The geology of
Hertfordshire describes the rocks of the English county
Hertfordshire which are a northern part of the great shallow
syncline known 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,
Cenozoic rocks made up of the
Paleocene age Reading beds and
London Clay that occupies the remaining southern
1 The Cretaceous
2 The Cenozoic
2.1 The Ice Age
3 See also
Chalk Pit, a nature reserve in Barkway in Hertfordshire
managed by the Herts and Middlesex Wildlife Trust. The exposed chalk
is a block displaced by an ice sheet which once covered the area
On the northern boundary and just inside the county, at the foot of
the chalk Chiltern Hills, near
Tring and Ashwell, there is a small
strip of exposed
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 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
Whipsnade and Ivinghoe.
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
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 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, 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.
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
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 as it flowed
About 478,000 to 424,000 years ago during the ice age period known as
the Anglian Stage, glaciers approached from the
North Sea and 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
Prior to the ice ages the
River Thames followed a path through the
southern part of Hertfordshire, running from the area of modern
Staines up the valley of the Colne to Hatfield and then eastward
across Essex originally towards the primeval
Rhine but 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
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.
Hertfordshire for a general description of the county.
Geology of the United Kingdom
Geology of England
^ a b c d e f g h i j Chisholm, Hugh, ed. (1911).
"Hertfordshire". Encyclopædia Britannica. 13 (11th ed.). Cambridge
University Press. p. 398.
^ a b c d e f g h i j
Hertfordshire Geological Society
^ a b Dacorum Landscape Character Assessment 2002
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
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).
Geology of England
Isle of Wight
Tyne and Wear
West Midlands (County)