With a length of about 80 kilometres, the range includes the state's second highest peak, Mount Trusmadi, after which it is named.

[1][2] Its geology comprises tertiary formation of mudstone, shale and argillite with subordinate beds of quartzite, sandstone, siltstone and limestone breccias.

[1] The range physical characteristics strongly dissected into steep sided valleys and narrow-crested ridges where it is separated from the Crockers by the Tambunan-Keningau-Tenom plain.

The largest catchment area is the headwaters of the Pegalan River with a number of tributaries including the Ambual, Kaingaran, Keinop, Linsudon, Mailo, Monsok, Sembuan, Sungei and Sook flow to the western direction into the South China Sea while the smallest catchment is irrigated by several tributaries of Liwagu River that flow into eastern direction towards the Sulu Sea.

[2] With its higher elevation, the range hosts around 172 ferns species in 75 genera and 23 families.