Geology

The geology behind chalk streams

Jane Tubb – Chair, East Herts Geology Club

In their natural state chalk streams are pure because they carry little or no sediment, and their water is  filtered as it percolates down through chalk hills; they are rare because chalk hills are rare.  

Chalk is a very fine grained limestone (the mineral calcium carbonate) formed between about 100 and 80 million years ago in the Cretaceous period when dinosaurs roamed the Earth. There was less exposed land  than now because sea levels were high during a time of extreme global warming sometimes referred to as The Cretaceous Greenhouse. With much of the Earth’s surface underwater there was little sediment being washed into the seas and life flourished in the warm clear waters.

Some chalk is hard enough to use as a building stone, but most is soft and powders easily. However there is more to the powder than meets the eye, most grains are fossils of minute algae called coccolithophores  which live in large numbers in surface waters. They form calcium  carbonate plates called coccoliths arranged around them in a cocosphere. The plates are renewed during life and most separate on death.  

Scanning electron micrograph  of a cocosphere. Diameter  about 0.016 of a millimetre

In the Cretaceous Greenhouse a constant supply of these plates sank to the sea  floor and became compacted forming deposits up to 400 meters thick.  After the Cretaceous when global temperatures and sea level fell, the  accumulated coccoliths were exposed on land as chalk.

Rain water is naturally slightly acidic, and more so when falling through pollution. Chalk dissolves in  acid and is easily eroded exposing older rocks, in other places it has been covered by younger rocks. Once  widespread over the Earth’s surface, most chalk has now been removed or buried. Exposures survive in  England, from the Wiltshire Downs to the white cliffs of Dover, to the Yorkshire coast; in Northern  Ireland, and in some other countries including Russia, the American Midwest and Australia.

Simplified geological map showing  where chalk is exposed at or near the  
surface in southern England. Hertfordshire is outlined in red (in most of the county chalk is covered by more recent sands, gravels and clays)

Chalk is porous (it can hold water in the pore spaces between grains) but not very permeable (water does  not move easily between pores). However fissures and cavities in chalk hills caused by natural jointing  and dissolution by rainwater have increased the permeability allowing water to percolate downwards to  the aquifer (the water saturated chalk) from where water emerges through fissures as springs or seeps into the soil, and flows down to streams in the valleys. The water is now more alkaline due to reactions  with the chalk, and contains dissolved calcium carbonate (‘hard water’).

The aquifer level varies with amount of recharge from rainfall, discharge in springs, and abstraction by  humans. When abstraction exceeds recharge the level drops, and flow rate slows.  

The Rib and Quin valleys

Jane Tubb – Chair, East Herts Geology Club

Geological history

Since the start of the ice age 2½ million years ago, there have been dozens of big swings in  climate between very cold (glacial) and warmer (interglacial) stages. During glacial stages sea  levels fell as water was locked up in land ice. Gradually the swings intensified until, during each  glacial stage the land of southern England was joined to that of mainland Europe. When snow  melted in summer, the ancestral Thames became a huge powerful river. It flowed north of its  present position, following a course through St Albans, Essex and Suffolk to join the Rhine past  where Dogger Bank is now. Through time, the river gradually migrated south east until, half a  million years ago, it flowed across Hertfordshire and Essex past Clacton and beyond.  

The Thames about one million years ago.  After Hey, Rose, Bridgland, Whiteman.Allen.

About 0.42 million years ago, near the end of the greatest ice advance – the ‘Anglian glaciation’ – an ice sheet pushed across Hertfordshire blocking the Thames and creating lakes which overflowed and diverted the river.

The ice sheet deposited rock-debris called till (also known as boulder clay) transported by the  ice from as far away as Scotland and Scandinavia.  

As the ice retreated the River Lea and its tributaries began to flow south through valleys which  had been part of the pre-Anglian northward drainage system. The melting ice produced large  quantities of fast flowing water which deepened and widened the valleys and deposited sands  and gravels which had been carried in the glacier, and are now being quarried for building  material. After four further glacial stages, the Thames eventually reached its present position as  a small ‘interglacial’ river and by about 7,000 years ago sea level had risen yet again.  

Images courtesy of Essex Rock and Mineral Society – http://www.erms.org

With thanks to Ian Mercer of Essex Rock and Mineral Society for his contribution to this page.  

The Geology of the present rivers

The Rivers Rib and Quin are not typical chalk streams as their sources are on top of thick glacial  deposits which overlay the major Chalk aquifer. Their upper reaches are predominately drainage  ditches, being fed by rainwater running off higher ground. Runoff into both rivers is increased  by agricultural activities. There is some contribution from a ‘perched water table’ within the  glacial deposits where water sinking through sands and gravels meets impervious till beneath the  river beds and overflows into the rivers when the level of the water table rises. These upper  reaches of both rivers run dry when the perched water table recedes below the river bed and  during sustained periods without rainfall.  

Under average conditions, the Chalk aquifer in the Quin sub-catchment is approximately 7m  below the river bed in the vicinity of Hare Street and 15m below the bed of the Rib at Chipping.  

The aquifer begins to contribute to the River Rib in the Standon and Puckeridge area, and to  the River Quin at Braughing, from which points they are classed as chalk streams and provide habitats for their rare and wonderful wildlife.  

With thanks to Dr Ilias Karapanos of Affinity Water for his contribution to this page. 

For more information about local geology visit East Herts Geology Club www.ehgc.org.uk