Hertfordshire Puddingstone is a ‘conglomerate’ of rounded flint pebbles in a matrix of fine sand all cemented together by silica. Despite its name it is also found in other parts of south east England and northern France. It was so named because it resembles an old fashioned plum pudding.
In the late Cretaceous 75 million years ago (near the end of the dinosaur age), world temperatures and sea levels were 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 (coccoliths). Later chemical reactions caused silica to dissolve, and re-precipitate as flints forming hard irregular lumps in the Chalk.
About 65 million years ago sea level fell and this area became dry land. Rivers washed away the soft chalk, leaving the hard flints on the surface. The sea re-advanced about 56 million years ago and flints were rounded by waves on a beach or gravel bar. At this stage the pebbles were black or grey, as on modern flint beaches. Over the next million or so years changing sea level alternately exposed and submerged this area.
The attractive colours of the pebbles are probably due to staining by iron compounds on the surface of the land in a sub tropical climate. The pebbles were mixed with fine white sand, probably on a beach. Silica in groundwater cemented patches of the pebbles and sand into hard coherent rock. As the pebbles, sand and 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 believe in East Herts puddingstone formed in the Lambeth Group of sediments, probably in a pebble bed which was deposited on top of the Upnor Formation and patchily cemented in early Reading Formation times about 55.8 million years ago. In places downcutting by the Reading Formation rivers deposited pebbles and puddingstones in channels within the Upnor Formation.
|(a) The lower Lambeth Group, with probable position of puddingstones.||(b) Why puddingstones are seldom found in situ.|