It is typically found conformally overlying the Marcellus Formation, and underlying the Tully Limestone (where present). The Mahantango has been divided into four members: 1  Tully Limestone: a fossiliferous calcaerous shale. 2  The Tully is a conspicuous formation, which separates the Mahantango from the Hamilton Group.3  Sherman Ridge: Olive gray, fossiliferous, claystone and siltstone with interbedded fine sandstones coarsening upward.t is named for the North branch of the Mahantango Creek in Perry and Juniata counties in Pennsylvania. It is a member of the Hamilton Group, along with the underlying the Marcellus Formation Shale.