The portion west of the main Periyar channel which flows from south to north here, is the true Idukki sub-plateau. This is the lowest part of the Cardamom Hills sloping towards the north in which the Idukki reservoir is located. It is separated from the rest of the Cardamom Hills by the deep channel of the Periyar River. The sub-plateau extends north from near Ayyappankoil to near Neriamangalam. Its western border is the clearly demarcated scarp face of the Ghats descending to the plains of Thodupuzha. This high western crestline separates the Periyar catchment from the Manimala, Meenachil and Moovattupuzha drainage basins. It ends in the north along the channel of Periyar from Neriamangalam to Panamkutty. The catchment of Kattappana Ar located beyond the southeastern corner of the Idukki reservoir also falls in this portion. A southwest to northeast ridge called the Chekuthan Mala separates the Idukki Sub-plateau from the Peermade Plateau as well as the main mass of Cardamom Hills. Its eastern boundary is a northwest to southeast ridge separating the Idukki reservoir from the Kallar/Perinjankutty tributary of Periyar which drains the main Cardamom Hills. The Idukki Subplateau is at an average elevation of . This area was heavily forested till the 1950s but now only three small fragments of forest vegetation alone remain. The largest is within the Idukki Wildlife Sanctuary. South of Vagamon, the tributaries of Meenachil, Manimala and Pamba have eroded away extensive areas of the Periyar basin between Ayyappankovil and Vallakkadavu. The Periyar only has a narrow strip of the Cardamom Hill tableland to flow northward here.