Lithostatic pressure, pore-fluid pressure, effective stress কী?

At the depth $z$, the weight of the rock above, per unit of horizontal area, is the vertical stress $\sigma_v = \int_0^z\rho g\,\mathrm dz'$, which is $\rho g z$ in a layer of uniform density $\rho$. When the stress is the same in every direction and equal to it, the rock is under the lithostatic pressure. The pressure of the water, oil or gas in the pores and cracks of the rock is the pore-fluid pressure $P$; it is hydrostatic, $P = \rho_w g z$, when the pores connect to the surface. The effective stress is the stress minus the pore pressure, $\sigma' = \sigma - P$, on every plane; $\lambda = P/\sigma_v$ is the pore-pressure ratio.