page_title: Geology of Hertfordshire
section_title: The Cretaceous
hierarchical_section_title: Geology of Hertfordshire / The Cretaceous
caption_attribution_description: English: Barkway Chalk Pit, a nature reserve in Barkway in Hertfordshire managed by the Herts and Middlesex Wildlife Trust. The exposed chalk is a result of the ice sheet which once covered the area
context_page_description: 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 chalks, which are exposed as the high ground in the north and west of the county, and the Cenozoic rocks made up of the Paleocene age Reading beds and Eocene age London Clay that occupies the remaining southern part.
context_section_description: 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 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 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.
caption: Barkway 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
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