Peninsular Thailand lies south of about 12° 50′ north latitude. Along the crescent of the coast it is almost 600 air miles from this latitude to the boundary of Malaya. The land varies in width from 10 to 135 miles. Plains fringe the coastal areas, and highlands form the backbone of the Peninsula. Mountains extend from north to south and form short ridges or ranges en echelon. Between these mountains are small plains or valleys, which are considerably dissected. Physiographically, as well as climatically, ethnically, and economically, there is a distinct difference between the Peninsula East Coast and the Peninsula West Coast. Here, for comparative purposes, the two can best be treated together.
Along the western side of the Peninsula the Tanaosi (Tenasserim) Range, which begins in the Western Mountains sub-province, continues to mark the Thailand-Burma border. South of Pračhuapkhirikhan, where the Thailand section of the Peninsula is less than 20 miles wide, the Tanaosi Range levels off rapidly to the narrow coastal plain, but just to the south a further range (or possibly an extension of the Tanaosi) begins, now running from northnortheast to south-southwest. This range begins in a high mountain (4,040 feet) but is not continuous in sizeable elevation, being hardly 300 feet high in the vicinity of Chumphon. Upon meeting the trough of the Kra or Pakčhan River, it splits into two sections, a western range in Burma, and an eastern in Thailand. The eastern, known as the Phuket Range, extends south from the Kra Isthmus and skirts the Indian Ocean into the Island of Phuket. There are numerous granite cores in the Phuket Range, many of them reaching altitudes of more than 3,300 feet. They are the source of the rich alluvial tin ores of the most important mining districts of Thailand.
To the east of the Phuket Range, at about 8° 20′ north latitude, a second Peninsular range commences. It begins in Khao Phanom Benčha, with an elevation of 4,583 feet, strikes south through Krabi, and ends on Lanta Island at a peak of 1,614 feet elevation.
An eastern range, the main range of the Peninsula, begins along the East Coast at about 10° 05′ north latitude on Tao Island. It continues through Phangan and Samui Islands to the East Coast mainland, east of Bandon, and parallels the coast all the way into Malayan territory. This is generally called the Nakhon Sithammarat Range. The greatest elevations in this granitic range are in its northern half. Peaks on Phangan and Samui Islands rise to 2,057 and 2,086 feet respectively. Southeast of Bandon, the peak reaches 4,396 feet; and majestic Khao Luang, west of Nakhon Sithammarat city, reaches 5,860 feet. Like the high Phuket Range, these formations yield very important alluvial tin ores. In the southern extension of this range most of the summits are below 3,300 feet, and in several places the range disappears completely below the level of the alluvial plains so that the southern section becomes a line of separate smaller ranges and peaks. Near its southern end, the peaks have an elevation of 2,000 to 2,300 feet.
Between the Nakhon Sithammarat and Phuket Ranges isolated peaks rise from the surrounding lowlands. One such peak, Khao Phanom Benčha, attains an altitude of about, 4,500 feet, but most of the peaks are much lower.
East of the southern section of the Nakhon Sithammarat Range are three further north-south parallel ranges. Separated by a broad alluvial plain in the vicinity of Songkhla, the fourth Peninsula range commences, continuing across the Thai border into Kedah, Malaya. A fifth range, the Pattani Range, lies just to the east, its southern section forming the boundary between Kedah (Malayan Federation) and Pattani čhangwat. Still further east lies a sixth range which sometimes is called the Taluban Range from the river of that name in southern Pattani čhangwat. In Malaya this range becomes the main range, extending south toward Malacca, and reaches a high point of 7,000 feet. Along the southern Thailand boundary several peaks in the Pattani and Taluban Ranges reach almost 5,000 feet.
The sinking of the Thailand-Burma boundary range provides an ancient trade route across the Peninsula via the so-called Isthmus of Kra. This isthmus is only about 15 miles wide from the Pakčhan River in the west to a smaller coastal river in the east. (Neither of these rivers can be navigated by modern ocean-going steamers.) Elevation at the pass in the center of the isthmus is about 230 feet. A canal through this isthmus--often proposed in the past-could have reduced markedly the travel time and shipping cost between Europe and the Far East and, at the same time, would have seriously damaged the commercial position of Singapore. For these reasons, the isthmus long has had a special political-geographical significance, especially to Great Britain, who understandably has been instrumental in discouraging any canal construction. In 1942, the Japanese built a railway on top of the highway from Chumphon to La-un where a harbor with wharves was developed on the Pakčhan River.
In general, the East Coast of the Peninsula is smooth and regular, with few bays, but many long beaches, especially at Nakhon Sithammarat, Songkhla, and Pattani. The coastal plain is 3-22 miles wide, and there are several river plains and basins which extend far inland. A large inland sea, Thalesap, lies north of Songkhla.
The West Coast shoreline is very irregular and much indented with estuaries. It is fringed with islands such as those in Phuket Bay. There are few beaches. The mountains extend down to the sea in many places, and the coastal plain is generally very narrow. Mangrove swamps are numerous. The coast of the Pakčhan River has the appearance of a drowned valley. Remains of buried mangrove trees have been exposed in hydraulic mining many feet blow the present sea level and have been found along the shorelines of Takuapa and Phuket.
On neither coast is there a well-developed sand dune tract landward of the broad sandy flats of the outer coast. This is probably because on both coasts the season of strong onshore winds is also the season of most rain. The sand is daily saturated by rain water and is too wet to be blown in the monsoon winds which are only moderately strong.
The plains of the Peninsula have been built up by alluvium from the rivers and creeks which flow over them. The enrichment of tin ore just above the bedrock suggests that the alluvium was worked over repeatedly. This displacement of alluvium proceeds with relatively greater speed during the rainy season. At that time of year enormous quantities of water come into the valleys, filling the deeply cut stream beds of the dry season and overflowing the entire plain. The alluvium is then saturated with water and consequently easily moved. During floods, stream beds have been known to shift sometimes to an entirely different portion of the plain.
The thickness of the valley alluvium is limited by the range of stream-water levels. In the rainy season streams may rise repeatedly, overflowing their banks, flooding the plain, and leaving sediment behind. However, during the dry season the contraction of the streams is so great that the water may not even cover the full width of the bed. Thus, during the high water of the rainy season, the stream bed erodes seriously, while in the dry season there is a heavy deposition of alluvium. The alluvium below the bed of the stream is saturated with water even during the dry season. With highly ionized water always in contact with the rocks below, they are exposed to intensive chemical weathering. Because the streams meander back and forth in the plain, there is an extremely gradual lowering of the general level of the plain. Consequently, profound weathering of the rocks takes place below the surface of the alluvium. This sort of plain formation applies not only to plains of erosion in the Peninsula but also to plains throughout Thailand which have been developed in low hilly terrain.