cretaceous Gault Clayand 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 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
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.
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 Tertiaryboundary to the south.
All the chalk was deposited between 100 million and 65 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 Tertiary rocks and chalk.
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, Sarrat, and Bedmond. The Reading Beds were laid down about 60 million years ago when the area was a river estuaryreceiving river sedimentfrom land to the west.
The 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 was once again under a deeper sea but was near enough to land to receive fine mud deposits.
The Ice Age
About 478,000 to 424,000 years ago during the
ice ageperiod known as the Anglian Stage, glaciers approached from the North Seaand reached as far south-west as 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 itself has been disturbed at Reed and Barley by glaciation.
Prior to the ice ages the
River Thamesfollowed a path through the southern part of Hertfordshire, running from the area of modern Stainesup the valley of the Colne to Hatfield and then eastward across Essex originally towards the primeval Rhinebut later down the valley of the modern 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 to cut the path of the modern Thames through central 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
loesswas deposited over the whole county, forming thin layers under a meter thick. This makes for fine, easily cultivated and fertile soils.
Hertfordshirefor a general description of the county.
Geology of the United Kingdom
Geology of England
* [http://www.hertsgeolsoc.ology.org.uk/IntroToHertsGeology.htm Hertfordshire Geological Society] - Containing maps and cross section.
* [http://enquire.hertscc.gov.uk/landscsh/features.htm Hertfordshire County Council landscape description] - Describing landscape and soil types.
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