The Philippine Sea is bounded on the west by the southern half of Japan, the Ryukyu Islands and the Philippines, and on the east by the arcuate seafloor ridge extending from Japan to the Palau Islands through the Mariana Islands. Both arcuate structures are associated with deep sea trench, island arcs, volcanic activity, deep focus earthquakes and negative gravity anomalies. This abyssal deep basin which consists of the oceanic crust is separated into the east and the west basin by the Kyushu-Palau Ridge. The east basin is somewhat shoaler, and can be divided by the central narrow zone at 23°N into two parts, Shikoku Basin and West Mariana Basin, which gradually rise toward the Honshu-Mariana Ridge. There are, however, several seamounts, called the Kinan Seamount Chain, arranged in NNW-SSE direction in the Shikoku Basin. And on its southern extension, there are several troughs, called the West Mariana Trough, arranged is N-S direction. The Shikoku Basin is composed of the smooth floor of the northeastern part and the mountainous area of the southwestern part. The West Mariana Basin is also divided into the abyssal plain on the east and abyssal hill region on the west. And the West Mariana Trough is running on its boundary. The west basin, Philippine Basin, is unusually deep. A region of rugged topography which can be termed the Daito Ridges lies at the northernmost part of the basin. These several parallel ridges elongate from the respective coral islands to southeastward intervening narrow depressions. There are flat plains at the depth of about 1500, 2000 and 3000 m on the northern side of each of the ridges. There is the striking feature, called Central Basin Fault, in the central part of the Philippine Basin. Central Basin Fault which is the low ridge associated with trough in the NW-SE direction separate the Philippine Basin into the north basin and the south basin. On the Philippine Basin floor, the most extensive area is abyssal hill region. Abyssal plains are developed only in limited localities such as the west side of the northern arc of the Kyushu-Palau Ridge. From the description on the submarine topography of the Philippine Sea the author concluded as follows. (1) A few ridges and seamounts not associated with trench are running parallel to the Ryukyu-Philippine Arc and Izu-Mariana Arc : one Kyushu-Palau Ridge, two Kinan Seamount Chain-West Mariana Trough, three Nishi-Shichito Ridge-West Mariana Ridge. These ridges close the space eastward. And basements of basins which intervened between each ridges rise toward the east. (2) There are another ridges having unusual trend. The Daito Ridges have E-W trend and the minor ridges on the Izu Ridge have the NE-SW trend. The numerous minor ridges which construct the abyssal hill region also have the E-W trend on the northern part of the Philippine Sea, although its trend changes to the N-S trend southward gradually. These trends are correspond to the Southwest Japan Arc. (3) Ridges with E-W trend are cut by the ridges with N-S trend, alike the Japan Arc crossed by the Izu-Mariana arc. (4) Kyushu-Palau Ridge consists of two arcs, northern arc parallel to the Ryukyu Arc and southern arc parallel to the Philippine Arc. These two arcs are separated by the Central Basin Fault. Another possible fault runs from the depression in the east side of the northern Kyushu-Palau Ridge to the junction of the Izu-Ogasawara Arc and the Mariana Arc. The southern arc (Mariana) remarkably projects to the southeast along this fault. (5) Abyssal hill provinces in the Philippine Sea occupy the western half of each basins usually. Some of the flat plains occupying the eastern half appear to the archipelagic apron which is constructed by the eruption on the concave side flank of the arcuate ridges.
The existence of sand waves in the region as shown in Fig. 2 was confirmed by the Joint Survey Team from Indonesia, Japan, Malaysia and Singapore during the preliminary hydrographic survey in the Malacca and Singapore Straits conducted from January to March 1969. The sand waves as appeared on echograms can be classified into the following categories : A. Round top type group I : a. Round top type (Fig. 3) b. Conical type (Fig. 4) C. Compound round top type (Fig. 5) B. Round top type group II : a. Asymmetric round top type (Fig. 6) b. Asymmetric compound round top type (Fig. 7) C. Pinnacle type group : a. Pinnacle type (Fig. 8) b. Asymmetric pinnacle type (Fig. 9) c. Asymmetric compound pinnacle type (Fig. 10) The distribution of sand waves according to their types is as follows : (a) Pinnacle types sand waves are developed in the area extending from the SE extremity of Malay Peninsula to Singapore offing, and their wave-heights are 8 to 15 metres. (b) In the area between SW tip of Singapore island and Benut offing, mixed existence of round top type group I and pinnacle type group is found. (c) Various types are found between Tg. Tohor and One Fathom Bank, but pinnacle type waves are prevailing in the offing between Tg. Tohor and Muar, where the largest sand wave (of a pinnacle type with a height of 20.6 metres and a length of 267 metres) was found. Sand waves of pinnacle type and round top type group II were seen off the coast from Muar to Cape Rachado, and their heights were 5 to 10 metres. (d) Between Cape Rachado and One Fathom Bank, those of pinnacle type and round top type group II with heights of more than 10 metres were found, among which where included those with heights reaching 15 to 18 metres. Of sand waves surveyed in the detailed survey area off Cape Rachado, 1, 144 waves were calculated as to their various factors for classification, and the following data were obtained : (1) Average wave-height and average wave-length. (2) Correlation between the height and length of waves (Fig. 14 and 15). (3) Relation between the water depth and existence ratio of each type of wave (Fig. 16). (4) Relation between the wave height and the existence ratio of each type of wave (Fig. 17). Causes for creation of sand waves and their movements were not studied in this paper, since this was the first time of study and, besides, detailed observations were not conducted for currents and tidal currents in the area in question.
From 1959 to 1960 the writer carried out observations of beach cusps that are developed along the normal high water line of the southern shore of the Futtsu cuspate foreland, Chiba Prefecture. The result revealed the following facts. 1) All the year round, development of cusps is best in the Experimental Station zone (from the west of the Summer Bungalow of the Japan National Railway toward the tip of the foreland). The Onuki zone is the second best in the cusp development. 2) Three types of cusps are recognized singular and lingulate type (A type), continuous and fan-shaped type (B type) and continuous and triangular type (C type). The A type cusps are distributed in the Summer Bungalow zone, B type ones in the Onuki and Summer Bungalow zones, and C type ones mostly in the Experimental Station zone. 3) Intercusp space is generally between 3 m and 27 m. But, in the A type cusps the predominant space is 8 to 11 m, in B type ones it is mostly 10 to 18 m, and in C type ones it is mostly 10 to 15 m. 4) The sediments constituting the surface of cusps comprise four types gravel type, shell type, sand type, and gravel-shell mixed type. In the report area the mixed type is predominant. 5) In the area where cusps are undeveloped the beach profile has a 2° to 3° gradient, whereas in the area where development of cusps is good the gradient is 4° to 7°, steeper than the former area. 6) The bottom profile of the surf zone is convex in the area where cusps are undeveloped, and concave in the area where cusps are developed well. As a factor of cusp formation, the difference in the bottom profile may be more important than the difference in the beach profile. 7) The consecutive observations of the same group of cusps showed that the horn direction of cusps and the intercusp space are generally constant, except for a period of abnormally high water due to typhoons or some other causes.