You might think a pond is just a pond. You would be incorrect. Martin Fone tells us the fascinating story of pingo and dew ponds.
One of the most striking, if profoundly disturbing, sights I have witnessed was a large piece of a glacier breaking off and falling into Alaska’s Glacier Bay, a visible manifestation of the impact of the warming of the Earth’s temperature. The current phase of climate change may largely be anthropogenic, but over this planet’s long history there have been natural climate cycles where temperatures have risen and fallen, each leaving their mark on the terrain.
Around 20,000 years ago, as the glaciers retreated, water flowing underground sometimes froze. Forced to the surface by artesian pressure, the ice created small, relatively low hillocks. First described in 1825 by John Franklin, they are known as pingos, a term coined by Arctic botanist, Alf Erling Porsild, borrowed from the Inuvialuit word for a conical hill, pinguq, for his 1938 paper on the mounds he had discovered in the western Arctic coast of Canada and Alaska.
This area, especially around the Mackenzie Delta in the Canadian North-west Territories, is home to around 1,350 identified pingos, around a quarter of the world’s total. Examples of pingos, looking like giant molehills, can still be seen in the London Wetlands Centre in Barnes.
While an indicator of permafrost regions where ground temperatures remained below 0⁰C, surviving pingos are relatively rare. As temperatures continued to rise after the Ice Age, the ice forming the core of the pingo would melt, collapsing the hillock and forming a shallow crater. With nowhere else to go, the meltwater filled the resultant hollow, creating a pond, known as a relic pingo, pingo pond or kettle pond.
Judging by maps of the county from the 18th century, one of Norfolk’s distinctive features was the number of its Commons. Very few survive today, most ploughed over to satisfy the increasing demand for fertile agricultural land. One notable exception, though, is the self-dubbed pingo pond capital of the UK, Thompson Common in Breckland, six kilometres south of Watton, and now managed by the Norfolk Wildlife Trust.
The extraordinary concentration of pingo ponds on the Common rendered the land too difficult to convert for farming purposes and although some on the periphery were filled in, the majority remain in their natural state. The shallow ponds offer an excellent habitat for some particularly rare species of wildlife. The downside is that they also make a great breeding ground for mosquitoes. An eight-mile circular walk, known as the Great Eastern Pingo Trail, allows visitors to observe pingo ponds in their natural environment and, as an added bonus, the opportunity to spot some long horned cattle.
A collapsed pingo, thus creating the pingo pond. Credit: Getty Images
While collapsed pingos have enabled Thompson Common to turn a geological phenomenon into a nature conservation project, they presented engineers building the Blackwall Tunnel some unanticipated problems. The plan for what would be the longest underwater tunnel in the world was to route it through fairly level bedrock. However, geographical surveys soon revealed a deep depression at Blackwall that went through the London clay at least 200ft into the chalk.
Filled with sand and gravel, the basin was thought to be a collapsed underground pingo. Finding a solution to excavate through it was one of the reasons why it took until 1897, six years after the start of the project, for the tunnel to be completed. The second tunnel, built in the 1960s, ran into similar problems and there is evidence of collapsed underground pingos in other parts of London, such as Battersea and Canning Town.
Pingo ponds are natural, but dew ponds are man-made, usually found in areas where natural water sources are scarce, often in upland areas and even on tops of hills. It has been suggested that it was a dew pond that Jack and Jill were visiting before their mishap.
Also known as cloud or mist ponds, the term dew pond was first used in print in 1865 in the Journal of the Royal Agricultural Society, reflecting a popular misconception that, given their position, their primary water source of water was dew or mist. There was some empirical evidence to suggest that, with Gilbert White noting in 1788 that during prolonged summer droughts, the artificial ponds on the downs above Selborne retained their water while larger ponds in the valley below dried up.
A greater understanding of how dew was formed and the realization that as water’s heat-retaining capacity is far greater than that of earth, the summer air above a pond was unlikely to attract condensation put paid to that romantic notion. Instead, the dew pond’s greater ability to retain water was primarily down to its distinctive shape. Shallow, saucer-shaped, and with a wide brim it could more easily capture and retain rainfall and run-off than other types of pond.
Constructing a dew pond was a skill that was passed down the generations, as a Sussex farmer recalled in The Field in its edition of December 14, 1907. After the basic shape of the pool had been excavated and a layer of straw laid around the basin, chalk would be tipped in and crushed to a powder by a team of oxen harnessed to a heavy broad-wheeled cart. Keeping the powder moist and working it most of the day, it was reduced to the consistency of a thick cream known as puddle chalk. Smoothed with the back of a shovel until it gleamed like glass and left to set, within a few days it would be as hard as cement and impermeable.
‘This old method of making dew ponds’, he lamented, ‘seems to have died out when the oxen disappeared from the Sussex hills, but it is evident that the older ponds, many of which have stood for scores of years practically without repair, are still more watertight than most modern ones in which Portland cement has been employed.’ Often snow was piled into a new pool and left to melt to fill it up.
When dew ponds were first constructed is another subject of debate. Rudyard Kipling in Puck of Pook’s Hill (1906) believed the craft went back to Neolithic times, attributing the Dewpond near Chanctonbury Ring to ‘the Flint Men’. A land deed dating from 825 AD, referencing Oxenmere at Milk Hill in Wiltshire, suggests that they were in use during Saxon times, but the more substantive body of evidence dates them to mediaeval times.
The heyday for dew ponds was the 18th and 19th centuries for two reasons. To feed the demand for wool from the burgeoning wool industry, there was a steep increase in the sheep population, many of which were forced to graze in areas which previously had not supported livestock and where natural water sources were scarce. Secondly, landowners, empowered by the Enclosure Act of 1773, in closing off common land often deprived communities of their only reliable sources of water. The construction of dew ponds offered a solution to both problems.
By the 20th century, with the development of a reliable rural water supply, many dew ponds became redundant and, after falling into disrepair through neglect, were filled in. Some, though, do survive, particularly in areas such as the South Downs, the Peak District, Wiltshire, and Hampshire. A list of dew ponds in England can be found here, but whether it is comprehensive is unclear.
Belying the view that a pond is a pond, pingo and dew ponds have fascinating stories to tell.
Martin Fone is the author of ‘Fifty Curious Questions: Pabulum for the Enquiring Mind’
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