The Mon State forms a large part of the coastal peninsula that extends from the Gulf of Martaban down to the Andaman Sea. The Andaman coastal area covers the southern part of Mon and Kayin States (Kayin is inland) along with Tanintharyi (formerly Tenasserim) Division and includes the more than 900 islands of the Mergui Archipelago. The region is a long narrow strip between the sea and the watershed of the Tenasserim Range.
The Andaman coastal area has complex geology. According to Waltham and Eavis (2004), there are major faults isolating scattered and unrelated outcrops of Permian limestones and Mesozoic granites within the dominant slates, shales and sandstones that are also mainly of Permian age. Karst and caves are therefore restricted to isolated patches. The strata is Upper Carboniferous and Permian, overlain by reef limestones from the Triassic period. The Moulmein limestones are highly jointed, sometimes in several directions (Bates & Nwe 2001). The Permian Moulmein Limestone is a continuation of the limestone-dolomite sequence extending from the Shan State south through Kayah and Kayin States into Tanintharyi.
Steep and craggy karst towers rise abruptly from the flat agricultural plains and alluvial plains of the Salween River. Most of the hills are isolated towers, some more than 400m high, running NW-SE. There is also ridge karst. Mt. Zwegabin, 722m high, is an impressive long ridge outside Hpa An. Some of the ranges have extensive forest cover whereas other outcrops have scant vegetation.
In the Mergui Archipelago, Tanintharyi coastal area there are isolated outcrops of coarsely crystalline thick limestone, intruded by granite. On the mainland they appear as precipitious hills, and offshore as a series of isolated islands. King Island is 767m high. These islands have hongs, some are only accessible at low tide. The coastal area has cliffs almost 400m high, which contain vast caves (Bates & Nwe 2001). The Tanintharyi coast is the wettet part of the country as it is exposed to the southwest monsoon.
Outcrops of Permian Limestone also occur in the Tenasserim Range but little is known about these deposits which are continuous with those found in the Khwae Noi and Khwae Yai drainage basin of western Thailand.
Moulmein (now known as Mawlamyine) is Myanmar’s third largest city and is capital of the Mon state. There are however no high rise buildings and there is a distinct lack of restaurants. Moulmein is on the left bank of the Thanlwin River, at the confluence of the Salween River from the north, Gyaing River from the east, and Ataran River is just south.
It is very confusing trying to identify caves from the colonial literature. Firstly different names and different spellings are used. Secondly many reports refer to the Amherst district, and this name is no longer used. Amherst town, now known as Kyaik Khami, was a small station on the coast, used in colonial times as a beach resort. Temple (1897) names 23 caves in the Amherst district.
The most famous caves in Mon state are the Farm Caves. These were well documented in colonial times.