Sedimentary rocks of Tertiary age have been reported from several areas of inland (or onshore) Peninsular Malaysia, though a review of published data indicates that only some of them are known, or likely, to be so (Table A).In the known and likely localities, which occur as a series of small basins along the West Coast, are found partly consolidated gravels and sands inter-bedded with soft, often carbonaceous, shales, clays and lignite seams, as well as rare limestone, calcareous shale and volcanic ash. The sediments are entirely continental deposits and mostly gently dipping, though dips of 30° to 45° occur with a synclinal or basinal structure shown by some of them. The sediments mostly unconformably overlie, much older, Palaeozoic to early Mesozoic, generally folded and partly metamorphosed, rocks. The sediments have been considered to have been of a more extensive distribution than they presently are, for in places dipping beds are truncated, at the surface, or along an unconformity, though it is doubtful ifthey ever formed a connected sheet.At Bukit Arang in Perlis and Kedah, the Tertiary strata form two broad, gently northward plunging, synclinal basins with moderate bedding dips « 35°). The strata consist of an upper sequence of poorly sorted gravels and boulders in a sandy to clayey matrix (Boulder Beds) that unconformably overlies a lower sequence of sand and clay layers with a few thin coal seams. The lower sequence has a minimum thickness of 130 m and was deposited in a lacustrine environment, whilst the Boulder Beds with a total thickness of some 90 m were deposited under fluvial conditions. At Enggor in Perak, the Tertiary strata form a small, circular, basin with a thin layer of surface wash sediments overlying a sequence of shales, sandstones and clays with two coal seams. The coal-bearing strata, which dip 10° towards the NW, have a minimum thickness of 65 m and unconformably overlie folded, Upper Palaeozoic sedimentary rocks.
