Abstract
In the Shevaroy and the Kollaimalai hills, Tamil Nadu, lateritic bauxite cappings occur at altitudes of about 1550 m and 1300 m, respectively. The gently undulating terrains on plateau landforms with flat-topped hills and wide shallow valleys and the altitudinal restriction of the cappings are suggestive of the prevalence of a planation surface/surfaces.
At first sight, the altitudinal difference of 250 m between the plateau landforms on the Kollaimalai and Shevaroys is suggestive of the prevalance of two independent surfaces. It is apparent that the two surfaces are erosional remnants of a single surface with a southerly slope of 0°15'. The sheared and ductile nature of the gneisses exposed in the east-west running Attur valley in between the two hill ranges suggests that the rocks represent deep tectonic levels of faults. The unsheared nature of the ultramafic emplacements (Chalk Hills of Salem, Precambrian in age) traced within the gneisses indicates that there was no large scale tectonic disturbance along the Attur valley in post-Precambrian times.
The plateau landforms with lateritic bauxite cappings at an altitude of about 2134m (7000') in the Nilgiri and Palni hills belong to the same planation surface as the one on the Shevaroys and the Kollaimalais.
It is suggested that lateritisation commenced towards the end phases of planation by erosion and continued after the formation of the surfaces.
