The idea that hillslopes in active mountain belts are limited to a threshold inclination by the rate of landsliding has guided landscape evolution models for over a decade. However, widely applied topographic diagnostics of threshold hillslopes remain debated and unverified. This study relates landslide occurrence to hillslope metrics of several active mountain ranges throughout New Zealand, which are formed in uniform greywacke and its schist derivative. All these ranges share a distinctive peak in hillslope inclination despite order-of-magnitude variations in rates of rock uplift and precipitation, landslide density, and extent of Quaternary glaciation. Comparison with landslide-dominated terrain in different lithologies highlights this peak as diagnostic of rock type, reflecting a conspicuous tendency in hillslope evolution to adjust to rock-mass strength irrespective of the intensity of tectonic and climatic forcing. This finding expands the model perspective that only undisturbed strength-equilibrium slopes may adjust to their rock-mass strength.