The South Mayo Trough, an early Ordovician sedimentary basin, was developed at the southern margin of the Laurentian plate. It controlled deposition of 12.8 km of sediment. Basic vulcanism accompanied the opening of the trough. This was followed by the deposition of turbidites and finally of fluvio-deltaic sediments. Initial island arc vulcanism was replaced by a bimodal basalt–rhyolite suite during sedimentation. The trough was bounded to the north and south by metamorphic source areas, of which the southerly, the Connemara Cordillera, was the more important. The Connemara Cordillera, comprised of Dalradian continental margin sediments, was deformed and metamorphosed before the trough's formation. Uplift and erosion of Connemara, opening of the trough, and change in vulcanism all occurred during late Tremadoc – early Arenig times. These events are related to the collision of the Iapetus ridge with a trench to the south of Connemara, which initiated a Gulf of California-type marginal basin, the South Mayo Trough. Comparisons between the morphology and sedimentary history of the trough with that of the Gulf of California and the geology of their adjacent source areas support this hypothesis.