The Onondaga Limestone is composed of four main subunits.
In descending order:
Seneca Member
Moorehouse Member
Nedrow Member
Edgecliff Member
The Seneca and Moorehouse members are sharply divided by the Tioga-B bentonite layer, which was formed in a very short time period as the result of a large volcanic eruption in what is now the state of Virginia.

Relative age dating of the Onondaga places its formation in the Eifelian to Givetian stage of the Middle Devonian period, or 391.9 to 383.7  Ma.
Radiometric dating of a sample from the bentonite at the top of the Onondaga placed it at 390 ± 0.5 Ma.
The formation is time equivalent with the Floresta Formation of the Altiplano Cundiboyacense, Colombia.
The Onondaga Limestone is composed of four main subunits. In descending order:
Seneca Member
Moorehouse Member
Nedrow Member
Edgecliff Member
The Seneca and Moorehouse members are sharply divided by the Tioga-B bentonite layer, which was formed in a very short time period as the result of a large volcanic eruption in what is now the state of Virginia.
Relative age dating of the Onondaga places its formation in the Eifelian to Givetian stage of the Middle Devonian period, or 391.9 to 383.7  Ma. Radiometric dating of a sample from the bentonite at the top of the Onondaga placed it at 390 ± 0.5 Ma. The formation is time equivalent with the Floresta Formation of the Altiplano Cundiboyacense, Colombia.
Onondaga Limestone