New paleomagnetic data for Britain establish the width of the Iapetus Ocean as ∼1000 km in mid-Ordovician time but do not yet allow precise estimates of the rate of its subsequent closure. The new lower Paleozoic data from northern Scotland are similar to data from the rest of Britain but are not compatible with North America data on the Bullard et al. reassembly. If this discrepancy is correct, lines of major transcurrent dislocation have yet to be identified either in the North Atlantic continental shelves and margins or in eastern North America, and the Great Glen fault was not the main locus of offset between Britain and North America. The tectonic complexities associated with this configuration require further evaluation.

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