Pingo 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 preponderantly be down to man’s actions, but over the long history of the planet 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, in permafrost regions, where ground temperatures remained below 0⁰C, water flowing underground froze. Forced by artesian pressure to the surface, the created small, relatively low hillocks known as pingos, a term coined by Arctic botanist, Alf Erling Porsild, from the Inuvialuit word for a conical hill, pinguq, for his 1938 paper in which he described the mounds he had discovered in the western Arctic coast of Canada and Alaska.
Nevertheless, the first to describe a pingo was John Franklin in 1825, which he had seen on Ellice Island in the Mackenzie Delta in the Canadian North-west Territories, an area which contains around a quarter of the world’s identified pingos, some 1,350. Closer to home, pingos, looking like giant molehills, can still be seen in the London Wetlands Centre in Barnes.
The surviving pingos are relatively rare because when the rise in the earth’s temperature would start to melt the solid ice within the pingo, causing the hillock eventually to collapse, forming a shallow crater. With nowhere else to go, the meltwater filled the resultant hollow, creating a pond, known as a relic pingo or pingo pond or kettle pond.
A feature of Norfolk, as seen on maps of the county dating from the 18th century, is the number of Commons there were. Very few survive today, mostly ploughed up as the demand for fertile agricultural land increased. One notable exception, though, was the self-dubbed Pingo Pond capital of the UK, Thompson Common in Breckland, six kilometres south of Watton, 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 and, as a word of warning, a great breeding ground for mosquitoes. There is even Great Eastern Pingo Trail which is a circular walk of just under eight miles, which, as an added bonus, also offers the opportunity to see some long horned cattle.
While collapsed pingos have enabled Thompson Common to turn a geological phenomenon into a nature conservation project as well as a tourist attraction, they presented the engineers building both Blackwall Tunnels unanticipated problems. Most of the route for the first, completed in 1897 and then the longest underwater tunnel in the world, went through pretty level bedrock with variations of no greater than five metres, but geological surveys revealed a deep depression at Blackwall that went through the London clay at least sixty metres into the chalk.
The basin was filled with sand and gravel and it is thought that it was a collapsed underground pingo. Finding a solution to excavate through that was one of the reasons why it took six years to complete the project. The second tunnel, built in the 1960s, hit similar problems. There is evidence of collapsed underground pingos in other parts of London, such as Battersea and Canning Town.
