SUMMARY The Ingleton Group is a sequence of steeply dipping turbiditic greywackes that crops out in two of the Craven inliers at the southern margin of the Askrigg Block, at Ingleton and in Ribblesdale. The structure has previously been interpreted as a series of concertina-like isoclinal folds, much more intense than the deformation recorded in Late Ordovician–Silurian strata of the overlying Windermere Supergroup. For this reason, the Ingleton Group was long regarded as late Precambrian in age. In 1982 the British Geological Survey (then Institute of Geological Sciences) assigned it to the early Ordovician on the basis of an acritarch assemblage recovered from a single borehole sample. This paper reports a resurvey based on the ample wayup evidence provided by turbidite bedforms. The greywackes are disposed in a large isoclinal fold pair, the previously recognized Skirwith Syncline in the Ingleton Inlier and the Horton Anticline mapped in Ribblesdale. Some 3 km of strata are exposed on the common limb, and assigned to five lithofacies. A start has been made on lithostratigraphical subdivision, three formations being recognized in the uppermost part of the succession. Isoclinal folding of the Ingleton Group predates the Acadian (Devonian) deformation of the overlying Silurian strata; the Acadian cleavage is superimposed obliquely across the isoclines and faces down on their inverted limbs. The group records a prelate Ordovician history comprising the deposition of penecontemporaneous continental volcanic arc detritus in a substantial turbidite basin, intrusion of mafic dykes, major folding and low-grade metamorphism, followed by deep erosion. Uncertainty about the depositional age of the Ingleton Group presents problems as to its affinities. Lithofacies and provenance characteristics match late Neoproterozoic Avalonian interarc basin deposits in the English Midlands and Welsh Borderland, but not the apparently contemporaneous Ordovician Skiddaw and Manx groups. Confirmation of an early Ordovician age would suggest that the deformation history of the Ingleton Group matches that of the Monian Supergroup of Anglesey. The problem is discussed in the context of Ordovician terrane assembly.

The central zone of the Outer Hebrides is composed of a gneissic (‘grey-gneiss’) complex containing evidence of the earliest as well as many of the later events in the history of the Lewisian. In northern South Uist, an old metamorphic complex is intruded by pegmatites which were strongly deformed by a tectogenesis equated here with the Scourian of more than 2600 m.y. ago. It is concluded that much of the original material of the greygneiss may represent the products of a pre-Scourian orogenic cycle of possible Katarchaean (>3000 m.y.) age. Widespread post-Scourian basic (tholeiitic) dykes which cut the grey gneiss are thought to be equivalent to the Scourie dykes of the Scottish Mainland. These intrusions were subjected to granulite-facies metamorphism, regarded as Early Laxfordian, followed by retrogressive amphibolite-facies metamorphism with local folding and migmatization, all of Late Laxfordian date (1500 to 1600 m.y.). A number of dykes described from Benbecula and South Uist illustrate stages of Laxfordian deformation in which perfectly preserved cross-cutting relationships are progressively obliterated, the final products in the southern zone of South Uist being highly deformed and disrupted amphibolite masses with concordant margins in mobilized Laxfordian migmatitic gneiss.

SYNOPSISThe symposium comprised seventeen contributions on the following subjects: palaeontological evidence for the age of the Dalradian, the Ordovician of western Ireland and other formations relevant to the dating of events (2 papers); the sequence of tectonothermal events in parts of the Moine Thrust Belt and the Moine Nappe (4 papers); deformation rates (1 paper); isotope ages and their relation to events in the Moinian and Dalradian areas (4 papers); early events in the Scandinavian Caledonides and their isotope dating (3 papers); and wider geotectonic aspects, including a plate tectonics synthesis (1 paper), a comparison of Newfoundland and Ireland (1 paper), and new palaeomagnetic evidence for the location of the main continental masses in the Palaeozoic (1 paper).The symposium failed to settle the question of the significance of the Precambrian isotope ages obtained from various parts of the Moine outcrop. Several new Precambrian ages were put forward to augment those already published, but at the same time further geological evidence was adduced to show that the earliest tectonic structures in the Moinian were post-Arenig.Evidence was presented to show that Dalradian sedimentation in Scotland and Ireland north of the Highland Boundary Fault lasted from perhaps 700 m.y. ago (top of the Lower Dalradian=660 m.y.) through the Cambrian into the Lower Ordovician (Arenig or Llanvirn). The main folding and metamorphism of the Dalradian north of the Highland Boundary Fault occurred around 500-505 m.y. ago. In western Ireland south of the Highland Boundary Fault, Dalradian sedimentation ceased before the Arenig and the main polyphase folding and metamorphism occurred about 520 m.y. ago. The younger D 3 deformation phase in the Scottish Dalradian may be about 460 m.y. old and possibly equivalent to the pre-Silurian deformation of the Ordovician of western Ireland.It now seems probable that early Caledonian folding in the Scandinavian Caledonides is confined to the North Cape and Bergen regions and may be about 530 m.y. old. Elsewhere in Scandinavia the main Caledonian folding is of Upper Silurian date; K-Ar ages around 435 m.y. are reported in the Trondheim area. The Varanger Tillite is 660 m.y. old.Palaeomagnetic evidence was put forward which demonstrated the existence of a vast Hercynian ocean which closed around the beginning of the Carboniferous, but there is no evidence for a Caledonian ocean between the presently exposed forelands in Britain that could have been wider than 2000 km.