From Trevor Ford, University of Leicester
The article on ridged icicles in your 13 July issue (p 21), reminded me of a similar phenomenon in stalactites, stalagmites and flowstone in caves, which I first noted some 14 years ago (Cave Science, vol 15, p 8). I also noted the phenomenon on soft limonitic deposits in an old coal mine and on the lime deposits found under concrete bridges.
I appealed then for explanations of the origin and periodicity of the ridges, but none was forthcoming. Like the icicles, these structures showed a rippled effect with the ripple crests generally around 10 millimetres apart.