Bulletin of the Geological Society of Malaysia, Volume 3, March 1970, pp. 27 – 51
University of Western Australia
Abstract: Marine intertidal erosion of limestone at two localities in Malaysia is described. At Semporna, Sabah, uplifted Quaternary coral rock forms a group of islands, and at Pulau Langkawi, Kedah, crystalline Palaeozoic limestones form part of an archipelago. The erosional features at Semporna are similar to those described in coral rock elsewhere; there is a visor, an intertidal notch, and a low tide platform which may extend 30 meters or more from the notch. At Langkawi many of the islands have sea cliffs, which continue below sea level; intertidal notches are cut into them but there are no low tide platforms.
The form of the notch is similar in the two places. Amplitude is related to tidal range and wave action; the height approximates spring tide range where the sea is generally calm, but increases with exposure to wave action. In the coral rock the surface of the upper notch is rough and pitted and there are few sedentary animals. The crystalline rock is generally much smoother above the level of mean high water neap tides while below this it is heavily encrusted with oysters and barnacles. At both places the surface rock is bored by endolithic algae and these are rasped by chitons and other browsing molluscs. The boring sponge Cliona, the bivalve mollusc Lithophaga and other boring organisms riddle the outer few cm of the Langkawi rock up to about mean high water neaps, but are much less common in the coral rock.
It is concluded that these boring organisms are the main agents responsible for erosion in the notches. The rate of this bioerosion is not known but is considered to be at least fast enough to have cut notches up to four meters deep during the Holocene. Massive breaks in the upper cliff appear to have resulted from undercutting by notching combined in some instances with the hydraulic action of waves.
Some examples of possible higher level notches are discussed, but no unequivocal examples were seen. Holes made by bivalve molluscs several feet above the level at which living animals occur afford more reliable evidence of former higher sea level. Oyster shell encrustations are similarly found several feet above living animals.