Landforms are shaped by tectonic movement and sculptured by denudational processes. Davis (1899) deduced landform development by denudational processes, postulating prolonged stillstand of a landmass following rapid uplift, but W. Penck (1924) emphasized that land forms were formed by tectonic and denudational processes proceeding concurrently at different rates. These two distinctive views of tectonics and denudation in geomorphology have been discussed many times, but actual conditions of these processes have rarely been assessed quantitatively.
Schumm (1963) and Bloom (1978) estimated modern rates of uplift to be much greater than those of denudation, and supported to some extent the Davisian assumption of rapid uplift of a landmass, which allowed little denudational modification of the area during the period of uplift. Recent geomorphological study has achieved many excellent results concerning tectonic and denudational processes and their products, but landform development by concurrent tectonics and denudation has scarcely been investigated intensively.
As a result of the author's estimate in Japan (Yoshikawa, 1974), modern rates of uplift are generally greater than those of denudation, but denudation rates are greater than or approximately equal to uplift rates in high mountains of Central Japan and on the Pacific slope of Southwest Japan; in these mountains both rates are usually of the order of 1mm/yr. These mountains have been rapidly uplifted and intensely denuded in the Quaternary. Landform development of these mountains, therefore, should be explained not by the Davisian scheme, but by the Penckian.
When a landmass is uplifted at a constant rate, the area increases its relief with uplift, being sculptured by rivers. Denudation rates become greater and approach uplift rates. Ultimately both rates become equal, and steady-state landforms in dynamic equilibrium of uplift and denudation are accomplished, as far as the landmass is continuously uplifted at the constant rate (Plirano, 1972, 1976; Ohmori, 1978). Landform evolution by uplift and denudation, therefore, can be divided into the following three stages; (1) the developing stage that landforms approach steady state by concurrently proceeding uplift and denudation, (2) the culminating stage that steady-state landforms are maintained in dynamic equilibrium of uplift and denudation, and (3) the declining stage that landforms are reduced down to sea level by denudation when uplift ceases. Landform evolution passes through these three stages in different duration periods according to various rates of uplift and denudation as well as duration periods of uplift.
Supported by the interpretation that erosion surfaces fragmentarily distributed in Japanese mountains are remnants of peneplains in previous cycles, the Davisian scheme of landform development has survived in Japan, where active uplift and intense denudation have proceeded concurrently in recent geologic time. It was, however, clarified in the upper drainage basin of the Waiapu River, northeastern North Island, New Zealand, that erosion surfaces in the hills, about 500 to 700m above sea level, were formed nearly at the present height probably by periglacial processes and fluvial transportation of debris in the last glacial age (Yoshikawa et at., in preparation). This suggests that there is a possibility that a considerable part of erosion surfaces in Japanese high mountains is also of the similar origin.
Geomorphological study in tectonically active and intensely denuded regions, such as Japan, will produce invaluable information of landform evolution by concurrent tectonics and denudation. This will contribute to further development of geomorphology.
