Selby established the fact that on many rock slopes a condition of ‘strength–equilibrium’ exists in which slope gradients are in adjustment with the mass strength of the underlying rock. An established assessment technique, the rock mass strength (RMS) classification, can be used to quantify rock slope condition and, in combination with slope gradient, determine the status of rock slopes with respect to a statistically-defined ‘strength–equilibrium envelope’. This technique has been applied to 59 glacial valley slopes in the English Lake District, an area with characteristic ‘U-shaped’ glacial valleys, deglaciated around 11 ka bp. Overall, two-thirds of the slopes examined plot within the limits of the strength-equilibrium envelope, whilst the remainder are ‘understeepened’ or ‘oversteepened’ with respect to their RMS. ‘U-shaped’ valleys possess slopes that are yet to respond measurably to post-glacial erosion or readjust significantly. Evidently, slopes in similar deglaciated environments with tectonic, climatic and relief characteristics comparable with those of the Lake District need more than 11 ka (and probably much more) to evolve into strength-equilibrium. The implication is that even if slopes plot within the strength-equilibrium envelope they are in a pseudo-equilibrium condition, indicating that the Selby strength-equilibrium theorem is inapplicable to landscapes deglaciated c. 11 ka bp.