A vent agglomerate is a coarse grained volcanic rock with rounded to sub-angular fragments. It forms from pyroclastic eruptions and generally fills in the vents of volcanoes during explosive activity or during caldera collapse. Often the origin of such deposits are uncertain and the term agglomerate has been used to define deposits which are actually vent breccias or debris flows like lahars.

On Arran, the Central Ring Complex contains vent agglomerate. These rocks are thought to be the result of the very explosive igneous activity which resulted in repeated caldera collapse. The agglomerate is often composed of a variety of igneous materials of different composition, along with xenoliths of country rock incorporated during eruption, set in a fine grained igneous matrix.

In Glen Craigag, south of the String Road, exposures of rhyolitic vent agglomerate from the Central Ring Complex can be found. These agglomerates contain a diverse set of clasts: quartzite, vein quartz, Dalradian schists, basalt, and sedimentary rocks of Devonian age, as well as the fine grained acidic matrix which gives the agglomerate its name.