The Tully was deposited as a carbonate rich mud, in a shallow sea at the end of the Middle Devonian.

[2] It is also found subsurface in western Maryland and northern West Virginia.

A number of fossil remains from marine organisms may be found in Tully outcrops.

There there are also layers with much higher clay contend resulting in a calcareous shale.

Tullypothyridina, Camarotoechia Mesocostale, Rhyssochonetes, Emanuella, Pseudoatrypa, Spinatrypa, Tylothyris, Mucrospirifer tulliensis, Cyrtina, Tullypothyridina, Echinocoelia, Strophodonta Loyalsburg Formation