Minimal geologic studies have been conducted at the Franklin Glacier Complex and its geology is therefore poorly known. It measures wide and long, covering a northwesterly elliptical area of . Volcanic rocks of the Franklin Glacier Complex consist predominantly of dacite breccia, minor dacite flows and a few hornblende andesite remnants. An absence of pumice, glassy blocks or bombs in the breccia suggests that it may be of epiclastic origin, possibly formed during collapse and infilling of a caldera. The complex has therefore been described as a caldera or a cauldron subsidence. Heavy erosion of the volcanic rocks has exposed a series of biotite-quartz-porphyry, biotite quartz-feldspar-porphyry and quartz monzonite subvolcanic plutons and dike swarms. They intrude through fractured and hydrothermally altered Mesozoic to early Tertiary granitic and metamorphic rocks of the Coast Plutonic Complex.