The late Precambrian/early Cambrian Monian basement to the lower Paleozoic cover in Anglesey does not form one coherent stratigraphic unit but is divisible into at least three separate suspect terranes. Where they are exposed, the boundaries to these terranes range from ductile shear zones to brittle faults, all of which have an undetermined amount of displacement. Terrane 1 is represented by the Monian Supergroup: a thick sequence of marine sedimentary rocks with subordinate basic volcanic rocks, gabbros, and serpentinites, overlain by olistostromic mélange. Terrane 2 is identified as the Coedana Complex of central Anglesey: high-grade gneisses and the ca. 600-Ma Coedana Granite with an associated hornfels. Terrane 3 is represented by the southeast Anglesey schist belt and includes the ca. 550 to 560-Ma Anglesey blueschists. These suspect terranes are bounded to the southeast by the Menai Strait Fault System, to the southeast of which rocks of the Sarn Complex are placed within a separate terrane which may represent the northwest margin of the Avalon terrane in Britain. Terrane 1 and possibly terrane 2 are traceable into southeast Ireland (Cullenstown Formation and Rosslare Complex). Monian terranes are not known from elsewhere within the British Isles, although broadly coeval rocks occur in different terranes as the Avalonian/Cadomian rocks of southern Britain and the Channel Islands, and the Dalradian Supergroup of Scotland and northwest Ireland. No Monian rocks have yet been identified from the Tornquist line in the east, whereas several attempts at correlation have been made between Anglesey and the eastern seaboard of North America. Such attempts have failed to appreciate that there is not one “Mona Complex,” but a collage of disparate terranes, some of which are juxtaposed by shear zones comparable in scale to major crustal shears developed along transcurrent plate boundaries.

Paleomagnetic data confirm geological evidence for tropical paleolatitudes for deposition of the Catskill Delta and related Old Red deposits in Europe. A tectonic model constrained by paleomagnetic data suggests that the Catskill deposits are the product of the Acadian orogeny of the Northern Appalachians as a complex continent-continent collision between Armorica (Hercynian Europe), Laurentia (cratonic North America) and possibly Gondwana, with the Traveler terrane (central New England and New Brunswick) rotated and compressed in between.

The Late Devonian Catskill Delta is made up of marine and non-marine facies built up on the flank of the tectonic Appalachian Peninsula during assembly of the Old Red (Laurasian) Continent. Much of the continent was under the influence of tropical climates showing a wide range of rainfall. Over the delta, the climate was either tropical wet and dry or desert, due in part, to a rainshadow effect caused by the mountains to the east. Streams showed great variations in discharge and an extended period of drought was an annual event over the region. Alluvial processes were dominant on land. Braided streams deposited the coarsest sediments on alluvial fans and sinuous, channelized streams deposited sand and mud on the alluvial plains. Interfluves on the alluvial plains were sufficiently long-lived to permit the formation of carbonate soils. Plants were most common near stream courses. Fine sand and mud were carried across the shoreline in distributaries to the floor of the adjacent Catskill Sea. Deltaic processes, wave-related processes, and tides shaped the shore. Wave-related processes and bioturbation modified and reworked the shallow marine sediments while turbidity currents and slow deposition from suspension were most effective over the rest of the basin.