The islands of Guanaja (also called "Bonacca"), Roatan, and Utila are on the emergent crest of a narrow ridge on the south side of the Bartlett Trough. On Utila, alkaline basalt has erupted through Quaternary coral reefs; no older rocks are exposed. On Roatan, a varied sequence of gently folded metamorphic rocks of unknown age is overlain by limestone and conglomerate of pre-Tertiary age. Serpentine has been intruded along south-dipping thrust faults. The metamorphic sequence includes pyroxene hornblendite in various stages of uralitization.

The island of Guanaja is composed of graywacke, silty shale, and chert which are metamorphosed progressively to quartz-mica schist and gneiss and grade into sodic granite. Hornblende gabbro is associated with serpentinite and aplite dikes.

One set of faults on Roatan and Guanaja parallels the escarpment of the Bartlett Trough. Clear evidence of lateral displacement is absent, but vertical movement resulting in uplift and southward tilting of the islands has taken place in very recent times. Other steeply dipping faults are oriented at high angles to the trough. On Roatan, thrust faulting appears to have been important. Folding of the metamorphic sequence on Roatan and Guanaja has been oriented about axes that plunge gently west-southwestward parallel with the trough.