A model for the early Palaeozoic metamorphic history of the Midland Valley and adjacent areas to the S in Scotland, England and Ireland is based on the results of new field mapping, thin section petrography, electron probe microanalysis, X-ray diffractometry, conodont and palynomorph colouration and graptolite reflectance measurement.The oldest metamorphic rocks of the Midland Valley of Scotland, excluding xenoliths in post-Silurian lavas, are possibly the blueschist occurrences in the melange unit of the Ballantrae complex. These may be tectonised remnants of (?)pre-Arenig ocean-floor subducted during closure of the Iapetus Ocean. In the early Ordovician, the melange terrane was dynamothermally metamorphosed during obduction of newly-formed ocean crust. The obduction process piled up a thick sequence of various ocean-floor types such that burial metamorphism in parts reached pumpellyite-actinolite facies; elsewhere prehnite-pumpellyite and zeolite facies was attained.Whilst the Midland Valley acted as an inter- or fore-arc basin during the Late Ordovician and Silurian and experienced burial metamorphism, an accretionary prism was formed to the S. Accretion, tectonic burial and metamorphism of ocean-floor and trench sediment was continuous in the Southern Uplands and the Longford-Down massif of Ireland through Late Ordovician to Late Silurian times. Rocks at the present-day surface vary from zeolite facies to prehnitepumpellyite facies. Silurian trench-slope basin sediments can be recognised in part by their lower grade of burial metamorphism. Greenschist facies rocks of the prism probably lie close to the surface.The Lake District island-arc terrane of Northern England has an early Ordovician history of burial metamorphism up to prehnite-pumpellyite facies. The Late Ordovician and Silurian metamorphic history is one of sedimentary burial complicated by tectonism and intrusion of granite plutons to a relatively high level. The Iapetus suture is marked by a weak contrast in metamorphic grade.

NE-SW faults in the Bail Hill-Abington area of the Northern Belt of the Southern Uplands define blocks up to 3.2 km wide. The strata, folded and locally overturned, young predominantly to the NW but successive blocks to the SW contain progressively younger sequences. Analogous configurations occur in modern accretionary margins. The oldest rocks are Arenig basalts, dolerites, cherts and brown mudstones underlying red shales, possibly Llanvirn, and black fossiliferous shales and cherts of Llandeilo and Caradoc age.Trench sediments overlying pelagic sequences represent a range of depositional mechanisms. Rudites and associated fine-grained lithologies of lateral origin relate to a lower trench slope canyon system, whilst axially transported sands, originating on the lower trench slope, were deposited by turbidity currents and related flows.The Bail Hill Volcanic Group (Upper Llandeilo) represents a mildly alkaline seamount in the Iapetus Ocean, with volcanic activity spanning the transition from pelagic plate to trench sedimentation before accretion.Faults, initially low-angle thrusts, and bedding were rotated through the vertical within the accretionary complex, pre-dating or accompanying slaty cleavage development. Soft sediment deformation, two fold phases and a kink-band set are recognized. Imbricate fault zones located in incompetent pelagic sequences are tentatively equated with tectonic m61anges of other accretionary complexes.Index minerals, illite crystallinity and 'vitrinite' reflectance establish metamorphic grade as a zeolite to prehnite-pumpellyite facies.