Tricouni Southwest lies in the southern Mount Cayley volcanic field (MCVF), south of Tricouni Peak. It is an elongated basaltic andesite lava flow whose source vent has not been identified, and forms the east wall of a north-south gully up to 200 m deep, near the head of High Falls Creek. The eastern margin of the flow, outside the High Falls Creek gully, has a more gentle morphology. Complex fine-scale cooling joints and the overall flow morphology indicate that this lava flow was impounded against ice along the length of the gully. Near the south end, lava flowed into fractures or crevasses in the ice, although most of these spire-like cooling features have been removed by erosion. Ice-contact eruptive features are present, and include narrow joints (indicating rapid cooling), unusually thick lava flows, and vertical cooling surfaces (indicating lava impoundment against a barrier); these indicate that the eruption took place during the waning stages of the Fraser Glaciation (about 10,000 years ago). The reason the west side of the flow impounded against ice while the east side did not is probably that the west side of the flow is in a deep north-south gully, which would have received less solar heat than the open slopes on the eastern margin of the flow, and thus retained stagnant ice at a time when slopes to the east were bare.

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