The lower valleys of Idamalayar and Pooyamkutty Ar constitute the Lower Periyar Valleys. The exceedingly violent impact of very heavy rainfall on a likely geologically unstable part of the Western Ghats where there is extensive faulting has caused accelerated weathering and erosion, carving out a complex of deep valleys on the western face of the High Range Plateau. This topographically complex segment of the Western Ghats extends from near Neriamangalam in the southwest to Panamkutty in the southeast along the Periyar River and continues north up to the watershed divide between the Periyar and the Chalakudy Basins. Its eastern border is the main Anamalai ridge. This section of western slope of the Western Ghats is deeply dissected and has geographical as well as ecological characteristics found nowhere in the Western Ghats. To the south of this section is the Idukki sub-plateau and to the southeast, the Cardamom Hills. The western edge of Panniyar and Muthirapuzha Basins form the eastern border of this section. The Parvathimudi-Umayamala ridge forms the northeastern corner of the Lower Periyar Valleys.