AbstractThe isotope characteristics of interbedded fine‐grained sedimentary rocks and pebbles from an Ashgill sequence in the Welsh Basin show that they have different provenance. Gondwana‐derived continental crustal material dominates the Nd‐Sr isotope signatures of the fine‐grained rocks whereas the pebbles sample the Avalonian basement. Detritus from Gondwana and Avalonia accounts for the isotopic composition of most of the lower Palaeozoic sedimentary rocks of Southern Britain and this suggests that any contribution from the reworking of Lower Palaeozoic volcanic rock is