 Above this is the 43m-thick Birkhill Shale, which consist predominantly of black graptolitic shale with subordinate gray mudstones and meta-bentonites.The base is identified by the appearance of the graptolites "Parakidograptus acuminatus" and "Akidograptus ascensus" at Dob's Linn.The Llandovery epoch is subdivided into three stages