Geology of Hertfordshire
Encyclopedia
The Geology of Hertfordshire describes the rocks of the English
county of 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
chalk
s, which are exposed as the high ground in the north and west of the county, and the Tertiary
rocks made up of the Paleocene
age Reading beds and Eocene
age London Clay
that occupies the remaining southern part.
, near Tring
and Ashwell
, there is a small strip of exposed Cretaceous 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 north-westwards, near 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 Chalk slopes southward towards the Tertiary
boundary 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 estuary
receiving river sediment
from land to the west.
The London Clay
is a stiff, blue clay that weather
s 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.
period known as the Anglian Stage, glacier
s 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 glaciation.
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 meter thick. This makes for fine, easily cultivated and fertile soils.
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
county of Hertfordshire
Hertfordshire
Hertfordshire is a ceremonial and non-metropolitan county in the East region of England. The county town is Hertford.The county is one of the Home Counties and lies inland, bordered by Greater London , Buckinghamshire , Bedfordshire , Cambridgeshire and...
which are a northern part of the great shallow syncline
Syncline
In structural geology, a syncline is a fold, with younger layers closer to the center of the structure. A synclinorium is a large syncline with superimposed smaller folds. Synclines are typically a downward fold, termed a synformal syncline In structural geology, a syncline is a fold, with younger...
known as the London Basin
London Basin
The London Basin is an elongated, roughly triangular sedimentary basin approximately long which underlies London and a large area of south east England, south eastern East Anglia and the adjacent North Sea...
. The beds dip in a south-easterly direction towards the syncline's lowest point roughly under the River Thames
River Thames
The River Thames flows through southern England. It is the longest river entirely in England and the second longest in the United Kingdom. While it is best known because its lower reaches flow through central London, the river flows alongside several other towns and cities, including Oxford,...
. The most important formations are the Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
chalk
Chalk
Chalk is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. Calcite is calcium carbonate or CaCO3. It forms under reasonably deep marine conditions from the gradual accumulation of minute calcite plates shed from micro-organisms called coccolithophores....
s, which are exposed as the high ground in the north and west of the county, and the Tertiary
Tertiary
The Tertiary is a deprecated term for a geologic period 65 million to 2.6 million years ago. The Tertiary covered the time span between the superseded Secondary period and the Quaternary...
rocks made up of the Paleocene
Paleocene
The Paleocene or Palaeocene, the "early recent", is a geologic epoch that lasted from about . It is the first epoch of the Palaeogene Period in the modern Cenozoic Era...
age Reading beds and Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
age London Clay
London Clay
The London Clay Formation is a marine geological formation of Ypresian age which crops out in the southeast of England. The London Clay is well known for the fossils it contains. The fossils from the Lower Eocene indicate a moderately warm climate, the flora being tropical or subtropical...
that occupies the remaining southern part.
The Cretaceous
On the northern boundary and just inside the county, at the foot of the chalk Chiltern HillsChiltern Hills
The Chiltern Hills form a chalk escarpment in South East England. They are known locally as "the Chilterns". A large portion of the hills was designated officially as an Area of Outstanding Natural Beauty in 1965.-Location:...
, near Tring
Tring
Tring is a small market town and also a civil parish in the Chiltern Hills in Hertfordshire, England. Situated north-west of London and linked to London by the old Roman road of Akeman Street, by the modern A41, by the Grand Union Canal and by rail lines to Euston Station, Tring is now largely a...
and Ashwell
Ashwell, Hertfordshire
Ashwell is a village and civil parish situated about four miles north of Baldock in Hertfordshire.It has a wealth of architecture spanning several centuries. The dates almost entirely from the 14th century and is renowned for its ornate church tower which stands at , and is crowned by an...
, there is a small strip of exposed Cretaceous Gault Clay
Gault Clay
Gault is a clay formation of stiff blue clay deposited in a calm, fairly deep water marine environment during the Lower Cretaceous Period...
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
Totternhoe Stone
thumb|250 px|right|The characteristic checkerboard design of many Bedfordshire churches built with alternating chalk and flint blocks: [[St Mary's ]]....
above it, lies at the base of the Chiltern Hills escarpment. This is visible as a terrace projecting north-westwards, near Whipsnade
Whipsnade
Whipsnade is a small village and civil parish in the county of Bedfordshire. It lies on the eastward tail spurs of the Chiltern Hills, about 2.5 miles South-South-West of Dunstable...
and Ivinghoe
Ivinghoe
Ivinghoe is a village and civil parish within Aylesbury Vale district in Buckinghamshire, England, close to the border with Hertfordshire and Bedfordshire. It is four miles north of Tring and six miles south of Leighton Buzzard, close to the village of Pitstone.The village name is Anglo-Saxon in...
.
Above these beds, the Lower and Middle Chalk, without flints, rise up sharply to form the steepest part of the Dunstable Downs
Dunstable Downs
Dunstable Downs are part of the Chiltern Hills, in southern Bedfordshire in England. They are a chalk escarpment forming the north-eastern reaches of the Chilterns...
, which are the easterly continuation of the Chiltern Hills.
Next comes the Chalk Rock, which, being a hard bed, caps the hilltops by Boxmoor
Boxmoor
Boxmoor, or Boxmoor Village, is a district of Dacorum in Hertfordshire, England. It is now part of Hemel Hempstead. It is a district of mainly nineteenth century housing and meadowland, repeatedly cut through by transport links from London to the The Midlands....
, Apsley End
Apsley
Apsley is a 19th century mill town in the county of Hertfordshire, England. It is a historic industrial site situated in a valley of the Chiltern Hills. It is positioned below the confluence of two permanent rivers, the Gade and Bulbourne. In an area of little surface water this was an obvious site...
and near Baldock
Baldock
Baldock is a historic market town in the local government district of North Hertfordshire in the ceremonial county of Hertfordshire, England where the River Ivel rises. It lies north of London, southeast of Bedford, and north northwest of the county town of Hertford...
. The Upper Chalk slopes southward towards the Tertiary
Tertiary
The Tertiary is a deprecated term for a geologic period 65 million to 2.6 million years ago. The Tertiary covered the time span between the superseded Secondary period and the Quaternary...
boundary 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 Tertiary
Hertfordshire puddingstone
Hertfordshire puddingstone is a conglomerate sedimentary rock composed of rounded flint pebbles cemented together by a younger matrix of silica quartz. The distinctive rock is largely confined to the English county of Hertfordshire but small amounts occur throughout the London Basin. Despite a...
. Examples of Reading Beds outliers occur in what are otherwise chalky areas at St Albans
St Albans
St Albans is a city in southern Hertfordshire, England, around north of central London, which forms the main urban area of the City and District of St Albans. It is a historic market town, and is now a sought-after dormitory town within the London commuter belt...
, Ayot Green
Ayot Green
Ayot Green is a small village in Hertfordshire, England and is near the A1 Motorway, close to Welwyn Garden City. It is a typical traditional English village, centred around a village green...
, Burnham Green, Micklefield Green, Sarrat, and Bedmond
Bedmond
Bedmond is a village in Hertfordshire, England.Bedmond is the birthplace of Nicholas Breakspear, the only Englishman to ever be Pope. Nicholas Breakspear was born at Bedmond Farm around 1100 AD, and the site where his home stood is marked by a plaque....
. The Reading Beds were laid down about 60 million years ago when the area was a river estuary
Estuary
An estuary is a partly enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea....
receiving river sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
from land to the west.
The London Clay
London Clay
The London Clay Formation is a marine geological formation of Ypresian age which crops out in the southeast of England. The London Clay is well known for the fossils it contains. The fossils from the Lower Eocene indicate a moderately warm climate, the flora being tropical or subtropical...
is a stiff, blue clay that weather
Weathering
Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...
s 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 ageIce age
An ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...
period known as the Anglian Stage, glacier
Glacier
A glacier is a large persistent body of ice that forms where the accumulation of snow exceeds its ablation over many years, often centuries. At least 0.1 km² in area and 50 m thick, but often much larger, a glacier slowly deforms and flows due to stresses induced by its weight...
s approached from the North Sea
North Sea
In the southwest, beyond the Straits of Dover, the North Sea becomes the English Channel connecting to the Atlantic Ocean. In the east, it connects to the Baltic Sea via the Skagerrak and Kattegat, narrow straits that separate Denmark from Norway and Sweden respectively...
and reached as far south-west as Bricket Wood
Bricket Wood
Bricket Wood is a village in the county of Hertfordshire, England, approximately 4½ miles from St Albans. It is part of the parish of St Stephen. Its railway station is served by a London Midland service that runs between St Albans Abbey and Watford Junction stations.Close to the village stands...
. 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
Reed, Hertfordshire
Reed is a small village and civil parish in North Hertfordshire. It is situated on a chalk ridge at almost the highest point in the county, approximately 3 miles south of the market town of Royston. The Prime Meridian passes just to the east of the village, between Reed and Barkway...
and Barley
Barley, Hertfordshire
Barley is a village and civil parish in the district of North Hertfordshire, England. According to the 2001 census, it has a population of 659. The place-name refers to a lea or meadow and not to the grain-producing plant...
by glaciation.
Prior to the ice ages the River Thames
River Thames
The River Thames flows through southern England. It is the longest river entirely in England and the second longest in the United Kingdom. While it is best known because its lower reaches flow through central London, the river flows alongside several other towns and cities, including Oxford,...
followed a path through the southern part of Hertfordshire, running from the area of modern Staines
Staines
Staines is a Thames-side town in the Spelthorne borough of Surrey and Greater London Urban Area, as well as the London Commuter Belt of South East England. It is a suburban development within the western bounds of the M25 motorway and located 17 miles west south-west of Charing Cross in...
up the valley of the Colne to Hatfield
Hatfield, Hertfordshire
Hatfield is a town and civil parish in Hertfordshire, England in the borough of Welwyn Hatfield. It has a population of 29,616, and is of Saxon origin. Hatfield House, the home of the Marquess of Salisbury, is the nucleus of the old town...
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
London
London is the capital city of :England and the :United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its...
. 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
Loess
Loess is an aeolian sediment formed by the accumulation of wind-blown silt, typically in the 20–50 micrometre size range, twenty percent or less clay and the balance equal parts sand and silt that are loosely cemented by calcium carbonate...
was deposited over the whole county, forming thin layers under a meter thick. This makes for fine, easily cultivated and fertile soils.
See also
- HertfordshireHertfordshireHertfordshire is a ceremonial and non-metropolitan county in the East region of England. The county town is Hertford.The county is one of the Home Counties and lies inland, bordered by Greater London , Buckinghamshire , Bedfordshire , Cambridgeshire and...
for a general description of the county. - Geology of the United Kingdom
- Geology of EnglandGeology of EnglandThe Geology of England is mainly sedimentary. The youngest rocks are in the south east around London, progressing in age in a north westerly direction. The Tees-Exe line marks the division between younger, softer and low-lying rocks in the south east and older, harder, and generally a higher relief...
- Woolwich-and-Reading Beds