‘It was low tide, and the water ran below glidders, or steep muddy slopes.’
Williamson has a helpful habit of using unusual words and then immediately providing his definition of them. Here he defines ‘glidder’ as a steep muddy slope, which differs a little from the OED definition of ‘a loose stone on a hillside’.
Clearly, the slope is the key element, whether made of mud or stones.
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. 77