The authors re-examine conventionally used terms for hills on the basis of a morphometric study of landforms in Japan. Relative relief and drainage density are calculated for every 250 m grid on topographic maps.
Figure 2 shows relative relief on the vertical axis and drainage density on the horizontal axis. Hills are clearly delineated from mountains by these parameters. The relative relief for hills is smaller than 110 m120 m, and the drainage density is greater than 4050 streams/km2. The Shiranuka Hills in Hokkaido should more appropriately be called the Shiranuka Mountains, because of their high mean relative relief (134.3 m) and lower drainage density.
Figure 2 also indicates a possible subdivision of hills into 2 types, or 'flat topped hills' with a relative relief of less than 60 m, and 'ridge hills' with a relative relief of 60 m 120 m. The standard deviation of relief energy can also be taken as a criterion to distinguish the 2 types of hill, namely, the flat topped hills have a standard deviation of 8.120.6, in contrast to the ridge hills, which have a standard deviation of 21.029.8.
The drainage density generally increases corresponding to the length of time through which terraces are transformed into hills and the amount of uplift. Therefore, hills have greater drainage density than terraces. But, it is worthy of note that the drainage density is never greater than 7080 streams/km2. The greatest density is found in the Oku-Noto Hills, one of the ridge hills 480, 000780, 000 years old. Indications are that the development of a drainage system culminates at some stage between flat-topped hills and ridge hills, and then valleys are gradually unified, resulting in a lower drainage density.
Both flat-topped hills and ridge hills may be derived from various origins such as terraces, depositional surfaces of pyroclastic flows, dissected Tertiary or early Quaternary soft rocks, or degradation of mountains with medium relief. But, the formation of ridge hills requires a greater speed of uplift, and rocks that are hard enough to sustain steep slopes with a relative relief of 80 m120 m. The conditions being the same, ridge hills are older than flat-topped hills.
