In central Pennsylvania, the basal "calico" limestone is a fossiliferous, medium-light- to medium-gray very thick bedded calcilutite containing numerous small irregular patches of clear calcite.

[3] The depositional environment of the Keyser is interpreted as shallow marine and tidal flats, occurring in cycles.

[4] Contact with Tonoloway Formation is probably conformable, but sharp enough to suggest an unconformity.

The type locality of the Byers Island Member is at Selinsgrove, Pennsylvania, where it is 93 feet thick.

[6] Relative age dating places the Keyser in the late Silurian to early Devonian, with the transition occurring near the top of the formation.