2.SUNDALAND

The Sunda area is the partly submerged south- eastern outgrowth of the Asiatic continent, being connected with it by the Malay Peninsula and the Isthmus of Kra. The so called "Sunda Land" comprises Malay Peninsula, Borneo, Java, Sumatra and the interjacent shallow seas from which emerges a number of smaller islands.
We will first discuss the physiography of this shelf-sea and the smaller islands; thereafter that of the three Larger Sunda Islands: Borneo, Java and Sumatra.

2.1. THE SUNDA SHELF AND THE SMALLER ISLANDS
2.1.1. THE SUNDA SHELF
The Sunda Shelf is a shallow sea, generally less than a hundred meters deep, comprising the Gulf of Thailand, Malacca Strait, the SW part of the South China Sea, the Java Sea and the adjoining southwestern part of the Makassar Strait. With its area of 1.850,000 sq km, it is the most extensive, coherent shelf in the world (K. KRUMMEL, Ozeanographie I, 1907, p. 113). It was EARLE (1845) who more than a century ago drew attention to this extensive submerged flat and to its counter part on the Australian side ("Great Asiatic Bank" and "Great Australian Bank"). About 75 years later, after the results of the Siboga Expedition had become known, MOLENGRAAFF & WEBER (1919) took up again EARLE'S conception. MOLENGRAAFF advanced a number of arguments for the idea that these shelf seas were submerged peneplains, resulting from the marine transgression after the Pleistocene glaciation.
In the relief of the bottom of the Sunda Shelf the course of submerged drainage systems can be distinguished: the river system in the Malacca Strait, the river system in the South China Sea, the river systems in the Java Sea.

Fig. 2.1. Map of Sundaland

The zoogeographic studies of WEBER demonstrated that there is a closer relationship between the fresh water fishes in the present rivers of West Borneo (Kapuas River) and South East Sumatra, than between the Kapuas and the Mahakam River of East Borneo. This proves that the rivers of SE Sumatra and West Borneo have formerly been the tributaries of a large river in the South China Sea. A symposium on the geomorphology of the Java- and Sunda Sea (held at Utrecht in 1944) has been edited by SMIT SIBINGA in 1947 (see BAARTMANS, etc., 1947).
The structural framework of the Sundah Shelf and vicinity was studied by Ben-Avraham (1973) based on several surveys.

2.1.2. THE ISLANDS ON THE SUNDA SHELF
The Sunda Shelf carries a number of islands, which were formerly high parts on the Sunda peneplain. Therefore, they are nearly all rocky islands, often covered by a deep crust of lateric weathering. The arrangement of these islands forms an indication of the major structural trendlines which connect SE Asia with the three Larger Sunda Islands.
The Natuna Islands in the North (total area 1,875 sq km) lie on a belt which points from the western promontory of Borneo (Cape Datuk) northwest- ward to Pulu Condor and Indo China. This rise of the shelf floor is transversely cut by the gullies of the submerged drainage pattern. BOTHE (1928, p. 147) mentions a slight recent elevation of Natuna major or Bunguran, which has rejuvenated the erosion at some places, but it seems also possible that such a rejuvenation has resulted from the recent abrasion along the coasts.
Midai, South of Natuna major, is a young (though extinct) basalt volcano.
The Anambas and Tambelan Islands (total area of 625 sq km) form the next belt with a NW -SE trend. In the chapter on the geological evolution arguments will be advanced that this belt is the central part of the Sunda area, from which orogenic systems have spread to the NE and the SW.
The Riau Archipelago (Bintan and surrounding islands, with a total area of 2,313 sq km) forms the southern extension of Singapore and the eastern part of the Malay Peninsula. The islands are covered in many places by a thick lateritic crust, which is mined in Bintan as bauxite ore. Workable tin ores are absent in the Riouw Archipelago, though cassiterite is occasionally found in alluvial deposits and in contact-metamorphic rocks. This belt, extending from eastern Malaya to the Riouw Archipelago, can be traced to the Karimata Islands off the coast of West Borneo, giving it a slightly convex outline to the SW.
The Lingga Archipelago (Singkep, Lingga, and surrounding islands, with a total area of 2,188 sq km) belongs to the great Tin-belt. This belt extends from the western part of the Malay Peninsula (with the Main Range) via the Lingga Archipelago with Singkep, to Bangka and Billiton. These islands belong to a mountain range which had largely been baseleveled 1) and which was partly abraded. It has been dissected into a great number of islands owing to a rise of the sea in late quaternary time. They represent a drowned topography.
Singkep, Bangka, and Billiton are surrounded by an aureole of submerged river valleys, containing alluvial tin-ores. Bangka and Billiton are the most important tin producing islands. Bangka has an area of 11,340 sq km, being the 19th island in size of the whole Indian Archipelago. The Maras Mts in North Bangka attain a height of 692 m above sealevel and the Pading Mts in the southern part of the island are 654 m high. Billiton measures 4,595 sq km and the two summits in its centre (Tadjem Laki and Tadjem Bini) both attain an altitude of 510 m.
The Karlmundjawa Islands in the Java Sea are a group of rocky islands consisting of pre-tertiary rocks covered by basaltic lava. The basalt occurs also as E- W trending dikes in the basement complex. These islands are situated south of the great Tin-belt. They belong to the southernmost belt of the Sunda landmass, ending at its western side in the massif of crystalline schists and granites in the Lampong Districts of South Sumatra, which is also covered by a shield of young basaltic effusions.
The Island of Bawean in the southeastern part of the Java Sea has quite another character. It is the only island in the Sunda Shelf area which consists of marine tertiary strata and alkaline volcanic rocks. This part of the Java Sea does not belong to the more or less stable Sunda Land, but it has been subjected to tertiary processes of diastrophism. It bears a close resemblance to the Muriah on the North coast of Java. This extinct volcano was also an island, but it has been linked to the mainland in historical time by the silting up of the Semarang- Rembang passage.

2.2. BORNEO
Borneo is the second largest island of the SE Asian Archipelago (736,000 sq km), being slightly larger than France. Sarawak, Labuan, and Sabah belong politically to Malaysia (196,000 sq km), West-, South- and East-Kalimantan are a part of Indonesia (539,500 sq km) and Brunei Darussalam

Fig. 2.2. Topographic map of Borneo

2.2.1. OROGRAPHIC AND HYDROGRAPHIC TRENDLINES
The island has roughly a triangular outline with three small peninsulas at its northeastern side (Mangkalihat Peninsula, and the two promontaries bordering Darvel Bay; Fig. 2.2). It has extensive hilly and mountainous relief which, however, in most localities does not exceed 1500 m. The altitude levels are indicated on the map Fig. 2.2.
This map shows that the orographic trendlines are not as prominent as in most of the other islands. The older maps of Borneo show mountain-ranges which are too high and much too coherent. The divide mountains of the older cartography still haunts the geotectonic conceptions; even the border ranges have often been inferred between the major administrative and political divisions. A broad mountain system traverses the island from the Kinabalu Range in the north (with Mt Kinabalu, 4,095 m, forming the highest summit of the island), via the Iran and the Muller Mts to the Schwaner Mountain (with Bukit Raja, 2,278 m) in. the SW. This complex mountain system forms the main divide of the island, from which other orographic units branch off to the East and West, whilst the North- and South trending Meratus Mountain (with highest summit Besar 1,892 m) in the SE-part of the island has a more isolated position.
Westward branches are:
a) the Upper Kapuas Mts between the Redjang Valley to the North. and the Upper Kapuas Basin and the Batang Lupar Valley to the South. and
b) the Madi plateau between the Upper Kapuas Basin and the Melawi River. The latter branch can be traced farther westward. along an axial depression, which is traversed by the probably antecedent Kapuas River, to the mountain complex in the western promontory formed by the Chinese Districts (highest summit Niut, 1,701 m). This mountain complex is dissolved into a great number of isolated summits. a peculiar topography of residual mountains. The belt curves northwestward via Cape Datu(k) to the submerged ridge in the Sunda Sea. carrying the Natuna Islands. Eastward branches are:
a) mountain system in North Borneo. ending in the peninsulas at both sides of Darvel Bay. and
b) another complex mountain system, ending in the Mangkalihat Peninsula.
The Kapuas River should be distinguished from another Kapuas River, which starts on the other side of the same mountain range in central Borneo, but flows to the south, merging with the Barito River and discharging into the Java Sea.

2.2.2. STRUCTURAL TRENDLINES

Fig. 2.3. SAR Image of Borneo

Ridges lineations onshore Borneo are clearly seen on satelite immageries especially by Synthetic Aperture Radar (SAR) image as shown in Fig. 2.3. The bending of Rajang-Crocker range is almost like the core of the island. Meratus Mountains in the southeast of the island also appear as long continuous ridges. In a detail image, the Samarinda Anticlinorium is also comes out as log ridges in the east of Borneo (Fig. 2.4). Karst topography occur in places like in Mulu (Sarawak) and Sangkulirang-Mangkalihat area in East Borneo. The highest peak of Borneo is Mount Kinabalu in Sabah. A granitic intrusion which reach 4095 meters above sea level.

Fig. 2.4. Samarinda Anticlinorium ridges.

It then appears that Northwest Borneo region forms a part of the major trendlines of the Philippine Archipelago, whilst the bulk of the island belongs to the Sunda structure.
The Palawan festoon ends in the Kinabalu Range and the Sulu festoon in the Darvel Bay area. The NNE-SSW trending Kinabalu Range consists of highly interfolded pre-tertiary and lower tertiary strata which are intruded by the granodiorites of the Kinabalu Massif.
The East-to-West trending ranges North of Darvel Bay consist also of pre-tertiary and lower tertiary rocks. Less folded younger tertiary strata are found on the flanks of these ranges and in the basin between them, which forms the southwestern extension of the Sulu trough.
This complex of Northwest Borneo region has geological affinities to the Philippine trendlines, and it is separated from the mainland of Borneo by a neogene tract, which extends across the island from the Sulawesi Basin, in the East, to Labuan Bay, on the NW coast.
The Sundaic part of the island consists of a triangular continental core in SW-Borneo, which is flanked by the tertiary basin of Sarawak on the one side and the tertiary basins of the Southern and Eastern Divisions of Borneo on the other.
Only the western part of Borneo, comprised by the triangle Müller Mts - Cape Datu(k) - Cape Sam bar is a proper continental mass. It contains at its eastern side the Melawi Basin with Lower Tertiary in brackish-water facies. FEHN (1933) is also of the opinion that only SW -Borneo may be called an old land ("Alte Rumpfebene").
This continental core forms a part of the old Sunda landmass. Its northern boundary is formed by the mountain complexes extending from Cape Datu(k) via Mt Niut and the Madi Plateau to the Milller Mts. Its southern margin is formed by the Schwaner Mts and the low mountainous land extending from there to the Southcoast. Both marginal zones of the old Sunda landmass are, moreover, characterized by volcanic intrusions and extrusions of tertiary age. These tertiary volcanic belts unite in the Muller Mts and extend thence farther northeastward via the Batuajan (1,652 m) to the Kongkemal (2,053 m), ending in the low Latong Mts, West of Tarakan. Near the northern edge of the continental mass of West Borneo Quaternary basalt flows are found around the old Niut- stock, and along its southwestern edge WITKAMP has described some Quaternary, though extinct, volcanoes near Lonigram (Murai, Beluh, Bawang Aso).

Fig. 2.5. Relief of Borneo Island

From the Kongkemal, a complex ridge branches off eastward to the Niapa Mts (1,275 m) and thence the basement complex definitely plunges under the tertiary strata of the Mangkalihat Peninsula.
The Sunda landmass penetrates into Borneo like a huge wedge, which has its base (of 600 km width) along the SW-coast between Cape Datuk and Cape Sambar, and thence it extends northeastward into the island, gradually narrowing. Northeast of the Schwaner Mts it begins to plunge under marine tertiary strata, but exposures of it can be traced farther NE, to the KongkemaI. Thence it tapers out to the Latong Mts in NE-Borneo.
This wedge of pre-tertiary rocks forms the structural backbone of Sundaic Borneo. NW of it lies a great. 1000-2000 m high mountain range, concave to the NW, which comprises the Kapuas Mts and the Iran (or Nieuwenhuis) Mts. This mountain range consists of pre-tertiary and lower tertiary marine rocks which are intensely folded and thrusted to the NW.

Fig. 2.6. Physiographic units of Borneo

It is separated by the Redjang Valley from a ridge, generally less than 1000 m high, which is also concave to the Northwest. This ridge is called in this book the "UIarbulu Ridge". It is an anticlinorium, consisting for the greater part of tertiary strata, and separated from the coast of Sarawak and Brunei by a rather narrow stretch of low hilly land.
Both, Kapuas-Iran Range and Ularbulu Ridge, are tertiary mountain ranges, belonging to the circum-Sunda Mountain System. Southeast and East of the structural backbone of Borneo the pre-tertiary basement complex disappears under the basins of the Southern- and Eastern Division of the island, on which were deposited many thousands of metres of tertiary sediments.
Thebasement complex rises again towards the East coast, lying less deeply buried along the Strait of Makassar, and being exposed again in the small islands of Pulu Laut and Sebuku, off the SE corner of Borneo.
In this marginal tertiary basin of SE- and E-Borneo is situated a SSW-NNE trending median ridge. It begins with the Meratus Mts in the South, which consist largely of pre-tertiary rocks, and joins on to the great Samarinda anticlinorium which separates the lakes district of the Mahakam River from the coast. This Samarinda anticlinorium has an axial depression in the Samarinda section, where it is cut by the antecedent Mahakam River, and thence the axis rises again northward. towards the transverse threshold, formed by the Kongkemal-Niapa-Mangkalihat system. This Meratus-Samarinda Range is the result of the tertiary orogenesis at the southeastern side of the structural backbone of the island. It forms the counterpart of the tertiary mountain ranges of Sarawak at its northwestern side. It appears from this sketch that it is necessary, for a clear insight into the arrangement of the orographic elements of the island, to consider also its paleo-geographic evolution. The reader therefore is referred to the paleo-geographic maps of the tertiary evolution, pictured in fig. 148.

2.2.3. THE THREE LARGEST RIVERS OF BORNEO
The three largest rivers of Borneo are the Kapuas, the Barito, and the Mahakam. The Kapuas source is on the Tjemaru (1,681 m) in the centre of the island. The river flows westward through the Western Division, emptying with some ramifications into the Sunda Sea near Pontianak. It is probably the longest river in Indonesia (1.143 km), being somewhat shorter than the Rhine in Europe (1.320 km). Its upper course crosses. between Putussibau (898 km from the mouth) and Semitau (623 km from the mouth), a marshy lake district, which is an intermontane basin surrounded by the upper Kapuas Mts in the North, the Miiller Mts in the East, the Madi Plateau in the South, and the Kelingkang Mts in the West. After cutting through some East-to-West trending ridges between Semitau and Singkang (469 km from the mouth), it reaches the Melawi basin. From Sekadau (348 km from the mouth) the river flows' through a low mountain land to Tajan (182 km from the mouth). There it reaches its delta. a large, marshy tract of land, with some isolated hills •of Pre-Tertiary. The delta is at present still growing by coastal accretion. This delta has an area of 5,400 sq km, according to FERN (1933, p. 14). Upon reaching the delta the average water supply of the Kapuas is 6000-7000 cbm per second.
The second largest river of Borneo is the Barito, which rises in the Muller Mts, from which it flows 900 km southward. from Muaratewe through the marshy Barito Basin. This basin is framed at its eastern side by the Meratus Range, whilst its broad western part is traversed by several other important rivers, flowing from the Schwaner Mts southward. This western part of the Barito Basin is covered by a rather thin section of tertiary and quaternary strata, which gradually increases in thickness towards the Barito. It is still a rather stable area, underlain by the basement complex of the old Sunda land, which tilts towards the axis of the Barito Basin, being hinged on the old landmass of Western Borneo.
The third largest river is the Mahakam (about 775 km long), which rises, like the Kapuas river, on the Tjemaru in the centre of the island. It cuts through the pre-tertiary axis of the island East of the Batuajan (1.652 m) and then reaches the tertiary basin of Kutei. Its middle course traverses a lowland plain with many marshy lakes. This intermontane depression is seperated from the Barito depression by a broad hilly tract of less than 500 m altitude.
Thereafter, the Mahakam cuts through the Samarinda anticlinorium and reaches, near the oilfield of Sanga-Sanga, its alluvial delta. This delta spreads like a broad fan over the shelf-sea, having a base of 65 km and a radius of about 30 km.

2.3. SUMATRA
This island has an area of 435,000 sq km being about the size of Great Britain. Economically it is the second island in importance, next to Java, and it offers great possibilities for further development.
The island has an elongated form, measuring 1650 km from Kota Radja in the North to Cape Vlakke-Hoek (Tanjung Cina) in the South. Its width amounts to 100-200 km in the northern part and-to about 350 km in the southern part. There are few indentures along the coastline.
The West coast is only slightly indented by the Bay of Tapanuli, whereas along the East coast the larger rivers form wide, but shallow estuaries at their mouths. Only at the southern end of the island, two important bays penetrate about 50 km inland. These are the Lampong Bay (with Teluk Betung and Oosthaven or "East Harbour") and the Semangko Bay (with Kota Agung). Natural harbours are scarce. The West coast is dangerous owing to the ocean breakers. Barus (20 north. lat.) is one of the very few safe landing points, which has been used, according to VOLZ, by the Malayans and Indians in former times. The East coast, along the Strait of Malacca, is safer and, therefore, it was visited in early historic times by sailors coming from India and China.
The heavily silt laden rivers along the shallow East coast have necessitated the dredging of navigation channels for modern ships. The East coast in historical time has shifted outward many kilometres due to deposition by these streams. In the middle ages the sea still entered with large embayments in the East coast, forming, for instance, the large embayment in the vicinity of Palembang. The large marshy islands off the central part of the East coast, such as Rupat, Bengkalis, Padang, Tebingtinggi, Rangsang, are still separated by narrow, shallow straits from the mainland, but they will be united with the mainland in the near geological future in consequence of deposition.
The main physiographic trendlines of the island are rather simple. Its backbone is formed by the Barisan Range along its western side, which is the divide between the West- and the East coast. The slope towards the Indian Ocean is generally steep and consequently the West coast belt is mostly mountainous, with the exception of two lowland embayments in North Sumatra (Meulaboh and Singkel or Singkil), which have a width of about 20 km.
The eastern side of the island is occupied by broad, hilly tracts of tertiary formations and alluvial lowland. This low eastern belt has at Diamond Point in Aceh a width of about 30 km; southward its width increases to 150-200 km in Central and South Sumatra.

Fig. 2.3. Physiographic and relief map of Sumatra

2.3.1. THE BARISAN MOUNTAIN RANGE
The most prominent orographic element of the island is the Barisan Mountain Range, 1.650 km long and about 100 km wide (highest summit is the Peak of Kerinci or Peak of Indrapura, 3,800 m). The Tigapuluh Mts occupy an isolated position in the eastern lowland, forming a dome- or horst-like elevation (90 km long and 40 km wide, highest summit the Tjengeembun, 722 m).

Fig. 2.4. Barisan Mountain / Range a 3D view of a satellite image, with vertical scale exageration.

In the older descriptions the Barisan Range is commonly described as a number of parallel ranges or "coulisses", which one after the other branch off from the main NW -SE trending range, assuming a more West to East direction, and plunging toward the eastern lowland (see map of the Malayan Archipelago by BLOM. 1934). The coulisses repeat on a smaller scale the arcuate outline of the Sunda Mountain System as a whole, changing from NW-SE in Sumatra to WoE in Java. This pattern would be suggestive for those geotectonic interpretations, that accept a general compressive force in the crust from the N or NNE as the chief cause of the formation of this mountain system.
However, on closer inspection this explanation is definitely inadequate.
In northern Atjeb (or Atchin) there is indeed a tendency of the ranges to curve from NW -SE into a West-to-East direction. This can be observed in the ranges, flanking the Atjeh Valley near Kotaradja, which pass along the Southern Mountains into sliver blocks which form the WoE trending Y-Mountains, X-Mountains, and the Central Gajo Range (or Intemintem Mts with the Abongabong, 2,985 m) (Fig. 14). These ranges skirt the southern end of the Andaman Basin. Also their stratigraphy and tectonic structure correspond more with the northern part of the Sunda Mountain System than with that of the Sumatran section. The Sumatran trendlines, paralleling those of the Malayan Peninsula, begin with the N -S trending van Daalen Range. The latter meets the above mentioned ranges at right angles. Here an intersection occurs of the pretertiary trendlines, which we find belong to two different centres of orogenic activity, that of Mergui and that of the Sunda land. The foothills, formed by truncated tertiary anticlines and skirting the central pre-tertiary mountains of northern Atjeh, have a prevailing WoE trend along the North coast and curve around into a NW -SE direction South of Diamond Point. Therefore, the separation between the pre-tertiary trendlines of the Mergui section of the Sunda Mountain System (around the Andaman Basin) and those of the Sumatra section (around the Sunda Land), can no longer be made for the tertiary trendlines, which are all governed by the Barisan Range.
The Pundak Lembu (2,983 m) is a central knot, from which the van Daalen Range extends northward, the Central Gajo Range westward, and the Wilhelmina Range southeastward. In Southern Atjeh, South of Blangkedjeren, the NW-SE trend of the Barisan System prevails.
Between the Wampu- and the Barumun River, the Barisan Range shows a typical oblong culmination (NW-SE axis of 275 km length, and 150 km broad) (Fig. 15.) This culmination has been called by the author the "Batak-tumor", In its top part. which is about 2000 m high (Sibuatan, 2,457  m), lies the great Toba cauldron with Lake Toba. The cauldron is 100 km long and 31 km wide, an area of 2.269 sq km, while Lake Toba has a length of 87 km and an area (inclusive of Samosir Island) of 1,776.5 sq km. A geographical description of the southern part of the Batak culmination was lately given by HELBIG (1940).
The Barisan System of Central Sumatra consists of a number of NW-SE trending block-mountains. It is narrowest at the transition into the Batak- tumor near Padang Sidempuan (75 km) and then gradually widens southeastward to 175 km in the Padang section. These block-mountain ranges are highest at the southwestern side of the Barisan System, where they attain altitudes of over 2000 m, becoming lower towards the East-Sumatran lowland (Lisun-Kwantan-Lalo Range. about 1000 m, and Suligi-Lipat Kain Range up to 500 m). The pre-tertiary core of the Suligi-Lipat Kain Range can be traced via some anticlinal ridges of tertiary formations to the northwestern corner of theTigapuluh Mts, which are situated in the middle of the tertiary basin of East Sumatra. The Lisun-Kwantan-Lalo Range, however, plunges southeastward, disappearing under a 50 km wide basin, called the Sub-barisan Depression, which separates the Tigapuluh Mts from the main Barisan System.
In this section of Sumatra TOBLER (1917) distinguished the following tectonical and morphological elements (from NE to SW):
a. Alluvial East coast plain.
b. Tertiary foreland (peneplain) with the Tigapuluh Mts.
c. Sub-Baris an depression.
d. Fore-Barisan with overthrust masses.
e. Schiefer Barisan with intensely folded and metamorphic pre-tertiary rocks.
f. High-Barisan with young volcanoes.
g. Alluvial West coast plain.
This is still a useful distinction of the main physiographic elements in Central- and South- Sumatra. The Fore-Barisan begins in the Umbilin area, East of Lake Singkarak, were it wedges out between the Lisun-Kwantan-Lalo Range and the Schiefer Barisan; southeastward it disappears under the tertiary deposits of the East-Sumatran basin.
The Schiefer' Barisan can be traced along the entire length of .the island. The High-Barisan is especially well developed in the southern half, South of Padang. In the northern half of the island no distinction can be made between Schiefer-Barisan and High-Barisan, because pre-tertiary rocks are exposed over the entire cross-section and they are capped by more or less isolated young volcanoes. In the chapter on the geological evolution of Sumatra (V B) the tertiary basin of East Sumatra (a-c) will be called Zone I. The block- mountain ranges North of Umbilin, such as Suligi- Lipat Kain and Lisun-Kwantan-Lalo will be called Zone II. The overthrust masses of the Fore-Barisan (d) are united in Zone III. The Schiefer-Barisan (e) comprises Zone IV, but North of Padang the Schiefer-Barisan (e) coincides with the High-Barisan (f). The High-Barisan sensu stricto (f) forms Zone V, South of Padang. See fig. 126.
The Zones II and III are foreign elements, lying on the East flank of the main Barisan Range, as will appear from the geological analysis in the sub- chapter VB. The geanticlinal arch of the Barisan Range, which was elevated in plio-pleistocene time, comprises the Zones IV and V.

2.3.2. THE SEMANGKO ZONE
There is one typical feature, which characterizes the Barisan geanticline along its entire length, namely a median depression zone on its top, called the Semangko(-rift) Zone after its type section in the Semangko Valley of South Sumatra.
This Semangko Zone begins in the Semangko Bay of South Sumatra and can be traced from there to the trough of the Atjeh Valley with Kotaradja at the northern end of the island. Some sections were filled and capped by young volcanoes.
The series of narrow trough-valleys and volcano- tectonic sinks, forming this median Semangko-rift Zone on the top of the Barisan geanticline consists of the following units (from South to North): Semangko Valley with the Suoh and Antatai sinks; Liwa Basin; Warkuk Valley; Ranau cauldron; Kuala-Mekakau Valley; between Pulaubringin and Tandjong Sakti it is filled by the volcanoes Bepagut (2,732 m) and Patah (2,817 m) and it reappears in the Upper Mana Valley; Keruh Valley; Upper Musi Valley with Kepahiang; the Ketaun Basin with Muaraaman; upper Seblat Valley; interruption by the volcanic Gedang Massif (2,446 m); Upper Dikit Valley (Mentenang) with Muara Manderas; interruption by volcanic massifs; Upper Merangin Valley with the lake of Kerintji and Sungeipenuh; interruption by the Kerintji volcano (Peak of Indrapura, 3800 m, highest summit of Sumatra); Upper Batang Hari with Muara Labuh; the lakes Danau-di-Atas and Danau-di-Bawah; the Singkarak Valley with Solok and Lake Singkarak; interruption by the Marapi volcano (2,891 m); intermontane basin of Fort de Kock; Upper Masang Valley with Bondjol; Upper Rokan-kiri Valley (Batang .Sumpur) with Lubuksikaping and Rau (Rao), Here the Semangko-Zone bifurcates; the main fault-zone, called "Ulu-Aer fault" by DURHAM (1940), extends first northward and then curves NE to Sibubuhan and Sipirok; West of Rau the Angkola fault- trough branches off in a western direction and then sweeps northwestward along the Batang Gadis and Batang Angkola Valleys, in the vicinity of Muara Sipongi, Siabu, Sarumai- tinggi, and Padang Sidempuan. Near the last mentioned settlement occurs the transition between the Barisan of Central Sumatra and the Batak culmination of North Sumatra. At this point the Semangko zone is capped by the volcanic massif of Lobukraja-Buabuali (described by HELBIG, 1940, p. 189). It then can be traced farther, across the Batak-tumor, along the Batang Toru Valley with Tarutung and the Renun Valley with Sidikalang, This part has a crescentic outline, concave to the SW. The great Toba cauldron has a position somewhat off-side the main depression zone, cutting across its northeastern fault margin. In Atjeh the course of the Semangko-rift Zone becomes more complicated. The Renun Valley joins on to the Alas Valley, which has also a crescentic outline, concave to the SW. The Alas depression can be traced farther NW to the intermontane basin of Blangkedjeren (760 m), and from there along the upper course of the Tripa and the Seunagan Rivers in a western direction towards the embayment of Meulaboh on the West coast. The continuation of this depression zone is the upper course of the Teunom and the Atjeh River. Finally, it ends in the marine trough between the islands Breueh and Peunasoe at its SW-side and the volcanic island of Weh at its NE-side.

2.3.3. MAIN STRUCTURAL TRENDLINES
After this analysis, the main trendlines of Sumatra may be outlined as follows: The West flank of the Barisan geanticline extending west of the Semangko Zone is rather regularly formed in the southern half of the range, South of Padang. In this southern part the West flank is formed by a long crustal block which has been tilted toward the Indian Ocean, whilst its elevated northeastern edge breaks down along the Semangko Zone. This tilted block, called the Benkulen Block, can be compared with the Southern Mountains of Java, which are blocks tilted oceanward and forming the South flank of the geanticline of Java. The escarpment along the Semangko Zone is in general the divide between the East- and the West coast. This is the Barisan sensu stricto or High-Barisan. The West coast rivers are short, having a steep grade towards the Indian Ocean. The rivers descending eastward are much longer, flowing through the erosional plain, which truncates the anticlines of the neogene basin, and through the wide alluvial lowlands, until they empty on the Sunda Shelf-sea and Strait Bangka (see, for instance, the description of the Musi drainage basin by LEHMANN, 1933).
The southern end of the Barisan in the Lampong Districts is nearly 150 km wide, and here one may distinguish between the West flank or Bengkulu Block, the top part or Lampung Block, and the Eastflank or Sekampong Block (see fig. 355).
North of Ranau the range narrows to less than 100 km, because the Sekampong Block disappears under the neogene oil basin of Palembang and the Lampong Block is also covered by neogene strata. The pre-tertiary basement complex of the latter reappears in the culminations of the Garba, Gumai-, and Tambesi-Rawas Mts, which belong to the Schiefer Barisan, whilst the edge of the Benkulen Block, capped by a series of young volcanic cones, forms the High-Barisan.
Between Padang and Padang Sidempuan the geanticlinal structure of the Barisan Range is less distinct. It is cut into a number of longitudinal block-mountains on the East flank (described by VON STEIGER, 1922), as well as the West flank. The latter are indicated, for instance, by the subsequent tributaries of the lower course of the Batang Gadis, after it has left the Batang Angkola trough of the Semangko Zone.
The next part of the Barisan Range, that of the Batak-tumor, is a great geanticlinal dome, traversed by an arcuate section of the Semangko-rift Zone (see fig. 15). The northern part of the Barisan Range, in Atjeh, is the most complicate portion, broken into a number of block-mountain structures. The Leuser Block and the West Mountains occupy a position on the West flank, comparable with that of the Benkulen Block in the South.
The Barisan Range forms a section of the volcanic inner arc of the Sunda Mountain System. It is separated from the old Sunda landmass by the oil-bearing basin of East Sumatra. This downwarp of the pretertiary basement complex is the "back- deep" of the Sunda Mountain System. This back- deep is filled by neogene sediments which were folded in plio-pleistocene time.
During or after the main phase of folding a dome was elevated in the centre of this back deep, now forming the Tigapuluh Mts. Also in other places the basement complex is exposed in the cores of the tertiary anticlines (Limau Mts., Duabelas Mts., Bukit Pendopo). The anticlines were eroded to baselevel during their folding, so that a primary peneplain of subaerial erosion truncates the tertiary anticlines. The pre-tertiary basement complex of the Sunda Land crops out at some places in the alluvial marshes along the East coast. These are in fact former islands in the Sunda Shelf Sea, which are now connected with the mainland of Sumatra by the deposition in subrecent time. Physiographic ally the back deep of the Sunda Mountain System now forms a low land in the Sumatra section, whilst in other sections, with less strong sedimentation in neogene time, it is represented by a sea basin; for instance, the Andaman Basin of the Mergui section, North of Sumatra.
West of the Barisan Range stretches the interdeep of the Sunda Mountain System, which forms the sea basin between Sumatra and the island-festoon to the West of it. The latter represents the non-volcanic outer arc of the Sunda Mountain System. Because it is not connected by a land surface with the central Sunda Land, like Sumatra, the physiography of this island-festoon in the Indian Ocean will be described under the heading of the circum- Sunda Archipelagoes.

2.4. JAVA AND MADURA
These islands, like Sumatra, are connected with the Sunda Shelf-sea, so that they belong physiographically to the central Sunda Land. They will be described in this chapter under the heading of the Sunda Shelf area. Geologically, however, they belong entirely to the young tertiary mountain systems around the pre-tertiary Sunda Land, forming, like Sumatra, a section of the Sunda Mountain System.

Fig. 2.4.1. Physiographic sketchmap of Java and Madura, modified from van Bemmelen (1949) using satellite images (above) and an example of a satellite image based relief map by Maphill (below) .

Java has an area of 127,000 sq km and Madura measures 4000 sq km, being in total nearly four times as large as the Netherlands 1). Java is about 1000 km long and Madura 160 km. The main structural elements of the island are the geanticline of South Java extending along the southern half of the island, and the geosyncline of North Java, occupying its northern half. From Serna rang eastward this geosynclinal basin becomes considerably wider, bifurcating into a northern branch, which occupies the hilly country of Rembang and Madura, and a southern branch which comprises the Kendeng Ridge and Strait Madura. The geanticline of South Java is less regularly built than the Barisan Range, which forms the geanticlinal backbone of Sumatra. This is because the top part of the geanticline of Java has broken down, now being physiographically a depressed zone with island-like elevations of the former geanticlinal crest. The southern flank of the Java-geanticline is formed by the Southern Mts. These are crustal blocks, tilted towards the Indian Ocean, like the Benkulen Block in South Sumatra. In the middle part of Java the Southern Mts have disappeared below sea level, so that here the median depression is bordered by the Indian Ocean. A similar phenomenon has been observed in North Sumatra, where the Semangko depression is bordered by the low- land embayments of Singkil and Meulaboh on the Westcoast. On account of physiographic and structural differences four sections can be distinguished:
1. West Java (West' of Cheribon)
2. Central Java (between Cheribon and Semarang)
3. East Java (between Semarang and Surabaja)
4. The eastern spur of Java (Neth. "Oosthoek") with the Strait of Madura and the Island of Madura.

2.4.1. WEST JAVA
The trendlines, typical for Java, begin east of the Wijnkoops Bay. The westernmost part of the island, called Bantam, has in some respects more affinities with the Strait Sunda area and Sumatra, than with Java.
In NW-Bantam some volcanic complexes rise above the northern lowland plain of Java: Firstly, the Gede (595 m) with the harbour of Merak at its western foot, and secondly the Danau complex with the cones of the Karang 0,778 m) and the Pulasari 0,346 m). These volcanoes belong to the eruptive activity which accompanied and followed the en- gulfment of the Sunda Strait, like the volcanoes of Prinsen Island, Krakatau group, Sebesi, Sebuku, Sangiang ("Dwars-in-de-weg") in the Sunda Strait, and the volcanoes Tanggamus, Ratai, Betung, and Radjabasa on the Sumatran border of this strait. The Lampong Districts and Bantam at both sides of the Sunda Strait are covered by acid, pumiceous tuffs, called respectively Lampong tuffs and Bantam tuffs, which are at least partly eruption products of cataclysmic outbursts in the Sunda Strait area during its engulfment in plio-pleistocene time.
The Udjung Kulon Peninsula (Pajong, 480 m) and the Hondje Ridge (620m) in SE Bantam were separated from Java by the sea in pliocene time, forming presumably the southeastern end of the Barisan Range of Sumatra. The connecting link between the Semangko Bay at the Sumatran coast and the Welkomst Bay at the Java coast broke down in the plio-pleistocene phase of diastrophism. It is at present the more than 1000 m deep southern part of the Sunda Strait.
The Hondje Ridge is connected by a low ridge of pliocene strata with the Bajah culmination in SE Bantam. This uplift forms a transition between the structural trendlines of the Sunda Strait area and those of Java proper. The Sunda Strait area is supposed to have been a large culmination in upper tertiary time, which formed a threshold between the geosynclinal basins of eastern Sumatra and of northern Java.
It is bounded on its northern side, like the Strait Sunda area, by a crescent of young volcanoes: Malang (909 m), Endut 0,297 m), Halimun I (1,929 m), Halimun II 0,750 m).
The lowland plain of NE Bantam, North of the Bajah dome and East of the volcanic Danau complex, consists of slightly folded young tertiary strata, overlain by quaternary tuffs and alluvial deposits. The trend of the folds is N-S, which is at right angles to the WoE trend of the folds in the geosyncline of North Java. This N-S direction appears also in the arrangement of the coral reef islands off the North coast of Java, North of Tangerang.
The proper trendlines of Java begin East of the connecting line between these "Duizend" Islands (Thousand Islands) and the Wijnkoops Bay.
The part of West Java between this line and Cheribon has a width of 150-175 km. It is formed by an alluvial lowland plain in the North and a mountainous belt in the South, which comprise respectively t and!' of the cross section.
The plain of Batavia is about 40 km wide, extending from Serang and Rankasbitung in Bantam to Cheribon. It consists largely of alluvial river deposits and lahars (mud flows) from the volcanoes in the hinterland, with occasional exposures of slightly folded marine tertiary sediments.
To the South follows a complex belt of hills and mountains, also about 40 km in cross section, which extends from the Djasinga area near the boundary of Bantam to the Pemali River and Bumiaju in Central Java.
This belt might be called the Bogor Zone, after the main place Bogor (Buitenzorg) situated in its western part. It is an anticlinorium of strongly folded neogene strata with many, intrusions of hyp- abyssal volcanic necks, stocks, bosses, etc. (e.g. the conspicuous Sangabuana Complex, West of Purwakarta). Its western part extends west to east, whilst its eastern part assumes a more WNW-ESE direction, giving it a slightly arcuate outline, convex to the North. Its eastern part is crowned by young volcanoes, such as the Sunda Complex, North of Bandung (highest summit the Bukittunggul, 2,209 m), the Tampomas (1,684 m), and the Ciremai (3,078 m),
The third physiographic unit is a longitudinal belt of intermontane depressions, from which emerge island-like ridges of tertiary strata. This belt has generally a width of 20-40 km. It extends from the Wijnkoops Bay in the East, via the Tjimandiri Valley (with Sukabumi, 600 m), the up- land plains of Cianjur (459 m), Bandung (715 m), and Garut (711 m), to the Citanduy Valley (with Tasikmalaja, 351 m) in the West, ending in the Segara Anakan (or "Kinderzee") at the South coast of Central Java.
This depressed zone might be called, after the main town in it, the Bandung Zone. It is structurally the top part of the geanticline of Java, which has broken down after or during its arching up at the end of the Tertiary. In some respects it can be compared with the Semangko Zone on the crest of the Barisan geanticline in Sumatra. However, the latter is only 5-15 km wide, whereas the Bandung Zone attains a width of over 40 km. This is because the Semangko Zone is only a belt of rifts and graben on the top of the Barisan geanticline, whereas the Bandung Zone comprises the top part as well as the North flank of the Java geanticline.
The border between the Bogor Zone and the Bandung Zone is capped by a series of quaternary volcanoes (Kendeng, 1.370 m; Gagak, 1.511 m: Salak, 2,211 m; Pangrango-Gedeh, resp. 3.019 m and 2.958 m; Sunda Complex. North of Bandung, with Burangrang, 2.064 m, Tangkuban Prahu, 2,076 m, and Bukittunggul, 2,209 m, Tjalantjang, 1,667 m; Tjakrabuwana, 1,721 m).
The border between the Bandung Zone and the Southern Mountains is also marked by a series of volcanoes (Kendeng, 1.852 m; Patuha, 2,429 m; Tilu, 2,040 m; Malabar. 2.321 m; Papandajan, 2,622 m; Tjikoraj, 2,821 m). The Garut section of the Bandung Zone is flanked by two transverse rows of volcanoes, one separating it from the Bandung plateau (with the Guntur, 2,249 m, and the Mandalawangi, 1.663 m), and the other forming a divide with the Citanduy Valley (with the Galunggung. 2,241 m, Telagabodas, 2,201 m, and the Sedakeling, 1.676 m). The extinct Sawal volcano (1,733 m) occupies an isolated position amidst of the Cidanduy Valley, North of Tasikmalaja.
The Bandung Zone is partly filled by young volcanic and alluvial deposits, but these upland plains are occasionally interrupted by hills and ridges of tertiary rocks. Among these are, for instance, the ridges of lower tertiary and Miocene strata near Sukabumi and the oligo-miocene Radjamandala ridge, South of Tjiandjur. A morphological analysis of this part of the Bandung Zone was given by PANNEKOEK (1946, Map A & C).
In its eastern part, similar island-like mountains rise above the marshy lowlands of the Citanduy, e.g. Mt Sangkur (365 m) near Bandjar at the western side of the great Lakbok swamp, and the low mountain ridge extending from Wonoredjo at the NE side of this swamp in a southeastern direction to Maos at the Seraju River. This ridge consists of lower neogene strata and volcanic rocks. It is bordered at its southwestern side by the railroad Meluwung-Sidoredjo-Maos, and at its northeastern side by the highroad Wonoredjo-Majenang-Jatilawang. Due to the presence of this ridge the eastern end of the Bandung depression zone bifurcates; the southern and widest branch follows the Citanduy Valley to the Segara Anakan, whilst a narrow branch follows the alluvial plain of the Tjikanring and the intermontane valley of Karangputjung- Lumbir-Karangajam-Wangon. The latter valley separates the median ridge from the southeastern end of the Bogor anticlinorium. This median ridge in the eastern end of the Bandung Zone might be called after its highest summits the Kebanaran (360 m) or Kutadjaja (339 m) ridge. It is arranged "en echelon" with the South-Seraju Range of Central Java. which is to be discussed in the next paragraph.
The fourth physiographic unit of West Java is formed by the mountain land of South Priangan, called Southern Mountains. This unit extends from the Wijnkoops Bay to Nusa Kembangan Islands, South of the Segara Anakan near Tjilatjap. It has an average width of 50 km, but for its eastern end, which narrows down to some kilo metres in the Island of Nusa Kembangan. As a whole it represents the southern flank of the Java geanticIine, being a crustal block which has been tilted some degrees to the South. Physiographically three parts can be distinguished. The western part, called the Jampangs, has been described morphologically by PANNEKOEK (1946). Its erosional surface rises gradually from the Indian Ocean to a height of about 1000 m, with some resistant volcanic necks of greater altitude (Mt Malang 1.305 m) and then breaks down with a fault or flexure to the Bandung Zone.
The central part or Pengalengan section is the highest one. It is crowned by several extinct volcanoes (e.g. Kantjana, 2,182 m) and then breaks down by stepfaults and flexures to the Bandung Zone. The transition between the elevated edge of the central part of the Southern Mts and the Bandung Zone. is masked by the series of quaternary volcanoes already mentioned with the Bandung Zone.
The eastern section of the Southern Mts, called KarangnunggaI section, resembles again the Djampang section, being a rather low mountain land, seldom reaching altitudes of over 1000 m (Bongkok, 1,144 m). This difference in altitude between the central part on the one hand, and the Djampangs to the West and the Karangnunggal area to the East on the other hand, must also have existed in the Neogene, for the upper-miocene transgression of the Bentang-Beser Series did not submerged entirely this central part, which was an island at that time.

2.4.2. CENTRAL JAVA.
The central part of Java is much narrower than West- and East Java, measuring only 100-120 km across. This is because the Java Sea extends inland with a broad bight between Ceribon and Semarang, so that the northern lowland is more restricted or even absent, and because the Southern Mountains disappear for the greater part below sea level between Nusa Kembangan and the Southern Mountains of East Java.
The northern coastal plain of Central Java has its maximum width (40 km) South of Brebes, where the Pemali Valley separates the Bogor Range of West Java from the northern mountains of Central Java. Farther East it narrows to about 20 km South of Tegal and Pekalongan, until it disappears completely East of Pekalongan where the headland of the mountains reaches the coast. Between Weliri and Kaliwungu another fertile alluvial stretch is formed by the delta of the Bodri River. The mountainland of Central Java is formed by two geanticlinal culminations, the North- and the South-Seraju Range.
The North-Seraju Range forms the connecting link between the Bogor Range in West Java and the Kendeng ridge in East Java (to be discussed in the next paragraph). The South-Seraju Range is a new element rising from the longitudinal Bandung-depression of West Java. The North-Seraju-Range has a width of 30-50 km. Its western end is capped by the Slamet volcano:~•(3,428 m), and its eastern part is covered by the young volcanic products of the Rogojembangan Mts (2,177 m), the Dieng complex (Prahu, 2,565 m), and the Ungaran (2,050 m). The border- line with the Bogor Range of West Java runs across Prupuk-Bumiaju-Ajibarang.
Between the North- and the South-Seraju Range there is again a longitudinal depression, the Seraju Zone, in which are situated the places Madjenang, Adjibarang, Purwokerto, Bandjarnegara, Wonosobo.
Between Purwokerto and Bandjarnegara the Seraju Zone has a width of 15 km; East of Wonosobo it becomes broader, but here the depression is partly filled and masked by the young volcanic cones of the Sundoro (3.155 m) and Sumbing (3,371 m). Orographically it appears again in the plain of Temanggung-Magelang, which is the first of a series of intermontane plains in East Java.
The South-Seraju Mts consist of a western and an eastern part. The western one (with Kabanaran 360 m) might be described as an elevation in the Bandung-depression Zone of West Java, or as a new structural element belonging to Central Java. It is separated from the Bogor Range by the Majenang plain and the upper course of the Cihaur and Pasir.
The eastern part of the South-Seraju Range forms a geanticlinal elevation in this depression zone of Bandung, comparable with the culmination of the Bajah Mts at its western end. The eastern part of the South-Seraju Range is completely separated from the western one by the Djatilawang Valley. It starts near Adjibarang as a simple, narrow anticline, transversely cut by the Seraju River. East of Banjumas this anticline develops into an anticlinorium of some 30 km width in the Lukulo (Loh Ulo) area, South of Bandjarnegara (Midangan 1,043 m). The eastern end of the South-Seraju Range is formed by the more or less independent dome of the West-Progo Mts (1,022 m), between Purworedjo and the Progo river. The Coastal Plain of South Central Java is 10-25 km wide. This part of the South coast forms a sharp contrast with the rocky South coasts of West and East Java, lying not more than 10 m above sea- level. Three shore bars with dunes of 5-15 m height and 100-500 m width run parallel to the coast, the youngest one still being subjected to changes. This low coastal stretch joins on to the Bandung Zone of West Java. It is interrupted in the middle part by the Karangbolong Mts (475 m), which are the physiographical and structural equivalent of the Southern Mts in West and East Java. Apart from this relic, these Southern Mts have subsided below sealevel between Nusa Kambangan and the mouth of the Opak river.

2.4.3. EAST JAVA
East of the line Semarang-Yogyakarta a number of parallel zones can be distinguished, the southern ones forming the direct continuation of the zones of West and Central Java, whereas the northern ones are new physiographic and structural elements.
In the North the Murjo or Muriah Massif (1,602 m), known for its leucite-bearing rocks, and the andesitic Lasem volcano (806 m) are situated off- side the main series of Javanese volcanoes.
The Muriah is now connected with Java by the alluvial plain of Semarang-Demak-Kudus-Pati- Djuwono-Rembang, but during the Holocene it was still an island (Present shift in the shoreline near Demak 30 m per year).
The hilly district of Rembang consists of a number of more or less East-and-West trending ridges, alternating with alluvial plains (near BIora, Djodjogan, and along the lower course of the Solo). This Rembang anticlinorium has an average width of 50 km, the highest tops reaching about 500 m above sealevel (Gading 535 m, Tungangan 491 m). The hills almost reach the North coast, from which they are separated by narrow sandy beaches with dunes.
The flat topped ridges near Tuban, consisting of reef limestones, look from the sea like gigantic coffins.
The Rembang hills are separated from the Kendeng ridge by a synclinal zone, called Randublatung Zone, which can be traced from Semarang via Purwodadi-Randublatung-Ngimbang to Wonokromo near Surabaja.
Near Randublatung and Ngimbang it has a cross section of some kilometres only, being wider at those places where it links up with the lowland plains between the ridges of Rembang (e.g. the lower Solo Valley). The structural importance of this synclinal zone follows from the fact that the direction of the folding movement in the Rembang area is generally southward, whereas the Kendeng strata have been pushed northward.
The Kendeng Ridge or Kendeng anticlinorium is the eastward continuation of the North-Seraju Range of Central Java. It has a length of 250 km and a width of 40 km South of Semarang, narrowing down eastward to 20 km. Its height is seldom more than 500 m. Near Ngawi an axial depression occurs. where the ridge is transversely cut by the Solo River. thus being divided into a western and an eastern part.
The eastern part attains its maximum width (30 km) near the small Pandan volcano (897 m) which pierces the tertiary strata at its southern rim. From here on the height and width of the Kendeng Ridge gradually decrease eastward. its anticlines disappearing near Modjokerto one after the other under the alluvial deposits of the Brantas delta. till only" two anticlinal ridges of about 10 km width reach the Strait of Madura near Surabaja.
Between the Kendeng Ridge and the eastern Southern Mountains a depression zone occurs. which is the physiographical and tectonical equivalent of the Bandung Zone in West Java. The latter bifurcates in Central Java into the Seraju Zone and the southern coastal plain of Central Java. thus embracing the South-Seraju Range. In the Progo Valley near Djokjakarta both branches unite. forming the wide Solo Zone of East Java. Like the Bandung Zone this longitudinal depression of East Java is partly filled and capped by a series of young volcanoes. It can be subdivided into three parallel strips. viz. the Ngawi Subzone. the Solo Zone (sensu stricto) and the "BIitar Subzone.
The Ngawi Subzone is the synclinal depression bordering the Kendeng Ridge at its southern side. It begins near Simo and can be considered as the eastward continuation of the Seraju Zone of Central Java. It can be traced via Sragen and Ngawi to Djombang where it links up with the alluvial plain of the Brantas delta. Structurally, however, this Ngawi Subzone has to be traced eastward across the saddle between the Penanggunan 0.653 m) and the Andjasmoro Mts via Bangil to the North coast of the eastern spur of Java ("Oosthoek"). The Solo Zone (sensu stricto) is formed by a series of giant quaternary volcanoes with intermontane plains,beginning with the Sundoro (3.135 m) and Sumbing (3.371 m) in Central Java:
The Brantas River is, after the Bengawan Solo, the second longest river of Java. It rises on the southern slope of the Andjasmoro; flows southward through the Malang Plain and bends sharply westward near Kepandjen; after a westward stretch of about 70 km it turns to the North near Tulungagung till it reaches the Kendeng Zone. which is partly covered by its alluvial deposits in the area of Djombang and Modjokerto; here the river takes an eastward course. Near Modjokerto the Brantas delta is reached and the river bifurcates into the Mas (emptying near Surabaja) and the Porong (reaching the Strait of Madura near Bangil). The present mouth of the Porong is situated at a distance of only 40 km from the head waters of the river." so that an almost closed noose is formed round the volcanic complex of the Andjasmoro Mts- Kelud - Kawi. The catchment area measures about 11.000 sq km, while the silt load of the Brantas River is 1.3 kg per cbm, less than half of the load of the Solo River which amounts to 2.75 kg per cbm. The shift in the coast line amounts to 7 m per year near the mouth of the Brantas and 9-15 m near the mouth of the Porong. In historical times (lOth century) the Brantas mouth still was a wide estuary, forming a good natural harbour.
South of the zone of quaternary volcanoes (Solo Zone s.str.) a third subzone, called Blitar Subzone, can be distinguished. in which the places Wonogiri. Balong, Tulungagung and Blitar are situated.
The Blitar Subzone is bounded on the South by the Southern Mts of East Java. Like those of West Java. the Southern Mts of East Java are in general an elevated block tilted oceanward in which the erosion has been rejuvenated. The northern border is marked by a complicated escarpment. The maximum width of these Southern Mts is 55 km South of Surakarta, whilst south of Blitar they measure only 25 km across.
The eastern part (between Opak and Patjitan), consisting partly of limestones with typical Karst- phenomena. is called "Duizend" (Thousand) Mts or Gunung Sewn (See morphological analysis by LEHMANN. 1936). Between Patjitan and Popoh the northern part of the Southern Mts consists of older volcanic deposits; here also remnants of the pre- quaternary peneplain can be observed (Gembes 1.243 m). The southern part is covered by the limestones of the Thousand Mts. The narrow stretch South of the Brantas River consists mainly of limestones, with steep abrasion cliffs along the Indian Ocean.
4.THE EASTERN SPUR ("OOSTHOEK") AND MADURA
Madura (area 4,382 sq km, length 160 km, max. width 38 km, max. height 471 m) forms the east- ward extension of the Rembang Hills. Its seperation from Java possibly occurred in 80 A.D. accor- ding to STUTTERHEIM (1929, p. 22, note 1). TheStrait of Madura, having nowhere a depth of more than 100 m and a maximum width of 68 km, forms the continuation of the Kendeng Ridge which plunges eastward under the Brantas delta. Eastward this strait passes into the Bali Sea which belongs to the backdeep of the Lesser Sunda Islands. The narrow North coast of the eastern spur shows folded plio-pleistocene deposits, capped by small volcanoes. From East to West the following elements can be distinguished: Bangil anticline, Semongkrong tuff hill (84 m), Hills of Probolinggo (38 m, 104 m), Lurus volcano (539 m), Ringgit- Beser Mts (resp. 1,250 m, 1,303 m), Baluran volcano (1,247 m). This coastal stretch represents the eastward extension of the southern margin of the Kendeng anticlinorium (which comprises Pandan volcano, the anticlines of Jombang and Mojokerto, and the Penanggungan volcano).
South of this narrow coastal stretch follows the eastward continuation of the Ngawi Subzone, which can be traced via Tampung plain (between Bangil and Lawang), and Grati Lake (between Semongkrong and Tengger) to the plains of Probolinggo and Bondowoso.
Then comes the Solo Zone (sensu stricto) which is composed of a series of volcano complexes and intermontane plains, just as in East Java. Table 4.
The Blitar Subzone of East Java extends east- ward by way of Kepandjen and Turen to Pasirian. Here the lowland of Lumadjang-Djember reaches the South coast at Puger, due to the absence of the Southern Mts, and by the same reason the lowland of Rogodjampi reaches the South coast at Gradjogan.
The Southern Mountains in the eastern spur of Java are discontinuous, consisting of three more or less isolated parts. The western part (South of Turen), about 25 km wide, is directly connected with the Southern Mts of East Java. Between Pasirian and Turen the Southern Mts are interrupted by the lowland of Lumadjang. However, the island of Nusa Barung (South of Puger) belongs structurally to them, forming a link with the next part of the Southern Mts between Puger and the river Baru (Betiri, 1,223 m). South of Rogodjampi the lowland again reaches the South coast, from which the isolated Pumpangpitu hill rises to a height of 489 m. Finally, the eastern-most fragment of the Southern Mts in this section of Java is represented by the Peninsula of Blambangan or Purwo (360 m), linked to the main island by an alluvial isthmus, 22 km wide. This breaking down of the Southern Mts continues eastward in Bali and Lombok, where the parts are separated by sea- straits in stead of alluvial plains.