DESSICRATION CRACKS (aka Mud Cracks) / ROCK TORS / LAVA TUNNELS
I have done this method before, although this time I forgot to add a layer of Toilet-Paper & PVA before adding Sharp Sand. It would have helped.
Sharp Sand is good because mixed with PVA it sets hard like cement. Last time I did this I mixed it in a pot first and spread it on the model, which gives a very thick and heavy layer of gritty skin. This time I painted the surface with PVA (it needs a thick layer) and sprinkled Sharp Sand onto it. It will probably flake off as it dries and will require another layer (I didn't make it thick enough).
It needs a surface-sealant layer on top of that in any case. Using slightly watered down PVA in a spray is best for this (careful not to soggy the cardboard). Sprinkling it instead of mixing a paste is worth it for reducing the thickness of the sand layer, although I regret not putting time into doing a layer of TP&PVA first to strengthen it and to help bond the sand to the surface.
I will make more of these. The next batch might be half the height, these Rock Tors are giants for the 28mm - 32mm scale. These models are useful because they are multi-functional as different terrain types. Also they can be stacked. They're versatile. Scrunching the card is fun. You have to cut triangles out of the edges then hot-glue the wedges in the gap, to create the drum shape.