As described in Chapter 10, the water phase pressure within a laterally extensive aquifer bounded by overpressured mudrocks follows the hydrostatic gradient. As a result, at structural crests, the effective stress is low because the pore pressure in the reservoir follows the hydrostatic gradient but the overburden stress follows the lithostatic gradient. There is a limiting case where the structural relief of the permeable body is so large that the aquifer pressure converges on the least principal stress and mechanical failure of the overlying seal occurs.