At first glance these ponds appear unremarkable, but the name and the aerial images hint at something much more interesting. They are actually sinkholes in the Magnesian Limestone
The holes are believed to have formed in an earthquake in 1179 and a huge amount of local folklore and legends have built up about site, which you can read on the ‘Old Corpse Road’ website. Links to Lewis Carols Alice in Wonderland have also been claimed as Charles Dodgson lived nearby as a child
The northern ‘Double Kettle’ is filled with water from surface run-off, however the water in the southern ‘Croft Kettle’ comes from subterranean springs, which is very noticeable on Google Earth imagery. The site is a SSSI as its the only place in Country Durham where this occurs.
The different colours of the water show the different supply?
Yes, heres the details from the SSSI listing.
Hell Kettles comprises two pools of contrasting ecological character. The southern ‘Croft Kettle’ is
fed by subterranean springs issuing from Magnesian Limestone. It has clear, highly calcareous waters
and supports a luxuriant growth of stoneworts Chara hispida and C. vulgaris. In contrast the larger
northern ‘Double Kettle’ is fed mainly by surface water drainage and has turbid water supporting only
a sparse growth of fennel-leaved pondweed Potamogeton pectinatus and the introduced Canadian
pondweed Elodea canadensis.
The marginal vegetation of the two pools also differs. Croft Kettle is fringed by a narrow swamp
dominated by saw-sedge but with common reed Phragmites australis, lesser water-parsnip Berula
erecta, mare’s-tail Hippuris vulgaris and tubular water-dropwort Oenanthe fistulosa; around
Double Kettle, common reed predominates, but with some saw-sedge and common club-rush
Scirpus lacustris.