"In the area between the Lorton Fells and Outerside, where the stratigraphic succession is most nearly complete, the graptolite zonal succession in descending order is as follows: zone of Didymograptus hirundo and the zone of D. extensus. The extensus zone is capable of threefold division into the subzone of Isograptus gibberulus, subzone of Didymograptus nitidus, and the subzone of D. deflexus. Below the subzone of D. deflexus the Skiddaw group has proved unfossiliferous to date."

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