Hertfordshire Puddingstone was so named because it resembles an old fashioned plum pudding.Geologically it is a conglomerate with well rounded flint pebbles in a matrix of fine pale sand all cemented together by silica. Most of the pebbles have black rims and are coloured ochre and/or red.
In the late Cretaceous 75 to 65 Million years ago (towards the end of the dinosaur age), world temperatures and sea levels were exceptionally high, and most of the British Isles was beneath a warm clear sea. Chalk formed on the sea floor from the shells of microscopic organisms called coccoliths. Later dissolved silica chemically formed into flints as hard irregular lumps in the Chalk.
About 65 million years ago world sea levels fell and South East England was lifted above sea level.
Rivers washed away the soft chalk, leaving the hard flints. Then about 60 million years ago the sea re- advanced and the flints were rounded into pebbles by wave action on the beaches of a fluctuating coastline.
At this stage the pebbles were black or grey, as on modern flint beaches.
The pebbles were stained by iron compounds giving them their distinctive black rims and attractive internal colours, and then they were embedded in fine pale sand. Later natural silica cement bound the pebbles and sand into hard coherent rock. As the pebbles, the fine sand and the cement are all made of silica this makes the rock hard and coherent and it breaks straight across matrix and pebbles, unlike concrete with flints, which breaks unevenly.
We are not sure where or when Hertfordshire Puddingstone formed, as it is elusive in situ, and usually found where it has been deposited after erosion and transport by rivers or glaciers.
We believe it is in the Lambeth Group (formally Woolwich and Reading beds) between the chalk and the London clay. A large volume of the rock recovered during construction of the A10 north of Ware suggests it was the fluvial Reading formation, which is above, and in places cuts down into the marine Upnor formation, which would date it to about 56 million years ago.
The position and beds of the Lambeth group in Hertfordshire
