The Sook Association is extensive in the Sook and Penawan Plains. It also occurs:
In the Ruku Ruku Valley,
In the Labuk estuary, and
In the valleys of the Bangan and Maliau Basins.
It occurs on valley floors and slightly dissected terraces of medium- to coarse-textured alluvium.
The landform comprises a series of low terraces and floodplains which in the Lokan Peneplain appear to represent old courses of the Lokan River and its tributaries. In the Bangan Basin and east of the Pinangah River, the association occupies concentrically arranged troughs bounded by sandstone ridges. The troughs contain minor streams with narrow floodplains and low terraces.
Lowland dipterocarp forest is widespread with patches of heath forest on terrace remnants. The forest on the floodplains has many small trees and is often swampy with sedge undergrowth.