A belt of piedmont surrounds the Bangkok Plain on the east, west, and north. Its area is limited and very much interrupted by marginal outcrops of rock, low hills, and mountains, which suggests that bedrock lies not far below the alluvium. Much of this surrounding marginal region has been formed as a result of rising crustal movements of the earth. Only that part with predominantly low relief and a generally prevalent accumulation of alluvium can be considered as belonging to the Marginal Plains and the Upper Plain. Coastal lands along the Gulf, especially those south of Phetchaburi, can be included with this region.
The Marginal Plains consist of a narrow strip along the west side of the Bangkok Plain all the way to the Gulf in the northern Peninsula. To the west of the Nakhon Chaisi and the Maeklong this region extends for an average distance of about 35 miles to where the Central Cordillera begins to rise. In the east the Marginal Plains include much of the upper valleys of the Tachin and its tributaries.
North and northeast of the Bangkok Plain the marginal regions include the long narrow valley of the Pasak River and the large triangle of the combined river plains of the Ping, Yom, and Nan. Collectively these make up the Upper Plain of the Central Valley.
In each of these regions, three associated forms repeatedly occur: narrow plains of deposition along the rivers, low outcrops of rock and foothill lands protruding here and there through the plains of deposition, and finally, higher elevations including separate mountains and mountain ranges.
Along the western Marginal Plains and the western Upper Plain, as well as toward the interior of the Peninsula, separate mountains or groups of mountains stand out clearly from the plains. On the basis of their morphological appearance these mountains resemble inselbergs, but many geologists working in the Peninsula have believed them to be old islands whose steep slopes had been cut by marine abrasion. Mountains with such sharp angular contact with the plains are always of Permocarboniferous limestone. Granitic mountains and those of quartzite have more gentle slopes. Yet they, too, are often scattered like inselbergs over the alluvial plains. Quite generally along the margin of closed mountain chains there are whole series of isolated hills and mountain ranges. The resistance against weathering of the towering masses of recrystallized limestones above the level of the plains is extraordinary. Solution phenomena, such as pipelike holes and hollow channels occurring at various levels in the slopes, indicate the former action of enclosed streams of water flowing underground during times when these remaining limestone mountains still stood adjacent to other limestone masses which have now disappeared.
The alluvium of the surrounding plains is underlaid by almost horizontal surfaces of limestone with pinnacle-points between which narrow solution pipes go down very deep. Such buried surfaces with pinnacle weathering have been disclosed by tin mining in the Peninsula. But there is no evidence of sea action, and no remains of marine life have been found in the material associated with the pinnacles. The accumulated sediments are similar to those which occur far in the interior, hundreds of miles from the coast and at different levels. The contrast between the plains and the steep residual limestone mountains is to be ascribed entirely to subaerial erosional processes.