‘Down a crumbling sog of peat and into the still brown-clear water.’
Of course, we speak of ‘soggy’ pasta or trifle sponges, but never had it occurred to me that what is sog-gy must have a ‘sog’ behind or beneath it. So, a sog is a marsh, quagmire, swamp or similar.
Now you can tell people to ‘sog off’. But only if they have behaved abysmally and you’re at risk of saying something worse.
See also ‘juggymire’.
Source: Henry Williamson, Tarka the Otter: His joyful water-life and death in the two rivers, illus. C.F. Tunnicliffe (Harmondsworth: Puffin Books, 1976 (1927)), p. 126