Most of the sedimentary rocks of Small Nggela are early Miocene in age. In the north, uniform volcaniclastic arenite (theSiota Beds) crops out. Much of the central, western, and eastern parts of the island contain diverse strata including pillow lava, calcarenite, volcaniclastic arenite, grit, and breccia (theGhumba Beds). The Siota and Ghumba Beds, which are separated by a 1–1.5 km wide, east-trending ophiolite wedge, are considered to have been deposited in a forearc basin. The ophiolite wedge arrested the northward transport of coarse sediment and was emergent during the late Oligocene and early Miocene. The sediment and the ophiolite wedge are considered to have overlain a southward-descending Benioff zone.