1.INTRODUCTION

Physiography (also known as geosystem or physical geography)  is one of the two major sub-fields of geography.It is a branch of natural science which deals with the study of processes and patterns in the natural environment like the atmosphere, hydrosphere, biosphere, and geosphere, as opposed to the cultural or built environment, the domain of human geography.


Within the body of physical geography, the Earth is often split either into several spheres or environments, the main spheres being the atmosphere, biosphere, cryosphere, geosphere, hydrosphere, lithosphere and pedosphere. Geomorphology, hydrology, biogeography, climatology, meteorology, pedology (soil science), paleogeography, coastal geography, oceanography, quarternary science, landscape ecology, geomatics, and environmental geography are parts of physiography. The following sections discuss the majority of these subjects in Indonesia and its vicinity.

1.1. SITUATION AND EXTENT

The main topic of this book is the geology of the Indonesian Archipelago which extend between 6^o08'northern and 11^o15'southern latitude, and between 94^o45'and 141^o45' eastern longitude. This archipelago, however, is not a regional unit in a geo-tectonical sense. It forms the central part of the great archipelago which extends between SE-Asia and Australia, and between the Pacific and the Indian Ocean.

The East Indian Archipelago in this sense comprises also: the Philippine Islands, Malaysian Northwest Borneo (Sabah and Sarawak), Brunei, Papua New Guinea, Christmas Island and the Andaman and Nicobar Islands.

Fig. 1.1. Eastern hemisphere geographic grid

For a better insight in its geological evolution it is necessary to consider this archipelago in its larger sense as the entire realm of islands extending between 21^o northern and 11^o southern latitude, and between 92 ^o15' and 150 ^o48' eastern longitude (Fig. 1.1). Moreover, the Malay Peninsula forms structurally a part of the Sunda Shelf area, so that a short discussion of its geology will be necessary. The total land area of the Southeast Asian Archipelago sensu largo amounts to more than 2,800,000 sqkm, which are divided among the political units, islands, and island groups as follows:

• Indonesia 1,904,569 km2 
• Papua New Guinea 462,840 km2
• Philippines: 298,170 km2
• Northwest Borneo (East Malaysia & Brunei): 198,847 km2
• Timor Leste: 15,410 km2
• Christmas Island: 161 km2

The total land area covers 2,879,997 km2.

Besides these 19 large islands, there are many thousands of smaller islands, ranging in size from several thousands of square kilometres to mere isolated rocks. Indonesia itself has  13,466 islands listed in Wikipedia. The physiographic position of the SE Asian Archipelago is shown in fig. 1.2.

Fig. 1.2. Eastern hemisphere of the globe

The cartographic basis for the geological maps of the East Indies has been provided by the excellent work of the Topographical Survey of the Netherlands Indies. The summary in the work of the Topographical Survey in the Netherlands Indies has been given by SCHEPERS (1941). During the 2nd World War the allied geographical section of the Southwest Pacific area issued a number of Terrain Studies on Papua, the eastern part of Indonesia and the Philippines, in which a wealth of geographical, ethnological and other data are collected, illustrated by excellent maps and air photographs. These days satellite images are available from public domain such as Google earth (Fig. 1.2). Higher resolution images are also available from several commercial, research and governmental institutions.

Fig. 1.3. SE Asian Archipelago (darker color) as treated in this book. The brown outline shows Indonesian border.

The area discussed in this book, with the major political boundaries, appears on fig. 3. The red outline shows Indonesian maritime boundaries in general.

The combined outline map of Europe, USA and Indonesia (fig. 1.4) demonstrates the dimensions of the latter, which should be kept in mind during the study of its geology. Total Area of Indonesia is 1,919,440 sq km (Land Area: 1,826,440 sq km; Water Area: 93,000 sq km).

Fig. 1.4. Areal comparison of Indonesia to USA (above) and to Europe (below) 

1.2. DENOMINATION

This book deals with the complex archipelago between SE-Asia and Australia. We might call it the "Australasiatic Archipelago" as was done by the SARASIN'S, or the "Indo-Australian Archipelago" (ZEUNER, 1943). However, these are somewhat uncommon names, and it is not clear that the Philippine Islands would belong geologically to this group. The term "Indian Archipelago" is easier to pronounce, but it neither clearly implies the Philippine Islands, nor Papua. Nevertheless, because of its shortness, this expression will often be used in this book for the whole of the island-system between the continents of Asia and Australia. The East Indies were called "Indonesia" by LOGAN in 1850, and by BASTIAN in 1884; this name has often been used in a political sense, but also as a geographical term in scientific papers. In August 1945, the people in this area have declare their independence and since October 1948 "Indonesia" became the official name for the Netherlands East Indies. The name "Insulinde" was created for the East Indies by the author DOUWES DEKKER (Multatuli) in 1860.

The term "Malayan Archipelago" has often been used for the East Indies and the Philippine Islands. However, New Guinea (Papua), not being inhabited prehistorically by Malayans, does not belong to this unit. This name seems to be inadequate as a name for the whole area, although it is a suitable name for the belt of islands between the continent of Asia and New Guinea. The name "Sunda Archipelago" also has a restricted meaning and should be used only for the islands grouped on and around the Sunda Shelf area of SE-Asia and not for the whole region between Asia and Australia, as was done for instance by CLOOS in his book "Einftihrung in die Geologie" (1936. p. 425). The name "Sunda Seas" was given by SCHOTT in 1935 to the water areas between the Strait of Malacca and the line Philippines- Papua. But the Moluccas lie outside the Sunda area, thus this name is inadequate. These seas between SE-Asia and Australia are more aptly called the "Austral- Asiatic Mediterranean" (Winkler Prins Encycl., Vth edit., 1937, Vol. 12. p. 625). The author decided to call this volume "The Geology of Indonesia and adjacent archipelagoes", thus laying stress on the Indonesia as the central area, and indicating that also the neighboring archipelagoes are treated in it. For simplicity Southeast Asia Archipelago will also be used.

1.3. GEOLOGICAL IMPORTANCE OF THE SE ASIAN ARCHIPELAGO

The SE Asian Archipelago is the most intricate part of the earth's surface. Even the Caribean Archipelago between North and South America, although bearing a close resemblance with it in many respects, does not attain such a diversity of forms and geological structures.

Fig. 1.5. Relief map of the Southeast Asian Archipelago and its vicinity.

MOUNTAIN SYSTEMS

In Indonesian region the interlacing of the Tethys mountain system with the western Pacific island festoons and the circum-Australian mountain system can be studied. This archipelago forms the border area between continental nuclei of Asia which belongs to the northern hemisphere, and the great Gondwana land of the southern hemisphere. In this archipelago both continental areas are being welded together by an active process of mountain building.

One may distinguish the more or less stable portion of the Sunda Shelf area in the NW, and that of the Sahul Shelf in the SE. The Sunda area is surrounded by the circum-Sunda Mountain System, which cuts across the trend lines of the circum-Australian Mountain System (Fig. 1.5).

The circum-Sunda System consists of two main parts; its northern portion (Philippine Islands) belongs to the island festoons along the western Pacific; its southern portion forms a part of the great Sunda Mountain System, which extends from the Southern Moluccas to the Bramahputra Valley in Assam.

This Sunda Mountain System has a length of about 7000 km, being traceable from the noose formed by the Banda arcs in the East along the Lesser Sunda Islands, Java, Sumatra, Andamans and Nicobars, to the Arakan Yorna in Burma, where it meets the Himalayan System with a sharp angle of intersection.

The Sunda Mountain System is one of the greatest coherent mountain belts of the world, comparable in length with the Cordillera de los Andes in South America. Along its entire length it consists of two parallel belts of mountain arcs, island-festoons and submarine ridges. The inner one has a volcanic nature whereas the outer one is non- volcanic.

The circum-Australian System extends along the central axis of Papua Island, and farther along the archipelagoes, situated East of Australia, to New Zealand. It may perhaps be traced along a sub- marine swell between Australia and Antarctica (Macquari threshold) to the Kerguelen rise in the southern part of the Indian Ocean. An indistinct branch of the median threshold in the Indian Ocean extends northeastward, via the Cocos Islands to Christmas Island, South of Java. The segment between Christmas Island and New Guinea is overlapped by the trendlines of the above mentioned Sunda Mountain System. Another geotectonic unit is formed by the mountain system which stretches from the Halmahera group via the northern part of Papua to the New Britain group.

OCEANIC BASINS

The Southeast Asian Archipelago is bordered to the NE and the SW by oceanic basins. The NE oceanic basin consist of the Philippine Basin and the Carolinan Basin at the Pacific side, and the Indo-Australian Basin at the side of the Indian Ocean.

The sea basins are 4000-6000 meters deep; however, they are presumably not primeval oceanic receptacles, but submerged borderlands of Asia and Australia. The Galathea Deep in the west of Philippines trench is 10540 meters deep. 

In his 1949 book van Bemmelen wrote: "Vertical oscillations of large blocks of the earth's crust, attaining a diameter of thousands of kilometers, may cause the subsidence of such blocks to oceanic depths or their uplift to high continental plateau. Such epeirogenetic movements are of another type and of larger extent than the crustal waves or "Plis de fond" which form mountain ranges and adjoining deeps. The latter are the expression of the process of mountain building or orogenesis in a stricter sense. Both, epeirogeny and orogeny are the effect of the endogenic forces of the crust. Both are at present active in the Indian Archipelago, causing actual rising and sinking movements, which are accompanied by normal and deep-focus earthquakes, anomalies of the isostatic equilibrium, and volcanic activity."

The plate tectonics concept, which was introduced in late 1950's (after the publication of van Bemmelen's book) suggests that these oceanic basins developed at the plate margins. The plate movement generate collisions and the oceanic plates in this region subducted underneath other plates. These subductions oceanic plates generated low reliefs. In many parts the collisions are still active and generate earth quakes (Fig. 1.6).The positive relief in Himalaya is caused by a collision of Indian-Australian continent and Asian continent.

SEISMICITY

Fig. 1.6. Epicentre distribution in Indonesia (source: USGS)

The mountain belts skirting the continental Sunda and Sahul blocks belong to the most seismic areas of the world. In Indonesia about 500 earth- quakes per year are registered (Fig. 1.6). The deep-focus shocks in the Flores Sea are the deepest of their kind (-720 km).

In 2004 a major earth quake in the Indian Ocean, close to Aceh has killed at least 280,000 people. Further detail of this incident is available in wikipedia (Click here for link)

GRAVITY ANOMALIES

Fig. 1.7. Gravity anomaly map of Indonesia by Sandwell and Smith (2009)

The rising outer arc of the circum-Sunda Mountain System is underlain by an uncompensated rnountain root, causing considerable negative isostatic anomalies (the negative anomaly belt of VENING MEINESZ). The anomaly found between Sulawesi and Halmaheira (-204 millidyne after application of VENING MEINESZ' method of regional isostatic reduction) is the largest isostatic gravity anomaly thus far known on the world. Figure 1.7 shows the gravity anomaly in the SE Asian archipelago.

VOLCANISM

Fig. 1.8. Major volcanoes of Indonesia (USGS, 2001)

The inner arcs of the orogenic systems in the Indian Archipelago are characterized by strong volcanic activity. The number of active volcanic centres in this area (at least 177) is greater than in any other coherent volcanic region of the world (Fig. 1.8). For more than twenty years the Netherlands Indies Volcanological Survey has systematically collected data on this orogenic volcanism. The Directorate of Vulcanology in Bandung is now monitoring the volcanoes and their observations could be found in their website. Its organization was unique, using Indonesian volcano-observers on permanent observation posts equipped with concrete refuge tunnels. Volcanic activity occurred in all stages of the geological evolution of this area. For the older stages the hypabyssal and abyssal intrusions of igneous rocks can be studied.

CRUSTAL RELIEF

The differences of altitude between the mountain ranges and the adjacent deep sea troughs in the present orogenic belts are enormous. The Emden Deep of -10,830 m in the Philippine trough is the greatest sea-depth ever measured. The Wilhelmina Range in New Guinea, now called Jayawijaya Mountain (with the Carstensz Summit of + 5,030 m, and it is called Puncak Jaya now), rises into the zone of perennial snow in this equatorial area.

STRATIGRAPHY AND PALEONTOLOGY

The study of the stratigraphy in this area has provided many interesting results. All types of facies are encountered, ranging from continental deposits to abyssal sediments. Quick changes of facies occur in the vertical section as well as in the horizontal distribution. The sedimental columns of the tertiary - (idio- ) geosynclines attain stupendous thicknesses (up to 10,000-15,000 m). The facies of the sediments reflects the differential vertical oscillations of the earth's crust, which were partly very rapid (orogenesis), partly slow (epeiro- genesis). Igneous intrusions have in places penetrated into these sediments changing their texture and composition, often to such a degree that the exogenous origin of the deposits becomes practically unrecognizable. The crystalline schists of the basement complex are often poly-metamorphic rocks, which were subjected to more than one cycle of mountain building. In some areas, tertiary rocks have already attained a phyllitic appearance.

The fossil faunae and florae have been described in numerous paleontological publications by a great many international specialists, thus greatly advancing this branch of science. We can mention the faunae of Foraminifera (DOUVILLE, RUTTEN, TAN SIN HOK, VAN DER VLERK, UMBGROVE, LEROY, etc.), Mollusca (MARTIN, OOSTINGH, BEETS, VAN REGTEREN ALTENA, etc.), Corals (UMBGROVE, etc.), Vertebrates (DUBOIS, VON KOENIGSWALD, HOOIJER, etc.), the permian and mesozoic faunae of Timor, Misool, Ceram, Buru, etc. (WANNER, and many others), the permo-carboniferous flora of Sumatra and New Guinea (JONGMANS).

CONCLUSION

It appears that the endogenic forces were extremely active in these areas since the oldest traces of its history. Moreover, at present the orogenesis still is in full swing in the crustal tracts between Asia and Australia.

Therefore, the Indian Archipelago is an extremely favourable object for the study of the tecto-genesis in relation with allied endogenic phenomena, like igneous activity (volcanism in its wider sense), seismicity, and isostatic anomalies.

It is to be expected that most branches of geological science will be advanced by the work done in this area. The East Indies are an important touchstone for conceptions on the fundamental problems of the geological evolution of our planet, as has been pointed out, for instance, by CLOOS in his book "Einfiihrung in die Geologie" (1936, p. 473).

1.4. FAUNA AND FLORA

Much work on our knowledge of the flora of the East Indies has been done in the past decades. A review of this work has been written by LAM (1948). Also many studies on the recent faunae appeared, as appears from the article by DE BEAUFORT (1948) in the report of the scientific work done in the Netherlands on behalf of the Dutch overseas territories in the period 1918-1943.

The fact that the Malay Archipelago separates the Australian continent from the Asiatic territory makes it a favourable object for the study of faunal migrations.

As A. R. WALLACE stated in his classical essay of 1860 (which laid the foundation for the modern science of zoogeography): "The western and eastern islands of the Archipelago belong to regions more distinct and contrasted than any other of the great zoological divisions of the globe. South America and Africa, separated by the Atlantic, do not differ so widely as Asia and Australia".

There is much truth in this statement. The boundary line between both faunal realms, known as "Wallace's line", has since been much criticized as well as defended. Of the more comprehensive zoogeographic publications on the East Indies we might mention the books by DE BEAUFORT (1926) and RENSCH (1936), and the symposium by SCRIVENOR et al. (1943). Some other recent papers were written by ZEUNER (1942, 1943) and MAYR (1944 a&b). In relation with the faunistic boundaries in this Archipelago, MAYR (1944 a) arrives at the following conclusions:

1. Wallace's line is not the boundary between the Indo-Malayan and the Australian Regions, but it rather indicates the edge of the area (Sunda Shelf) that was dry at the height of the pleistocene glaciations.

2. The equivalent line along the edge of the Sahul Shelf separates New Guinea and the Aru Islands from the Moluccas and Kai Islands.

3. Weber's line separates the islands in the West on which the Indo-Malayan element is predominant from the islands in the East on which the Australo-Papuan element has a numerical superiority.

Fig. 1.10. Zoogeographic border lines in the Malay Archipelago.

These zoogeographic border lines are marked on the map (fig. 1.10) which also shows clearly the continental shelves as shaded areas.

Besides the migration of the faunal elements, also the spreading of the plant species presents many interesting problems.

In this connection we might mention the papers of BACKER (1929), DOCTERS VAN LEEUWEN (1936), ERNST (1934) who studied the returning fauna and flora of Krakatau; JONGMANS & GOTHAN (1935), and JONGMANS (1940 & 1941) who made important contributions to our knowledge of the late-paleozoic flora in the East Indies; MUSPER (1938 b, 1939 b) who studied the stratigraphy of tertiary fossil woods; POSTHUMUS (1945) on the paleobotanical research in the Netherlands Indies; VAN STEENIS (1934/1936) on the origin of the Malaysian mountain-flora.

A synopsis of some important books on pure and applied botany in Malaysia, which appeared in the period 1921-1939, has been given by VAN STEENIS (1939).

The present flora of the Indian Archipelago is estimated to comprise at least 24000 species of flowering plants, belonging to circa 2200 genera.

No comprehensive treatments on this huge flora have thus far been published. The dozens of theories advanced for an explanation of the plant geography of the Archipelago in the static sense (floristics) and the dynamic sense (history and origin) have not been based on a complete survey of the flora. A first attempt consisting of an analysis of the complete flora, based on the statistics of the genera, led VAN STEENIS (1948) to a delimitation of the area and a distiction of provinces and districts. This paper is preliminary to a full treatment of the floristics in volume 3 of the forthcoming Flora Malesiana (see VAN STEENIS, 1947).

1.5. CLIMATE

In the past decades many important papers have appeared on the climate and meteorology, especially by the staff of the Royal Magnetic and Meteorological Observatory at Batavia (now Jakarta). A number of articles were also written in Europe by W. VAN BEMMELEN, B. BRAAK, S. W. VISSER, and E. VAN EVERDINGEN (see review by BRAAK, 1948).

The Indian Archipelago lies completely between the tropics and within the Indo-Australian monsoon region, which is characterized by high temperatures, high humidity, and abundant rains. The average sunshine is about 50-70 % in the coastal plains.

Fig. 1.11. Rain fall distribution in Indonesia (source BMG)

Due to the influence of the continents of Asia and Australia it is the most typical monsoon region in the world. Figure. 1.11 shows Badan Meteorologi, Klimatologi dan Geofisika (Indonesian Agencey for Meteorology, Climatology and Geophysic)  rain fall distribution in Indonesia. D. Kirono has provided the average seasonal rain distribution map of Indonesia from 1979 to 2001 (Fig. 1.12).

The Philippine Islands are often struck by typhoons. These are cyclones revolving counter clockwise, which form over the Pacific Ocean, as a rule east of the Ladrone Islands. Of those passing across the Philippine Archipelago, practically all occur North of Mindanao, and most of them strike Luzon.

In the southeastern part of the Indian Archipelago the climate is drier due to the influence of the Australian winter anticyclone. Therefore, a savanne landscape prevails in the eastern part of the Lesser Sunda Islands. See fig. 1.12.

Elsewhere the Archipelago is covered by dense forests. Part of these forests have been destroyed or replaced by man. But even on Java, with a mean population density of many hundreds pro square kilometre, still 20-30 % of the surface is covered with forests. In the sparsely populated eastern part of Borneo forests cover more than 80 % of the land.

Fig. 1.12 Seasonal changes of rain distribution in SE Asia from 1979-2001 (D. Kirono, Pers Comm, based on the CMAP data set of Xie et al 2003).

On the average two thirds of the Malayan Archipelago is covered by forests. See fig. 8. The mean annual temperature at sea level is slightly above 26° C (78.8° F) and the mean humidity is 80 %. The high humidity coupled with moderate heat is oppressive, but the heat is often tempered by breezes and the living conditions are greatly ameliorated thereby. Of the larger dwelling places along the coast, Surabaya has an average temperature of 26.4° C (79° F) and Manila of 26.6° C (79.9° F), which classes these cities as the hottest. The averages of some other coastal places are 26.2° C (79.2° F) for Batavia, 25.8° C (78.4 ° F) for Makassar, and Menado, 25.r C (78.3° F) for Balikpapan, and 25.2° C (77.4 ° F) for Medan. The mean temperature of dwelling places in the mountains more inland is considerably lower, e.g. at Bandung, at 730 m above sealevel, it is 22.10 C (71.8° F) and at Tosari, at an altitude of 1,735 m, only 15.9°C (60.6° F), these climates being more temperate. The decrease of temperature is from 5t to 60 C (10-11° F) for a rise of 1000 m.

Comparing the climatic living conditions of Indonesia with those in the neighbouring countries, BRAAK (1929) arrives at the following conclusion:

"Although the heat of the coast plains is far from pleasant, yet the climate compares favorably with that of the neighboring areas at a greater distance from the equator. As a matter of fact, the mean annual temperature decreases as the latitude increases, but the favourable effect of the cooler winter months is more than counter balanced by the unbearable heat of the hottest summer months. In this case a more equable temperature distribution over the year is better than the more usually praised variety. We may conclude from the wet-bulb temperatures that there exists on both sides of the equator a zone with more oppressive weather in the hottest month than is found on the equator. The following figures, which represent the mean wetbulb temperature in the hottest month, may serve as a proof (in degrees of Celsius): Jakarta 24.4, Shanghai 24.8, Manila 25.2, Hongkong 25.4, Port Darwin 25.4, Nhatrang (Annam) 25.8, Bombay 25.9, Madras 26,2, Calcutta 26.4, Lahore 26.6, Hanoi 26.9. Whereas it Jakarta the maximum heat, although disagreeable, can be endured without too much discomfort, the same cannot be said of most other places. At Calcutta, for instance, the climate is almost unbearable at the most oppressive time of the year".

The 1934 rain-gauge statistics for the Indonesia shows the majority of the recorded annual rainfall to be more than 2000 mm:

Palu in the Moluccas has the lowest average rainfall (557 mm per annum) and Tenjo in Central Java the highest (7,026 mm per annum).

The average annual rainfall for the Philippine Archipelago is 2,366 millimeters (94.6 inches). The greatest annual rainfall, 9,038.3 mm, was recorded at the Baguio weather station in the highlands of Luzon, in 1911. The greatest rainfall at Baguio for a single period of 24 hours was 1,168.1 mm (46 inches) (SMITH, 1924, p. 35).

Tropical rains, generally torrential, though mostly of short duration, are of geological importance. These, in combination with the high temperature and high humidity, cause rapid weathering of rocks, resulting in a denudation which is much more effective than in more tempered climatic zones (BEHRMANN, 1921; SAPPER, 1935).

1.6. DENUDATION

The main factors, responsible for a rapid denudation, are the tropical climate and the active process of mountain building. The high temperature and high humidity cause rapid weathering of a chemical character, whilst torrential rains cause leaching and surface erosion. The denudation is promoted by uplift of mountains and/or unconsolidated sedimentaries. L. M. R. RUTTEN (1917, 1938) collected some data on rivers of Java and Sumatra and found an annual denudation considerably above similar figures for rivers of Europe and North America

Many drainage basins show values of over 1 mm per annum 1), and in one case, that of the Pengaron River near Semarang, it amounts to 4 mm. The drainage area of the Pengaron is only 40 sq km. Here a mean denudation of 1 mm in one day has been calculated, corresponding with the denudation by the Marne in two centuries. Such areas on Java with excessive erosion are called "stervende landen" (dying lands). Recently VAN DUK & VOGELZANG (1948) have published some.i data on one of these dying lands, the Tjilutung drainage basin on the SW slope of the Tjarerne volcano in West Java. Measurements on the erosion were carried out in 1911/1912 and 1934/1935:

It appears that the gradually increasing deforestation, reckless cultural methods, and pasturing after 1917 caused doubled soil erosion. Under the conditions now prevailing and calculated over the whole area, a soil layer of 10 em depth is removed in about 50 years for the entire area. However, the erosion is almost exclusively confined to the most erodable soils from the Miocene marly clays. According to the values given by RUTTEN (1917), it may be presumed that the rate of erosion on these soils surpasses that on volcanic soils by about ten times.

The Cilutung basin consists for 34 % of quaternary volcanic rocks, 60 % of Miocene breccias, sandstones, and marly clays (VERBEEK'S m₁-Formation), and 6 % of creeping Miocene argillaceous marls (VERBEEK'S m₂- Formation).

It may be safely estimated that 90 percent of the eroded material originates from the Miocene deposits, covering nearly 2/3 of the total area. Hence under' conditions prevailing at present a loss of arable soil of 10 em depth is to be registered here in about 35 years. The quantity of bed load in the river has not been determined, so that the calculated rates of erosion are surely not too high.

Exceptionally strong floods cause an excessive devastation of the land, as occurred during the floods on Java in 1861 (KLINKERT, 1917). Heavy showers are about 60 times more numerous on Java than in Germany, and 11 times more numerous than in the most rainy southeastern part of the United States of North America, according to VAN KOOTEN (1927, see COSTER 1938, p. 459).

Therefore, the maximum water transport of the rivers on Java, and especially of the smaller rivers, is very much greater than elsewhere in the world, where the rainfall is less intensive.

The minimum flow-off in the dry seasons is strongly influenced by the geological formations. ROESSEL(1941)pointed out, that the "forest-sponge" theory is no longer up to date. This theory for the regulation of the water run-off in drainage basins has long been advocated by foresters as an argument for the preservation of protective forests in the catchment areas. The forests and the vegetation in general are certainly of importance for the maximum run-off after heavy rainshowers, but the minimum flow-off depends in the first place on the permeability of the underground and the infiltration-capacity of the surface. ROESSEL found no clear relation between the minimum run-off in the dry season on the one hand, and the percentage of protective forests in the catchment area on the other for several drainage basins in the young vol- canic area of the Andjasmoro Mts in East Java. On the other hand, there is a conspicuous difference in the minimum run-off of the dry seasons between the young volcanic areas and the young-tertiary marl areas of Java. Many of them have completely dry rivers in the dry season, and this is independant of the fact whether or not forests are present. This shows that there is no direct relation between forests and drought in the dry season, but the relation between geological formation and drought is evident.

Fig. 1.13. Comparison of SE Asian rivers (with red underline) and other rivers in the world.

Some data on the denudation in the Philippines are given by FELICIANO & CRUZ (1933). The Angel River, NE of Manila has a drainage area of 732 sq km and an average run-off amounting to 83,631 cb m per second. Near Matictic (Prov. of Bulacan) it transports yearly approximately 5,343,610 tons of load and dissolved matter into the ocean. This means a denudation of nearly 3 mm per annum, if the average rock density is taken at 2.5.

Coleman and Huh from Lousiana State University did a comparison of world river systems, including a number of rivers from Southeast Asia. Fig. 1.13 shows examples of the graphs they prepared on river length and average annual discharge. The Southeast Asian rivers are generally have small catchment areas and relatively short. The size of islands limit the river system. Mekong River is an exception as it is located in the Asian continent. On the other hand Chao Praya, which is also in the Asian continent is relatively small as it is controlled by fault zones.

LIXIVIATION OF THE SOIL

The heavy rainfall of 1 to 7 metres per year, which is particularly characteristic for the Indian Archipelago, strongly affects the soil and, consequently, the vegetation. For the abundance of rain water not only wets the soil, but most distinctly leaches it at the same time. All substances that are soluble in water, however slight the solubility may be, are dissolved in the long run. They are carried away into deeper levels and to springs, and thence to rivers and the sea.

This process also takes place in the very damp areas of the temperate zones, but there it works more slowly; firstly, because the rainfalls are less, and secondly, because the temperature is lower, a condition which greatly decreases solubility.

Among these soluble substances are those which serve to feed the vegetation. Hence the soil in these tropical regions is constantly being impoverished, a fact which has been stressed by MOHR in numerous publications. Finally, real laterites are formed on which vegetable growth is nearly impossible, like the aluminous laterites of Bintan, described by the author (1940 e).

REJUVENATION OF THE SOIL

Fortunately there is a number of factors which greatly, in some cases vey greatly, retards the process towards this fatal end, or even vey largely prevent its accomplishment. In the lowlands the silt of water floods may enrich the soils. But this means only a postponement or prevention of complete exhaustion. There is, however, pone radical factor which may at any time bring about a fun damental change in the whole situation, namely, the action of young volcanoes, ejecting great quantities of ashes, sand and stones over the surrounding country.

At first everything in the immediate neighbour- hood of the centre of eruption, on the slopes of the mountain, is in ruins, buried under all those ejecta. But it is surprising how quickly a new surface becomes covered with a fresh mantle of vegetation. This fact was noted in connection with the eruption of Krakatau in 1883 (BACKER, 1929; DOCTERS VAN LEEUWEN, 1936), and those of the Kelud in 1902 and 1919. If there is no immediate recurrence of the eruption, the new soil remains extraordinarily fertile for centuries, to be finally subjected once more to gradual impoverishment as a result of leaching by tropical rains.

MOHR'S opinion seems to be somewhat pessimistic, because not only leaching out of the soils occurs, buyt on the other hand erosion brings continuously fresh rocks within the reach of the process of weathering and soil formation. The active mountain building in this archipelago creates considerable relief, so that hypabyssal and plutonic intrusions are exposed by erosion, the mineral content of which supplies new feeding substances for vegetable growth.

Nevertheless, there is such a close relation bet ween the presence of young volcanoes and the density of the population, as has been pointed out for instance by MOHR (1938 b) that the process of lixiviation of the soil occurs apparently at a greater rate than its rejuvenation by the exposure of fresh rocks. From a human point of view the volcanic activity is the most important factor for soil rejuvenation. The population density varies from less than 1 to more than 1000 souls per sq km. In other words, the differences are enormous. According to the Census of 1930 in theIndonesia, the average density of population was 31.89. For Java, with its numerous volcanoes, it amounted to 316.11 and for Borneo, where not a single active volcano is known, it was only 4.02. 1)

DENUDATION AND MOUNTAIN RELIEF

The very high rate of denudation and baseleveling in the Indian Archipelago is confirmed by the study of the geological sections through young mountain ranges, which have been elevated in plio-pleistocene time. In some instances thousands of metres have already been removed by the combined effect of gravity flow (viz. creep) and surface erosion. Consequently these young ranges were already more or less baseleveled during their elevation ("Primare Rumpfflache" in the sense of W. PENCK). Many such young peneplains, highly dissected by the rejuvenated erosion, showing narrow divides and numerous gullies, are to be found in extensive areas of the Indian Archipelago. 2) In Sulawesi they reach altitudes of well above 2000 m, and in Ceram to about 1000-1200 m. PANNEKOEK (1946) gave a morphological analysis of the changes in the pliocene peneplain of SW Java by the rejuvenated erosion due to the uplift and tilting of the crustal block of the Southern Mountains.

The young volcanic cones represent another typical feature of the East Indian landscape. What would be left of the charming and grandiose landscapes of Sumatra, Java, Bali, and Lombok, without their imposing volcanic cones? These are very young structures, often built upon pleistocene plains. Only the very active volcanoes show superb conical outlines (Merapi, Semeru, Mayon). When the activity decreases they are quickly worn down by erosion. Older quaternary, extinct volcanoes are at present mere ruins; neogene volcanoes are mostly reduced to their very basement, exposing the feeding stocks and other hypabyssal intrusions. Therefore, the volcanoes are indeed only an ephemeral feature of the landscape.

The strong denudation in this tropical area, removing and impoverishing the soils, forms a social problem of great importance. It appears that the terracing of the wet rice-fields provides an ideal protection against erosion and floods, seeing that each rice field (sawah square) forms a water reservoir capable of absorbing a considerable rainfall before overflowing.

DENUDATION AND VEGETATION

It is thought necessary to preserve a forest cover in the mountainous catchment areas of the rivers in order to protect the lowlands against floods from the torrential rains ("bandjirs"), and to prevent excessive erosion of the soil.

The experimental station for forestry at Buitenzorg (W Java) has studied the influence of forests on the hydrology and erosion (DE HAAN, 1933 & 1936; COSTER, 1938).

From all observations the main fact stands out that the run-off and the erosion are determined in the main by one supreme factor, namely the extent to which the mineral soil lies bare. According to COSTER (1938) the surface run-off of the rainwater is small on a good forest soil (less than 1-2 %) and there is practically no surface erosion. Upon removing the vegetable growth the surface run-off increases to 30-50 %, and the erosion to 5-12 kg/sq m/year. On loose sandy ashes of volcanoes the erosion may assume catastrophic proportions. Comparing the denudation on Java with that of the Alps in Europe, DE HAAN (1936) arrives at the following conclusion:

Fig. 1. 14. Forest distribution of Southeasian Archipelago (University of Maryland).

"The factors which influence stream flow and erosion on Java are quite different from those in the Alps. In a volcanic area, such as Java, the slopes of the mountains are not steep and diminish gradually, so that here we do not find the erosion cones ("Schutthalden") which are so common in the Alps. Furthermore, in the tropics with a high and equable temperature and heavy rainfall, chemical decay is predominant against mechanical decay in the Alps. This results in a more or less thick layer of soil in the mountains of Java against a thin layer of soil or even bare rocks in the higher mountain regions above the tree limit in Europe.

The vegetation in the tropics is dense and nearly unbroken, the types of vegetation are very varied, and agriculture is possible up to a great altitude. In the Alps the forests do not mount above 2000 m; the higher regions are covered with grass, stones, snow, and glaciers. The flow of a glacier river is determined by the melting of snow and ice.

The flow of a river from the middle mountains is of a mixed type, influenced by the melting of snow and by rainfall. With the first type the vegetation will be of no consequence, with the second one its influence will be greater. The rivers on Java belong to the pure rain type, a third type, where the vegetation in the basins influences greatly the stream flow.

Inconsequence of the greater amount of precipitation and the thicker layers of soils, the minimal flow of the rivers on Java is much greater than that of Alpine rivers. But the maximal flow also is higher on Java than in the Alps, because of the long periods of heavy rainshowers.

In the Alps the rivers transport mostly coarse material, gravel and stones; on Java mostly sand and mud. These fine particles can not be chequed by engineering works ("Wildbach- Verbauung"). The only practical means to combat this kind of erosion is by keeping the soil-cover closed and dense, or by reafforestation. Only in exceptional cases technical works may be of value.

As the irrigation of the agricultural crops on Java depends on the local rivers and streams (irrigation water can not be transported .easily), a sufficient care of the agricultural land in the plains requires good and dense vegetation in the mountains all over the island. In general we may expect that the soil cover in the tropics has a greater effect on stream-flow and erosion than in the Alps. Engineering works may be useful, but do not stand on the first plan."

DENUDATION BY CREEP

The presence of a thick cover of weathered rock drenched with water causes also a considerable creep of the soils towards the floors of the valleys. STAUBER (1944) draws attention to the fact that also in the Alps the engineering works ("Wildbach- Verbauung"), which have cost in the past century more than 200 million Swiss francs, have had little effect on the gravity movements of the cover of detrital matter, which blankets the lower parts of the mountain slopes (land-slides, earth glaciers and mud flows). The detrital material of the mountain flanks often slides in large portions into the ravines, and are thence removed by river erosion.

The humid tropical climate of the Indian Archipelago is responsible for the formation of a deep mantle of disintegrated rocks up to high on the mountains. Moreover, the active process of mountain building has created considerable relief. It is clear that the combination of both factors highly promotes the occurrence of hillside and mountainside creep, which is accompanied by frequent landslides, cold "lahars" (mud flows), and the like.

This kind of denudation is quantitatively much more effective than mere surface erosion, the value of which is estimated by such experiments as has been made on Java by the Forestry Station. 

In the Karangkobar area of Central Java, where the core of the mountains consists of neogene mudstones, the creep of the surface layers is so strong that the sawah fields have to be reparceled from time to time (HARLOFF, 1930, VAN BEMMELEN, 1937 d). In the tin-islands, Bangka and Billiton, the process of creep has been studied in relation with the formation of the "Kulit" and "Kaksa" ores (ADAM, 1932-1933).

This hillside creep towards the valley floors is also very effective in those instances, where unconsolidated sediments are folded up or elevated above the local erosion base. In such cases it is not the disintegrated rock formation, but the primary deposit, yet unaffected by exogenic weathering, that is subjected to gravity flow.

In the chapter on the geological evolution of the regional units it will be demonstrated that this gravity flow is of utmost importance for the restoration of gravitational balance (secondary tectogenesis).

There is a gradual transition in the quantity of mass transport from the process of surface erosion, via hillside and mountain side creep, to the gravitational extension and spreading of elevated areas. The difference between both is that the process of denudation (by surface erosion and creep) is influenced by climatic factors, whereas gravitational tectogenesis depends only on the field of gravitational stress-gradients created by differential vertical movements and the physical properties of the elevated formations.

DENUDATION AND GEOLOGICAL FORMATIONS

Last but not least, there is of course a close relation between the rate of denudation and the geological formations. The rate of denudation will be much more rapid in unconsolidated sediments which are subjected to the erosion due to the process of mountain building, than in exposures of solid igneous rocks or those of the crystalline basement complex. RAVEN (1944) is of the opinion that the rate of denudation is approximately twenty times more rapid for the former than for the latter. This he considers is a conservative (low) estimate.

THE HUMAN STRUGGLE AGAINST DENUDATION

The process of erosion has far reaching cons quences for agriculture, forestry, cattle breeding, various types of civil engineering, particularly for irrigation and transport. Therefore, the knowledge of the causes, results, and methods of fighting erosion are of great cultural importance. In view of this the Government of the Netherlands Indies sent in 1946 a special commission of scientists to the United States of America to study the modem methods of combating erosion in that country. The original report of that commission, which consisted of nine members, has been summarized by VAN BAREN and was issued by the Department of Economic Affairs in 1947.

1.7. SOIL SCIENCE AND SEA SEDIMENTS

Though not belonging to the scope of this work, something has to be said about the study of the soils in Indonesia. A few decades ago private and government experimental stations for agriculture were established. The publications of these experimental stations give us an uninterrupted picture of the stages of development of scientific and practical soil science in the Netherlands Indies. M. TREUB founded in 1905 the Laboratory for Agrogeology and Soil Research. Its task was to become the link between geology in the widest sence of the word on the one hand, and of agriculture on the other.

Fig. 1.15. Asia soil map by ISRIC, 1997

A review on the development of the tropical soil science in the period 1918-1943 was recently published by MOHR (1948).

An excellent treatise on the soils of the East Indies has been written in 1933-1938 by Prof. Dr E. C. J. MOHR, the nestor of the pedologists who worked in the Netherlands Indies. Recently EDEL- MAN (1941) published a book on this subject with an extensive list of literature references. Further might be mentioned the work of VAN HEURN  14  PHYSIOGRAPHY   (1923) and DRUIF (1932-1934) on the soils of Sumatra's Eastcoast; that of SZEMIAN (1929/30 a, 1930) and IDENBURG (1937) on the pedological survey of South Sumatra, besides the pedological notes by SZEMIAN, accompanying the explanations of sheet 3 (Bengkunat) of the 1 : 200,000 geological map of S-Sumatra, and the sheets 30 (Purwakarta), 36 (Bandung), and 58 (Bumiaju) of the 1 : 100,000 geological map of Java.

There have been some discussions whether the pedological surveys were to be conducted as a branch of the geological survey, or by the agrogeological laboratory (VAN BEMMELEN, 1928 b&c; DE IONGH, 1929/30, SZEMIAN 1929/30 b, WHITE, 1930; OPPENOORTH, 1930; discussions "Algemeen Landbouw Weekblad" 15, 1930, by WHITE, DE IONGH, DEN BERGER, REITSEMA, SCHEIBENER, BOTHE, Roos, BERNARD).

In the decade before the outbreak of the war with Japan the general survey has been done by the Agrogeological Institute ("Bodemkundig Instituut") at Buitenzorg (Bogor). The privately financed experimental stations for sugar, coffee, tobacco, etc. had their own pedologists for the more local researches of the soil. The institute is now called Institut Pertanian Bogor (IPB = Agricultural Institute of Bogor).

Beside the survey of the soils of the islands, we may also mention in this paragraph the research of sea sediments. A map of the sediments in the Java Sea has been composed by MOHR (1919). The bottom samples of the Moluccan Seas, collected by the Snellius Expedition in 1929-1930, have been studied by NEEB (1943). See fig. 10.

MYERS (1945) wrote an article on the sediments of the Java Sea and their significance in relation to stratigraphic and petroleum geology.

In 1948 the Swedish deep sea expedition of the Albatros under the leadership of H. PETTERSON visited the East Indies. This expedition collected samples of sea-sediments of 20 metres depth. The profiles obtained by the Snellius expedition were 2.5 metres deep, which was a record depth at that time. Also seismic measurements of the thicknesses of the sediments at the sea floor will be made. So the results of the Albatros expedition, which at present are not yet available, will greatly augment our knowledge of the history of the sea floors (HARDENBERG, 1948).

1.8. MAJOR PHYSIOGRAPHIC DIVISIONS

The major relief features of the Indian Archipelago are fundamental for its division into regional physiographic units. There can be distinguished a partly submerged land mass in the West, the Sunda Shelf area, and the partly submerged northern extension of the Australian Continent in the East, the Sahul Shelf area. These are separated by an intervening belt of deep-sea basins and island-festoons.

The shelf seas are generally less than 100 m deep, although the edges of the shelves are indicated on the map by the 200 m isobath, as is common use. The islands emerging from the shelf seas are mostly less than 1000 m high.

These shelf seas largely are old peneplains, which are only gently warped by later epeirogenic movements, being more or less stable land masses with low seismicity, low isostatic gravity anomalies and no active volcanoes.

During the tertiary cycle of mountain building the marginal parts of the Sunda Shelf area have subsided considerably. In these marginal troughs thousands of metres of sediments have accumulated, on which are located the productive oilfields of NW and E-Borneo, N-Java, and E-Sumatra.

There can be distinguished in the Sunda Shelf area, an old central land mass (comprising the Malay Peninsula, the Riau-Lingga Archipelago, Bangka, Billiton, Karimondjawa Islands, Karimata Islands, Tambelan Islands, Anambas Islands, Natuna Islands, and the western part of Borneo) and more unstable marginal parts, which have been subjected to the tertiary cycle of mountain building (the remaining part of Borneo, Bawean Island, Java and Madura, Sumatra). As the latter are physiographically connected with the Sunda Shelf area, their physiographic description will be given under that heading; but, geologically, they belong to the circum-Sunda Mountain System, to be discussed hereafter.

The Sahul Shelf area comprises the Arafura Shelf Sea, the Aru Islands and the southern part of Papua (Merauke swell). Perhaps also the shelf-sea, extending West of the "Vogelkop" (Birds- head) to Misool, may be assigned to it. North of the Merauke ridge the pre-tertiary basement complex plunges down under the tertiary geosyncline of New Guinea, which forms a part of the circum- Australian Mountain System.

SUBMERGED BORDERLANDS

The crustal blocks of SE-Asia (i.e. Sunda) and of NW-Australia (i.e. Sahul) have a mean elevation corresponding more or less with the sealevel. The bordering parts of the Pacific and Indian Ocean are considered by the author to be crustal blocks of former border lands, which have subsided to oceanic depths, now forming the China Basin, the Philippine Basin, the Carolinan Basin, at the north- eastern (Pacific) side of the Archipelago, and the Indo-Australian Basin, at its southwestern (Indian) side. The floors of these basins are rather level, showing differences in depth which are generally less than 1000 m. The floor of the China Basin is at about 4000 m depth, and that of the Philippine Basin at 5000-6000 m; the Carolinan Basin, forming the northern part of Melanesia, is about 4000 m deep.

The part of the Indo-Australian Basin, extending between the Cocos or Keeling Islands and Australia, is 5000-6000 m deep, whilst the part of it extending from the fore-mentioned islands north- east- and northward to the Gulf of Bengal gradually shoals in that direction from 5000 to 3000 m.

There are several geological and geophysical reasons for supposing that these crustal blocks have been above sealevel in pretertiary times, forming-parts of the Asiatic continent and of the former Gondwana continent. They subsided, later on, to oceanic depths, but the discussion thereof is outside the scope of this chapter (See chapter IV). These differential vertical movements of such extensive crustal blocks, measuring thousands of kilometres across, are major geotectonic movements separated by very long phases of relative stability. They are generally described as epeiro- genic movements. In this book they will be called "geoundations". For the description of the present physiographic situation they can be considered as more or less stable crustal parts, lying at various depths with respect to the datum plain which is given by the sealevel.

OROGENIC BELTS

Between these crustal blocks are belts of much stronger relief, characterized by island-festoons or submarine ridges, which are paralleled by deep-sea troughs or trenches. The width of the unstable tract varies from some hundreds of kilo metres at the northern end of the Philippines, to more than two thousand kilometres between central Borneo and the Aru Islands. These belts are zones of active orogenic movements along which ranges have been elevated at times to some thousands of metres above sealevel, whilst the intervening basins sub- sided, reaching depths of 5000 to more than ,10,000 m (-10,830 m in the Philippine Deep).

An important advance in our knowledge of the oceanography of the Indo-Australian Mediterranean has been made by the cruise of the Snellius Expedition in the eastern part of the Netherlands East Indies in 1929-1930. A list of publications concerning the Snellius Expedition till 1943 is published in Vol. V, Geological Results, pp. 266-268 (KUENEN & NEEB, 1943). See also fig. 78 on pl. 8.

The bathymetrical results of the Snellius Expedition include more than 30,000 echo soundings. On these figures is based the large bathymetrical chart designed by the expeditionary staff and published by VAN RIEL (1934). This chart has been copied in varous publications (e.g. the "Atlas van Tropisch Nederland", 1938) and has thus become widely known.

It is, however, not the only bathymetrical chart designed from the depth figures of the Snellius Expedition. The late P. J. B. VAN KESSEL of the Topographical Service at Batavia was of the opinion that the course of the isobaths on the Snellius chart was too much influenced by preconceived ideas about submarine folds. This led him to design a bathymetrical chart based on the same soundings, but eliminating as much as possible any indication of the direction of the folds. The chart constructed by VAN KESsEL in 1933 was published posthumously by PANNE- KOEK (1941). On this chart most of the shallows are circular, even though there may be strong arguments in favour of a shallow being elongated in a certain direction. Because the shallows were rounded and but rarely linked together in the form of longitudinal ridges, the deeper parts of the seas occupy more space than on the Snellius chart.

A. J. PANNEKOEK, who was VAN KESSEL'S successor at the Topographical Service, compared critically both constructions of the isobaths, based on the same depth figures, and came to the following conclusions (1941): "Although, generally speaking, VAN KESSEL's construction of the isobaths seems to be less probable than that of the Snellius Expedition, it is, nevertheless, an interesting piece of work in that it shows up immediately where the Snellius chart may be subject to doubt. For certain areas it may even be said to be an improvement over the Snellius chart. Particular attention is called to the different representations of the central ridge of the Banda basin between Buru and the Tukangbesi Islands, Southeast of Sulawesi."

1.9. PHYSIOGRAPHIC DISTINCTION OF THE MAIN UNITS
THE GROUPING FOLLOWED IN THIS BOOK

As has been pointed out in the preceding .paragraphs, the main physiographic units are: 1) The continental platforms, 2) the oceanic basins (or engulfed borderland), and 3) the orogenic belts. These three groups can be subdivided into a number of smaller physiographic units. Thus we come to the following scheme:

A.            The Sunda area.
a.            The Sunda shelf and smaller islands.
b.            Larger Sunda Islands bordering the Shelf Sea (Borneo, Sumatra, Java and Madura).

B.            The circum-Sunda orogenic belts.
a.            Sin Cowe Reefs in the South China Sea.
b.            The Philippine Archipelago.
c.             Sulawesi.
d.            Moluccas.
d₁. Northern Moluccas.d₂. Southern Moluccas.
e.            Lesser Sunda Islands.
f.             Ridges South of Java and West of Sumatra.
g.            Andamans and Nicobars.

C.            The circum-Australian belt.\
a.            New Guinea.
b.            Sahul Shelf with the Aru Islands.
c.             Christmas Island. This grouping will be followed in this chapter.

From this physiographic description it will appear that the structural relations are somewhat more complicated than this simple scheme suggests. Therefore, in the part, on the regional geology (Chapter V) a slightly different scheme had to be used.

In the first place, Sumatra, Java and Madura, though bordering on the Sunda Shelf, belong almost entirely to the young Sunda Mountain System. Therefore, they will be treated under the heading of the circum-Sunda orogenic belts.

In the second place, a short discussion of the geology of the Malay Peninsula will be necessary for the understanding of the structural relations in the Sunda area.

In the third place, the Aru Islands on the Sahul Shelf do not belong to the young circum-Australian orogenic belts, being a marginal part of the Australian continental block. Consequently, the scheme for the discussion of the regional geology in Chapter V will be:

A. The Sundaland

1.            Sunda Shelf.
2.            Smaller Islands on the Sunda Shelf.
3.            Borneo.
4.            The Malay Peninsula.

B.  The Circum-Sunda orogenic systems

1.            The Philippines.
2.            Northern Moluccas.
3.            Sulawesi.             .
4.            Southern Moluccas (Banda Arcs).
5.            Lesser Sunda Islands.
6.            Java.
7.            Sumatra and the Islands to the West of it.
8.            Andamans and Nicobars.

C. The Circum-Australian orogenic systems

1.            New Guinea.
2.            Christmas Island.
3.           The Sahul area                   a. The Sahul Shelf.                   b. The Aru Islands.

The geological analysis in Chapter V shows that the geotectonic relations are still more complicated.  Beside the continental nuclei of SE Asia and Australia, we can distinguish four main orogenic systems, which meet and interlace in the focal part of the Archipelago, between Borneo and New Guinea. These four major mountain systems are:
1.            The Sunda Mountain System.
2.            The East-Asiatic Arcs.
3.            The Melanesian System.
4.            The circum-Australian System.

A synthesis of the general geotectonic picture will be given at the end of this volume, Chapter VI. FIG. 11. Main physiographic and tectonic outlines of the Indonesian and adjacent archipelagoes.

Fig. 1.16. Simplified tectonic map of SE Asia

Figure 1.16 illustrates the main physiographic and tectonic outlines of the SE Asian Archipelago and its geotectonic relations. The numbers and letters indicate the subchapters in which they are treated. The roman cipher I refers to the chapter on physiography, II to the chapter on stratigraphy, and V to that on the regional geology. The subchapters on stratigraphy, marked on this map, refer only to the discussion of the tertiary stratigraphy, in sofar as this has not been treated in chapter V.

We will now continue with the description of the physiographic features of the various units. It has to be born in mind, that this description is meant as an introduction to the regional geology. Therefore, stress is laid on the structural trendlines which can be derived from the orographic features.