When making paddy fields on a slope, the paddies, each with a level surface, must be made one by one in the form of a series of shelves or terraces. Paddy fields made in this manner are called “terraced paddy fields” or “shelf paddies” “tanada” in Japanese. Such paddy fields are found in various parts of the mountainous regions of Japan, and may roughly be divided into two categories, depending on the nature of the place where they are found. The first category includes terraced paddy fields that stretch in long, narrow lines along the bottom of small valley among the mountains. To the second category belong those that are found stretching on wide slopes. Terraced paddy fields belonging to the first category are found everywhere in mountainous regions. They are easy to make and their irrigation presents no problems. In the case of terraced paddy fields of the second category, however, because of the general complexity of the relief of the slopes where they are found, their making in such places requires great labor, and their irrigation is a difficult matter. The present writer conducted a study of 26 of the main terraced paddy fields belonging to the second category, and in each case, was convinced of the great pains and resourcefulness required to ensure the supply of irrigation water. In Japan, rice has been the main food crop since ancient times ; and until the Meiji period, it was the practice among farmers to pay their taxes in rice. Rice was the all-important element of economy, both for the central and local feudal lords until the Meiji period. This was the reason that, throughout the nation, the development and expansion of paddy fields were the two main concerns of the rulers as well as the farmers. This state of affairs continued until recent times. The present writer believes that it is possible to consider the existence of terraced paddy fields as one of the manifestations of the history of this type of rice-centered policy in the form of cultural landscapes seen in the land. In the present paper, the writer has chosen three typical regions, and he purposes to describe the development of the irrigation of the terraced paddy fields found in each of these regions. (1) The Wajima Region in the northern part of the Noto Peninsula. This region is the location of “Shirayone no Senmaida” (or “Numerous Small Terraced Paddy Fields of Shirayone”), which in recent years have come to attract the attention of tourists. There are monographs and books which state that the terraced paddy fields in this region are supplied with irrigation water by springs and rain water. The writer, however, has made clear in the present paper the fallacy of these claims. In the Edo period, in order to irrigate the terraced paddy fields in this region, long irrigation canals were dug to draw irrigation water from the upper reaches of rivers among the mountains, while irrigation reservoirs were made among the mountains to provide irrigation water. Examples of these irrigation systems are shown in Figures 1 and 2. The terraced paddy fields in this region are developed on slopes formed as a result of landslides. (2) The Obasute Region in Nagano Prefecture. This region is the location of terraced paddy fields widely reputed as “Tagoto no Tsuki” (or “The Moon Reflected in Each of Many Paddy Fields”). In this region, too, are found terraced paddy fields on slopes formed as a result of landslides. Concerning the terraced paddy fields in this region, there are books which state that they were supplied with irrigation water by springs and rain water, but the irrigation system in this region is not such a simple matter.
The term “pediment” used in this paper is limited to the piedmont gentle slope that truncates hard bedrocks and is formed by the process other than a periglacial action. Longitudinal profiles of pediments in Japan, Korea and the Liaotung Peninsula are concave in shape and more inclined closer to the knickpoint. the inclination of pediment decreases downslope gradually. In Japan the upper part of pediments is 13°-12°in gradient and the lower part is 8°, and the longitudinal length is about 1000-1500 meters in almost all pediments. In Korea and the Liaotung Peninsula the inclination of pediments is 10°-6° in the upper part and about 3°-1° in lower and the length is about 4000-5000 meters in most pediments, where the pediments continue to the plains which are supposed to be pediplains. All the pediments already stopped to grow and they have been dissected by stream-lets. The development of these pediments belongs to the Riss-Würm Interglacial and they stopped to grow a period before the Würm Glacial. It is concluded that the pediments had been formed under the arid climate with the following facts : 1) Most part of granitic bedrocks of the pediments are weathered more than 30-40 meters in thickness, but partially not weathered and solid. Quarries are located on these solid bedrocks. But differences between weathered and solid bedrocks cannot be seen in the shapes of pediments. So pediment surfaces are smooth from weathered to solid bedrocks. There is no soil (or A and B horizon of pedology) on the bedrocks covered with The deposits, and upper parts of the hedrcoks are like Zone 1 of weathering zones being divided by B. RUXTON and L. BERRY (1957). These facts indicate that chemical weathering of the granitic bedrocks started after pedimentation truncating solid bedrocks had stopped. According to N. M. STRAKHOV (1967), chemical weathering are not active in the regions of desert, semi-desert, steppe and tundra. 2) The facies of the deposits on these pediments are similar to those on pediments which are developing in arid and semi-arid regions nowadays. 3) Under the humid climate in East Asia at present, landslide and mudflow are the most active erosional process on the escarpments behind these pediments. These landslides and mudflows often occur on weathered shallow valley bottoms, so the escarpments behind these pediments are eroded linearly.
From the results of the bathymetric survey conducted by the Survey Ship SHOYO from 18 April to 13 May 1974, the following ocean bottom features have been confirmed : 1. All of the Kyushu-Palau Ridge, West Mariana Ridge and the knolls at the central part of the Philippine Basin present their profiles as gentle on their western slopes but steep on their eastern side. 2. On the western part of the bottom of each of the basins separated by the above mentioned ridges, a rugged sea knoll group exists, while a comparatively flat bottom extends at the eastern part. 3. The average depths of the Philippine Basin, West Mariana Basin and Mariana Trough are 5, 800 m, 4, 500 m and 3, 500 m, respectively, presenting a stepped feature separated by the ridges.