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In the early days of mineral exploration on behalf of the Netherlands East Indies Bureau of Mines and of petroleum exploration by the oil companies it was recognized that Pre-Tertiary rocks were extensively exposed in the Barisan Mountains in the western part of Sumatra (Fig. 1.4). These rocks are variably metamorphosed and were termed the 'Barisan-Schiefer' and the 'Old-Slates Formation' (Veerbeek 1883) in Central Sumatra, and the 'Crystalline Schists' in the Lampung area (Westerveld 1941). Locally these rocks contain fossils, and it was recognized that Carboniferous and Permian rocks occur within this Pre-Tertiary basement. Some basement units were defined during the mapping of Sumatra by the Netherlands Indies Geological Survey between 1927 and 1931, but the definition of units according to modern stratigraphic principles began in the early 1970s, with the commencement of systematic mapping by the Indonesian Geological Survey in collaboration with the United States Geological Survey, in the Padang area of West Sumatra (Kastowo & Leo 1973-Padang; Silitonga & Kastowo 1975-Solok; Rosidi et al. 1976-Painan and Muarasiberut).
Mapping and the definition of further units was continued in northern Sumatra by the Indonesian Directorate of Mineral Resources/British Geological Survey (DMR/BGS) between 1975 and 1980 as part of the Northern Sumatra Project and was extended into southern Sumatra in the 1980s and 1990s by the Indonesian Geological Research and Development Centre (GRDC), DMR amd BGS. The results of these surveys, which established the distribution of the basement units, are published by GRDC as 1:250 000 Geological Map Sheets covering the
