ridge, going south immediately to the west of Munnar and continuing on to Parvathi Mala forms the western boundary of the High Range Plateau. This ridge separates the High Ranges from the Lower Periyar Valleys further to the west. The eastern edge of the High Range plateau runs north from Kolukkumala, at the junction of the High Range Plateau with the Cardamom Hills, and continues onto the Top Station along the border between Kerala and Tamil Nadu. The High Range portion is essentially the south and west draining slopes of the main Anamalai ridge with the Cardamom Hills located to its south. All the northern slopes of the main Anamalai ridge falling within Kerala drain into Idamalayar and Pooyamkutty Ar, both tributaries of Periyar. The High Range Plateau has been almost totally transformed by tea plantations and hydroelectric dams. The original forest was mostly of the Shola-Grassland type. From the biodiversity value, this unit is of special importance because of its specialized high altitude flora and fauna. Lower Periyar Valleys 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. Towards north, the Lower Periyar Valleys and the Periyar Basin extend up to the Idamala gorge cut by Periyar’s northernmost tributary – Idamalayar. The basin is bounded in the north by the ridge separating Sholayar from the Idamala River. This ridge is a spur hill extending from near Valaparai in Tamil Nadu, projecting far west