The lower fluviatile stage consisting of 200250 m thick granular sediment is overlain by lacustrine sedimentary stage of 200-300m section of clay sediment.
Although much of its present hummocky topography is related to a large volcanic debris avalanche occurred in the zone, this lacustrine area was not originated by such event.
With the exception of the river valleys and lacustrine basins, it is a rolling to mountainous peneplain that ranges from 800 to 1400 feet above sea level.
