The earth beneath our feet
- Credit: Archant
Hertfordshire’s landscape and clues in its old buildings tell the story of millenia of climate change, which altered the earth beneath our feet and sculpted the county. Liz Hamilton of the Campaign to Protect Rural England Hertfordshire group looks back in time
St Albans-upon-Thames seems an unlikely prospect. Yet an ancient course of the river - dubbed by geologists the ‘proto-Thames’ - once ran through Hertfordshire. The valley it created (except its far eastern end) is called the Vale of St Albans. Gravels deposited in the county by this river include pieces of rock from north Wales, supporting the view that the ancient river gathered its headwaters from a much more extensive area than the modern Thames.
The proto-Thames left its present-day course at Goring in Oxfordshire and flowed past the sites of modern Hertfordshire settlements including Watford, Stanborough, Brickendonbury, Bengeo and Eastwick. Then it crossed into what is now the county of Essex between Sawbridgeworth and Harlow, eventually reaching the North Sea. You can trace the sides of its valley through parts of the county: look out for the especially distinct low scarp between Radlett and North Mymms. The river’s diversion to its present course through London arose from climate change and a giant ice sheet.
Tropical to icicle
Fossils found in the locality dating from around two and a half million years ago include those from an ancient species of cheetah, as well as porcupine and tapir, all species which would have inhabited tropical forest. Soon after this date, the climate began to cool across much of the globe. This was the start of what is known as the Ice Age, in reality a series of alternating cold and warm periods. This pattern of climate has been attributed to wobbles in the earth’s orbit around the sun, changes in the positions of the continents and several other contributory causes. During the coldest periods, temperatures in Britain were certainly below freezing for many months, while the warm or interglacial periods were as warm or warmer (up to four degrees C) than Britain today.
An especially long and severe cold period, known as the Anglian, was at its height around 450,000 years ago when an ice sheet reached from Scandinavia into Hertfordshire and beyond. It was perhaps 250 metres thick where it flowed over and partially eroded the chalk scarp in the north of the county. To the south-west of Ware, ice entered the proto-Thames valley, blocking the flow of the river to create a large lake. Another tongue of ice flowed down the line of the present-day Lee Valley and south-west to Finchley. Ice also flowed into the modern Hitchin-Stevenage gap, into Lilley Bottom and along the base of the Chilterns towards Tring, but never topped the Chilterns west of Luton. Ice also flowed as far as Hornchurch in Essex, thought to be the furthest south reached by the ice sheet in eastern England.
Later, the ice blocking the proto-Thames valley moved further to the south-west, and a second lake formed at what is now Moor Mill near Colney Street. This lake overflowed to the south and effected the diversion of the proto-Thames to a temporary course through Uxbridge. Later, the river became diverted again to its present course. In a small final advance, the ice reached Bricket Wood Common, the south-westerly limit of the ice sheet in Hertfordshire.
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Moving ice picks up material in its path, which is then deposited as the ice slows and melts. A large chunk of north-east Hertfordshire is covered in this material, once called boulder clay but now generally referred to as ‘till’, a mixture of clay and larger rocky material. Meltwater flowing under and out from the ice also laid down sands and gravels beyond its edges.
Pieces of rock carried by the ice (known as erratics) are common in Hertfordshire. Some originated as far away as Norway and Scotland. The famous Royse Stone, now in the centre of Royston, is a large piece of millstone grit from the Pennines of Yorkshire or Derbyshire. Smaller erratics were incorporated into church buildings. At All Saints in Sandon (meaning sandy hill) there are pieces of brown sandstone from the East Midlands, and at St Mary’s in Great Wymondley the Norman apse is built with neat rows of small rounded stones – quartzite from the West Midlands. These might have been carried by river action towards the east before being picked up by a glacier flowing south-west from Scandinavia.
Many of the sands and gravels left by both the proto-Thames and the glacial meltwater streams have proved to be extremely valuable sources of aggregate for the construction industries and quarried in great quantities in many places in the county.
After the Anglian ice disappeared, there were more periods of intense cold in Europe, the most recent being 20,000 years ago when ice covered much of Britain but did not reach Hertfordshire. Nonetheless, the cold created permafrost conditions in the county and periods when the upper ground layers thawed and became saturated with water. On hillsides, this thawed material often slid down-wards. The steep-sided valleys, or ‘coombes’, along the edge of the Chilterns perhaps owe their origin to this process.
In this period, vegetation was often sparse or absent, leaving bare expanses of ground. With nothing holding the topsoil, wind picked up fine clays and silts, later dropping them to form deposits of ‘loess’ or brickearth, which in places in Hertfordshire are up to six metres deep.
The modern era
Around 12,000 years ago, the climate warmed very quickly, some sources say by as much as 10 degrees C in 100 years. Since then, there have been further fluctuations in temperature. Following a sudden cooling around 11,000 years ago, when summers were on average eight degrees C colder than today, glaciers returned briefly to Britain. Around 6,000 years ago, average temperatures in Britain were perhaps as much as three degrees C higher than today: this period until recently was regarded as the ‘climatic optimum’ of the current interglacial, which was followed by a trend towards lower temperatures. However, now that man-made climate warming is widely acknowledged to be taking place, the future is uncertain. Will permafrost or even an ice sheet once again come to our county? Or has human impact broken the cycle of warm and cold phases established more than two million years ago?