The Cretaceous
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 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.
Read more about this topic: Geology Of Hertfordshire