The western belt of Peninsular Malaysia has been considered to be a single intact block which formed part of the integrated Shan-Thai or Sibumasu block. Lithological and palaeontological evidence from the Permo-Carboniferous sequences of northwest Peninsular Malaysia, however, indicate that these sediments were deposited in two palaeogeographically distinct settings or terranes, here named the West Langkawi terrane and the Perlis-Kedah terrane respectively. The West Langkawi terrane which covers western as well as central parts of Langkawi Islands is represented by the Singa Formation and the Jong limestone (new name). The Carboniferous and Lower Permian (the Singa Formation) of this terrane is characterised by thick pebbly mudstone of glacio-marine origin, containing characteristic cold temperate Gondwanan or peri-Gondwanan faunas. The Perlis-Kedah terrane covers a wider area in the States of Kedah and Perlis as well as the eastern part of Langkawi Islands. This terrane is represented by the Kubang Pasu and Semanggol Formations and the Chuping and Kodiang Limestones. The Carboniferous and Lower Permian of this terrane consists of shallow water Kubang Pasu Formation in the west and the deep-water Semanggol Formation in the east. So far, no well-defined glacial diamictites and cold water faunas have been recorded from this terrane. The Lower Permian fauna in Perlis is dominated by taxa suggestive of a transitional climatic zone (i.e. between typical cold-water Gondwanan and palaeo-tropical warm-water Cathaysian faunas). The Kubang Pasu Formation unconformably overlies the Lower Devonian Setul Limestone, while its lateral equivalent, the basal part of Singa Formation unconformably overlies the Upper Cambrian Machinchang Formation. In the eastern part of Langkawi Islands, the Permian of Langkawi terrane were overthrust by the Lower Palaeozoic rocks along the Kisap Fault zone. Although there are many different types of fault movements and directions within the fault zone, several lines of evidence in support of westward transport have been observed at Pulau Dayang Bunting and Pulau Timun. In the northeastern part of the main Langkawi Island where the fault movements have been questioned by many authors, there is still an undeniable fact that the Lower Palaeozoic rocks have been brought up at least to the same elevation as the Permian sequence. The two terranes are believed to have been brought close to each other by the post-Permian Kisap Thrust, a fault complex which has thrust the eastern block (i.e. the Perlis-Kedah terrane) over the western block (i.e. the West Langkawi terrane).