The Iapetus Suture (Solway) line coincides with a magnetic low, which lies between magnetic highs over southwestern Scotland and the Lake District-Isle of Man region. Although topography on deep magnetic basement can account for these long wavelength geophysical variations, an explanation which involves lateral basement magnetization contrasts is preferred on the basis of (a) correlations between inferred magnetization boundaries and major structures delineated from other evidence, and (b) the apparent westward continuation of the Solway low through Ireland and Newfoundland across areas with very different subsidence histories but similar position with respect to the collision of Laurentia and Avalonia. In the preferred model, relatively magnetic continental crust beneath the Southern Uplands and Lake District terranes is separated by a zone of less magnetic crust interpreted as sedimentary rock of Avalonian affinity carried to deeper structural levels within the Iapetus Suture Zone. The magnetic unit beneath the Southern Uplands is bounded to the south by the northward-dipping Iapetus Suture and to the north by a structure which may have been reactivated in late Caledonian times to produce the Moniaive Shear Zone in the overlying rocks; this unit may represent the ‘missing’ arc terrane inferred from provenance studies. Alternatively, the two magnetic basement domains may have originally been part of the same terrane, with that portion beneath the Southern Uplands rifting from the Avalonian continent during its northwards drift and being subsequently trapped in the hanging wall of the Iapetus Suture. The southern margin of the Lake District domain appears as a discontinuity in the magnetic anomaly pattern, with long wavelength anomalies to the south having a southeast ‘Tornquist’ trend.

Late Caradoc graptolite assemblages across the Iapetus Ocean in Wales and Scotland became progressively more disparate despite the narrowing of the ocean. We compare faunal distributions in continuous sections from opposite sides of Iapetus, at Whitland in South Wales and Hartfell Score in the Southern Uplands of Scotland. A comprehensive graptolite range-chart is given for each. The graptolite assemblages from the clingani Biozone are subdivided into a lower Ensigraptus caudatus Subzone and upper Dicellograptus morrisi Subzone at both localities, though the faunas differ in detail. Higher in the sequence, the distinctive Scottish linearis Biozone fauna is not recognizable at Whitland, its presumed equivalent being a fauna dominated by variable morphotypes of the genus Normalograptus. This suggests that environmental gradients (depth and/or temperature) were changing more rapidly than the geography. Significantly, the subsequent inception of limestone deposition at Whitland was approximately contemporaneous with widespread replacement of black mudstones by oxic, bioturbated sediments elsewhere in the Welsh Basin and in Scotland. This was possibly a response to an early phase of cooling prior to the end-Ordovician glaciation. In Wales the claimed hiatus at the Caradoc-Ashgill boundary may rather reflect biofacies variation.