The Lower Rhyolitic Tuff Formation (up to 600 m thick) represents an eruptive cycle of acidic ash-flow tuff which is stratigraphically associated with marine sediments and subaqueously emplaced basalt lavas. The formation comprises volcaniclastic and pyroclastic megabreccias and breccias, massive welded and non-welded acidic ash-flow tuffs, reworked tuffs and tuffities, siltstones, rhyolite intrusions and extrusions. Its basal contacts vary from conformable, to disconformable and unconformable. The inter-relationships of these variations to pre-, syn- and post-emplacement structures define a submarine, asymmetric downsag caldera. The main eruptive centre, coincident with the thickest accumulation of intracaldera tuffs, lies close to its north margin, on the north side of the Snowdon Massif. To the SW, the intracaldera tuffs thin progressively and much of the formation comprises tuffs reworked in the vicinity of a Caradocian shoreline. To the NE and E, outflow tuffs escaped into a deeper marine basin. Many of the features of the caldera are similar to those of subaerial calderas, and it is concluded that the enveloping sediments and lavas, and the character of the reworked tuffs, hold the key to the recognition of its submarine development. Subsequent resurgence resulted in only local and short-lived emergence of the intracaldera tuffs.