The Sinarun Association occurs:
In the Tenom, Keningau, Tambunan and Penawan Plains, and
In the Labau Pandewan valleys.
Parent materials comprise medium- to fine-textured, sometimes pebbly, alluvium overlying sandstone and mudstone. The alluvium often forms cappings on the hills with sandstone and mudstone outcroppings on lower slopes.
It forms on strongly dissected terrace remnants of formerly extensive inland plain at heights of about 450 m. These high level terraces are mainly surrounded by mountains rising to about 1,050 m but where the Labau has incised deep gorges, the terraces themselves from the positive relief. The dissected terraces are in the form of low hills and alluvial flats with an amplitude of about 30 to 45 m, convex crests and short moderate to very steep slopes.
Dipterocarp forest is widespread with swamp forests on some valley flats.