Our broad and brookless vales—
Only the dewpond on the height
Unfed, that never fails—"
My Twitter game friend, who lives in Gothenburg, Sweden, is actually English and as a boy lived in Sussex. When I told him of my find and that I had posted it on my website, he kindly offered up more information. He wrote me:
"Sussex is crossed east to west by a chain of chalk hills, the South Downs. The chalk was laid down at the bottom of an ancient sea and the lines of the Downs roll just like a frozen seaswell, which gives us our "broad ... vales". (Also "Our blunt, bow-headed, whale backed Downs".) When it rains, of course, the rainwater soaks straight into the chalk, hence "no waters to delight Our ... brookless vales". There are, though, plenty of springs and streams around the edges of the Downs where the rainwater absorbed by the chalk meets the underlying impermeable clay and emerges to water the Weald (which is the big valley north of the South Downs), and Sussex itself is crossed by a number of rivers that have cut through the chalk.
However, here and there on the Downs you find mysterious ponds. High up in the hills, well above the margin where the Wealden springs emerge, but always in grassy dells, they are not obviously fed either by streams, springs or even rainfall, so local tradition decided they must be places where dew collected - hence "dewponds". When Kipling writes "the dewpond ... that never fails" he's repeating the tradition but in fact they do fail - in the sense dry out from time to time - though magically reappearing again in the same spot.
At school I was taught that, in these spots, a dip in the chalk matched an irregularity in the underlying clay so that the chalk dipped below the top of the watertable. The dewponds grew or shrank and disappeared depending on the quantity of water in the watertable. I was never entirely satisfied with that explanation because there are also plenty of places in the Downs (locally called Dykes or Ghylls) where the chalk dips much further down, but where no dewponds form. And now I find (thank you, Wikipedia!) dewponds are actually man-made, specifically to hold water for farm animals. They can be ancient, though, and there doesn't seem any consensus on where the water comes from that fills them, or why it doesn't evaporate as quickly as conventional wisdom says it should. Perhaps there's something magic about the dewponds after all."