Hill tops are characterised by exposed sheet rock, although some possess a thin crust of soil and consequently have some grass covering. There are many boulders and rocky blanks on the slopes. The more eastern parts of the sanctuary have lateritic soils, possibly due to lower rainfall. The area is drained by several rivers, some perennial, and include the Thekkedy, Parambikulam and Sholayar, which flow westwards to converge at Orukombankutty and flow into the main Chalakudy River. The crystalline rocks of the area are poor aquifers and water is mainly confined to the 'joint plaines' in the rocks.
Weather conditions are more stable in the valleys.
However, the higher slopes become fairly cool during the monsoons. Mean annual rainfall is 2590mm, with a range of 1200-5000mm. Western and southern parts of the sanctuary receive higher precipitation than eastern parts. May. The slopes with evergreen or moist deciduous forest cover have higher relative humidity than the teak plantations in the valleys. Comprises a variety of natural and man-made habitats.