Orderly series of stages through which land-forms were believed to pass from their initiation following uplift to their final planation by erosion. The main stages were those of youth, when hillslopes were steep and river profiles irregular; of maturity, when river profiles were smoothly concave-up and incision had markedly slowed; and of old age, when the landscape was reduced to a gently undulating surface or peneplain. This framework for land-form studies has now largely dropped out of use.