Relief and structure of Meghalaya
Meghalaya is an upland area formed by a detached block of the Deccan plateau. Its summits vary in elevation from 4,000 to 6,000 feet (1,220 to 1,830 metres). The Garo Hills in the west rise abruptly from the Brahmaputra River valley to about 1,000 feet (300 metres) and then merge with the Khasi Hills and Jaintia Hills, adjacent highland systems that form a single massif of tablelands separated by a series of eastward-trending ridges. The southern faces of the plateau, overlooking the Bangladesh lowlands, is particularly steep.
Meghalaya Plateau belongs to the part of Super Continent of Gondwanaland, i.e., the Peninsular table land, but is detached from the latter by the intervening spread of the alluvium of the Ganga and the Brahmaputra. The structural history of the region reveals several phases of erosion, sedimentation, diastrophism, intrusion, movements of land and sea, and emission.
The plateau is mainly made up of rocks of the pre-Cambrian age. The pre-Tertiary and Tertiary rocks occur above these rocks on its western and southern margins. Stratigraphically the rocks of the plateau belong to five broad geological formations, namely the Archaean gneissic complex, the Shillong group of rocks, the lower Gondwana, the Sylhet traps, and Cretaceous – Tertiary – Quarternary Sediment.
The Archaean-gneissic complex occupies the central and northern parts of Meghalaya Plateau. The rocks include gneiss, granite, quartzites, schists, etc. The Shillong group of rocks lying unconformably over the gneissic complex, occurs in the central and eastern parts of the plateau These include phylite, quartzites, schists and conglomerates.
The lower Gondwana rocks are found in the western part of Garo Hills which include pebble beds, sandstones and shale. The Sylhet trap is exposed along the southern border of the Khasi Hills in an east-west direction, and rests unconformably over the eroded pre-Cambrian basement rocks. These rocks are predominantly basalt, rhyolite and acid tuffs.
The Garo Hills region of the State, in its greater portion, is formed of gneissic rocks overlaid by sandstones and 1 conglomerates of Cretaceous-Tertiary system.
The sediments of this system of rocks are known as the Garo group in the region. This group of rocks is again divided into the Simsang, Baghmara and the Chengapara formations. The Simsang is the oldest formation in the Garo group which lies conformably over the Kopili series (the youngest formation of the Jaintia group of Cretaceous – Tertiary sediments), and consists of siitstone and sandstone.
the Baghmara formation includes sand, pebble, conglomerates and clay which lie conformably over the Simsang formation. The Chengapara formation consists of sand, siitstone and clay. On the top of these Cretaceous-Tertiary formations rests limestone of Numulitic age, while sandstones of upper Tertiary origin form low hiiis along the Mymensing border.
The Khasi Hills are located east of the Garo Hills. The other Khasi tribes did not have princes but their twenty petty states (hima), and sometimes even smaller tribal divisions, are led by one or two Chiefs -selected in various ways- usually styled Siem, Syiem. The names of these chieftainships are : Bhawal, Cherra (or Sohra; capital Cherrapunji), Dwara (capital Hat Dwara), Jirang, Khyri(e)m (capital Cherrapunjee, under a Radja), Langrin, Lungiong, Maharam, Malai Sohmat, Marriw, Mawdon, Mawiang, Mawlong, Mawphlang, Mylliem (including Shillong city, the colonial capital of all Tribal Assam), Nobosohphoh, Noglwai, Nongkhlaw, Nongspung, Nongstoin, Pamsanggut, Rambrai, Shella, Sohiong. or Sardar .
The Jaintia Hills are located further to the east from the Khasi Hills. The twelve Chiefs of the elaka (tribal province) of the Jaintia, a Khasi subtribe of the) Pantars = Syntengs tribes, are styled Dolloi, and the land is called after them in Khasi: KA RI KHADAR DOLLOI ‘Land of 12 Tribal Chiefs‘) – they are in Nartiang itself (see the Raja, uniquely also styled, as premier Chief: U Kongsong), and in Amwi, Jowai, Lakadong, Mynso, Nongbah, Nongjngi, Nongphyllut, Nongtallang, Raliang, Shangpung, Sutnga (see below; also cited as seat of a Syiem) Above them is the only true princely ruler of the area, the Raja of Jaintiapur.
His winter capital is now in Bangladesh, with his summer residence shifted from Sutnga (where the family started as Syiems) to Nartiang; also a palace in the commercial center Borghat. The Jaintia Hills used to be a part of the Jaintia Hills District. The district has been bifurcated into two separate districts,namely, East Jaintia Hills and West Jaintia Hills on 31 July 2012.