The rocks of the English county of Hertfordshire belong to 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 Tertiary rocks made up of the Paleocene age Reading Beds and Eocene age London Clay that occupies the remaining southern part.

The Cretaceous
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 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.