Discussion

Abstract

W. S. McKerrow & L. R. M. Cocks write: Contrary to the statements by Mitchell, the geological data do not permit the interpretation of the Northern Belt of the Southern Uplands as a forearc basin to the north of the Lake District arc, nor do they permit initiation of rifting between this arc and the Southern Uplands in the earliest Caradoc, nor can continental collision have occurred in the latest Ordovician. Moreover, the progressively younger sequences towards the south point to accretion on the south margin of a continent, not on a north margin. Mitchell notes that ‘a possible difficulty with the above interpretation is the presence of Ordovician fossils of North American (Laurentian) affinity in the Northern Belt and European (Cadomian) faunas in the Lake District (McKerrow & Soper 1989)’. However, he does not present any solution to this ‘difficulty’; nor does he comment on the fact that in the Central Belt there are several sequences of graptolitic Moffat Shales showing continuous deposition from the Caradoc to the Llandovery. There could have been no substantial late Ordovician tectonic activity anywhere close to the Moffat Shales. The information from Newfoundland cited by Mitchell is not particularly relevant to the problem since the date of the Iapetus closure is difficult to deduce from that area alone (McKerrow & Cocks 1977). We would repeat briefly that the shallow shelf faunas of the Lake District and the Northern Belt are distinct throughout the Cambrian and the Ordovician until the Caradoc and later (Fortey