GEOMORPHIC CYCLE

also called geographic cycle, or cycle of erosion theory of the evolution of landforms. In this theory, first set forth by William M. Davis between 1884 and 1934, landforms were assumed to change through time from youth to maturity to old age, each stage having specific characteristics. The initial, or youthful, stage of landform development began with uplift that produced fold or block mountains. Upon dissection by streams, the area would reach maturity and, ultimately, would be reduced to an old-age surface called a peneplain, with an elevation near sea level. The cycle could be interrupted by uplift during any period of the life cycle and thus returned to the youthful stage; this return is called rejuvenation. The geomorphic cycle could be applied to all landforms such as hillslopes, valleys, mountains, and river drainage systems. It was assumed that, if the stage of a landform was known, its history followed directly according to a predetermined framework. Though Davis acknowledged that rock type, structure, and the processes of erosion play a part in landform determination, he emphasized that time was the primary factor. It is now believed that time is no more important in landform development than the other factors. The cycle-of-erosion theory has long been accepted in the face of accumulating quantitative data that refutes it. It is generally held now that the initial conditionsor upliftin a region do not necessarily predetermine the end products. Rather, there tends to be an eventual attainment of dynamic equilibrium between landforms and the processes that act upon them. When this happens, the physiographic history of a region is erased.