Subsidence is only possible where the ground material
can be displaced into some sort of underground voids,
which can only occur in certain rock types.
Macrovoids, large cavities: solution caves in limestones
(section 29); much rarer natural cavities in other rocks,
including salt and basalt; mined cavities in any rocks of
economic value (sections 30, 31).
Microvoids in very porous, deformable rocks: most
important in clay (section 28); in peat, some silts and
some sands; in made ground and backfill (section 30).
Subsidence cannot occur on solid, unmined rock, except
by shear failure and rotation to the surface under excess
load, or by landslides within slopes