[1][2] This col is an important vertebrate fossil locality at which at least a dozen specimens of the mammal-like reptile Thrinaxodon were found.

[3] The fossiliferous sedimentary strata exposed at Thrinaxodon Col consist of alternating mudstones and sandstones of the lower Fremouw Formation.

Diabase sills and dikes cross-cut the Fremouw Formation throughout the Thrinaxodon Col area and region.

An individual cycle consists of a basal fluvial sandstone that grades upward into an upper overbank mudstone.

The Thrinaxodon liorhinus remains occur in Triassic sandstone that fills the upper portion of a paleochannel.