Active fold-and-thrust belts and accretionary wedges along compressive plate boundaries are analogous to the wedges of soil or snow that form in front of moving bulldozers. Such wedges deform until they attain a critical taper, which corresponds to an internal state of stress that is everywhere on the verge of Coulomb failure. The critical taper depends on the wedge cohesion S ₀ as well as the internal and basal coefficients of friction μ and μ_b and the internal and basal Hubbert-Rubey pore-fluid pressure ratios λ and λ _b; an exact relation can be obtained if the cohesion increases...