‘Below the stone setts and pobbles of the wall’s apron …’
Couldn’t find this ‘pobble’ in the 600,000 words of the OED, until I realized it must be referring to ‘setts and cobbles’. A ‘sett’ is a cobblestone and the two are often mentioned together.
I prefer ‘pobble’ which must be a pebble-cobbled cobblestone. Or a fortuitous typo?
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. 191