Hertfordshire is not normally noted for shark-infested waters. But fossil remains have revealed how 50 million years ago the seas covering what became our county were home to ancestors of today's sharks.

This was one among many intriguing insights into the county's geological history provided by chalk micropalaeontologist Haydon Bailey when he gave the online Gerald Salisbury Memorial Lecture on the evening of Wednesday 23 February.

Covering 100 million years in 50 minutes, he focussed initially on the Late Cretaceous period when a series of different of chalk strata were laid down. The lowest chalk layers found in and around Hertfordshire contain fossils of ammonites, bi-valves and sponges that lived in a sea 500 feet deep. This phase was halted by a drastic reduction in water oxygen levels amid global warming that wiped out the marine creatures.

Picture imagines sea life 50 million years ago during formation of the London Basin

As life returned, sea worms and crustaceans were able to burrow their way into the sea floor leaving traces that can still be seen in exposed chalk at sites such as Hill End near Hitchin, Springfield Quarry near Flaunden and the Redbournbury Quarry near St Albans.

About 88 million years ago, the youngest chalk that can still be found in Herts was laid down. Chalk strata continued to form for another 20 million years, but these 'younger' deposits were subsequently eroded.

Jumping ahead to the early Eocene 55 million years ago, Dr Bailey described a period of spectacularly high temperatures when shoreline flint pebbles became cemented into the silcrete rocks known as Hertfordshire Puddingstone.

Photo: Hertfordshire Puddingtone outside Kingsbury Mill, St Albans (©Haydon Bailey)

On a continuing seaside theme, he also highlighted results from cosmogenic dating that point to sands at Little Heath near Berkhamsted being Pliocene beach deposits that date from 2.57 million years ago.  If correct, they suggest that the Chiltern Hills must have risen a remarkable 165 meters since then. 

The final focus of the lecture was on the Anglian Ice Age, a mere 450,000 years ago when local wildlife included well-insulated Mammoths. It was then that the descending ice-sheet diverted the Thames and other rivers into their current courses and a huge lake formed along the Colne Valley. This back-filled with the gravels that are now a characteristic surface feature of the Vale of St Albans.

In conclusion, Dr Bailey semi-seriously considered the consequences for Hertfordshire if global warming returned the planet to temperatures 100 million years ago, when there were no polar ice caps. He predicted that sea levels could rise 100 meters and that the Chilterns would become an archipelago of small islands. On a more positive note there would be no need to commute round the M25 and Fishpool Street in St Albans would live up to its name!

The Gerald Salisbury Memorial Lecture is jointly organised by Hertfordshire Natural History Society and Welwyn Natural History Society. To view a recording of the lecture contact the HNHS Secretary David Utting.