J. R. LOGAN AND THE ORIGIN OF THE TROPICAL INSELBERGS KNOWN AS NUBBINS (OR KNOLLS)

Abstract

Pulau Ubin is located in the Johor Strait between the Malay Peninsula and Singapore Island. The topography shaped in a granitoid complex of Permo-Triassic age, comprises jungle covered and isolated blocky hills, a flat and marshy central valley floor, and a coastline in which grooved boulders are prominent. In papers published in 1847, 1849 and 1851 (but here reporting their content in modern terminology) J. B. Logan attributed the island landscape to deep differential chemical weathering, followed by exposure of the resulting bedrock morphology. This is characterized by massive blocks protruding between a saprolithic cover—in modern parlance a two-stage etch origin of a group of inselbergs of nubbin (or knoll) type covered by blocks and boulders derived from the breakdown of the outer shell or shells or sheet structures. The Pulau Ubin nubbins carry remnants of a saprolithic veneer and are of ‘wet’ type by contrast with the saprolith-free ‘dry’ forms of arid or seasonally arid climes. The conversion of nubbins to bornhardts in arid climes also is noted. Logan identified the crucial aspects of what is still a highly favoured hypothesis of inselberg origin based on field evidence from many parts of the world and articulated by Falconer in 1911. But there was in Logan’s time no agreed and appropriate technical terminology. Moreover, some of Logan’s reports such as that dated 1849 are disorganized and diffuse, but contain occasional insightful observations and deductions.