The Position of the Wenlock/Lundlow Boundary in the Silurian Graptolite Sequence

SIR,-We are currently preparing a detailed account of the faunal stratigraphy of the Wenlock/Ludlow passage beds of Wenlock Edge and Hopedale, Shropshire.Our communication is prompted by the letter of Warren, Rickards, and Holland in this Magazine (1966,103, p. 466-7).The comments of these authors pertinent to our study involve the assignment of the Wenlock Limestone to the graptolite zone of Pristiograptus ludensis ( = vulgaris) and the reference of this zone to the Wenlockian.We concede that the base of the Ludlovian Series as defined by Wood (1900, p. 422) at the base of the P. ludensis (M.vulgaris) zone is no longer tenable following the report by Warren, Rickards, and Holland that species indicative of this zone occur below and within the Wenlock Limestone of the Ludlow area.We fail, however, to agree with their statement (p.467) that the base of the Ludlovian as currently defined at Ludlow by Holland, Lawson, and Walmsley (1963, p. 139) " would appear to lie more or less at the base of the Pristiograptus nilsonni zone ".The Wenlock/Ludlow junction in fact occurs at the base of the " Barren " beds of Alexander (1936) (= Lower Elton Beds), from which graptolites have not yet been recorded at Ludlow.In our study of the passage beds between the limestone facies of the Wenlock Limestone and the Lower Elton Beds of Wenlock Edge we have found no evidence to suggest that the latter beds (which are the direct correlatives of the lower Elton Beds at Ludlow) lie within the zone of P. nilsonni.In fact there is evidence to the contrary from the report of Das Gupta (1932, p. 351-2) who identified the graptolite species Monograptus vulgaris ( = Pristiograptus ludensis), M. colonus var.ludensis ?(= P. ludensis) and M. dubius (= P. dubius) from an horizon within the Lower Ludlow Shales 100 feet above the Wenlock Limestone at Millichope.One of us (J.H. S.) has determined Das Gupta's horizon to lie within the lithological transition from Lower to Middle Eltonian and immediately below beds with a P. nilssoni zone graptolitic assemblage.The Middle Elton Beds at Ludlow have yielded a similar P. nilssoni zone assemblage.(Holland, Lawson, and Walmsley, 1963, p. 107.)This evidence leads us to believe that the Lower Elton Beds represent a dominantly shelly lateral equivalence of the uppermost P. ludensis zone as it occurs in the basin facies, and that the Wenlock/Ludlow boundary as defined by Holland, Lawson, and Walmsley lies within this zone.If, as Warren, Rickards, and Holland suggest, the base of the P. nilssoni zone is accepted as the base of the Ludlovian, we consider that the shales (Lower Elton Beds) immediately overlying the Wenlockian limestone facies should then be regarded as the highest division of the Wenlockian, in which conclusion we voice the opinions of Ketley (1865) and Davidson and Maw (1881).