Geologically the area can be grouped into two main rock types.
Laterite is a ferruginous, reddish, soft rock with irregular galleries filled with yellow clay, which hardens on exposure to atmospheric agencies. Laterite formation is attributed to monsoon conditions repeated through ages.
Decomposition of gneiss and partial rearrangement by the mechanical action of water give rise to lateritic rocks.
The mountains of the Western Ghats are composed of ancient crystalline and metamorphic rocks of Achaean age, mainly quartzose gneiss.
The principal rock types are granites and their gneissic variations. The gneissic rocks are biotite gneiss, their chief constituents being quartz, felspar, biotite and garnet.
The main feature of gneiss in the Western Ghats is the tendency to weather or decompose, generally into white, yellow or red felsparic clayey rocks, which in many places often very extensively, become lateritic.
The soil types met with in the sanctuary are mainly laterite soils, red soils and forest and hill soils.
Lateritic and red soils are found within the lower reaches where the soil depth is fairly good.
As the elevation increases the soil changes to forest and hill type. Also, at the lower reaches, soil contains appreciable amounts of gravel, which indicates good internal drainage.
The accumulation of humus in the topsoil gives it dark reddish brown to dark brown colour, which changes to red in the sub soil.
The surface soil has granular structure, which favours good root development. The soil is predominantly non-calcareous in character and acidic in reaction.