Following a change in the rate of tectonic activity, such as a change in the rate of convergence in a continent-continent collision zone, the rate of rock uplift changes. Because the erosion rate varies directly with relief, this will result in a change in the rate of erosion. Eventually a new steady state is reached in which the rock-uplift rate equals the erosion rate. An analysis based on perturbation theory and using an erosion law of the form E=Se^aR, where E is the erosion rate, R is relief, and S and a are constants (Summerfield and Hulton, 1994), predicts that the time constant for this adjustment is about 0.5 x 10⁶ years. While appreciable, this is short compared with the time scale of orogeny. If a linear erosion law, E=ßR with ß=1.5 x 10^-7 a^-11 (Ahnert, 1970) is used instead, the time constant is two orders of magnitude larger. This is probably unrealistic; it is attributed to a paucity of data from high mountain areas in Ahnert's data set.