Thanks very much guys!
DFM, yes, it's representing melted snow and ice that has re-frozen. I have used other materials in the past, including bicarbonate of soda (bicarbonnate de soude), but to get the right, almost transparent effect at the edges, this time I used PVA glue, which was painted in various shades of white and grey. The ice-covered 'puddles' were painted, then glazed with Johnson's 'Klear', and the cracks painted in. The 'fresh powder snow' was made using talcum powder, which resists decomposition. Bicarbonate can, eventually, discolour, even when treated with hairspray, varnish or spray adhesive.