Does anyone how deep the frost line is in West Virginia ? I 'm north- west in the state , between Clarksburg & Parkersburg .
Do you have a permitting authority in your area? If so, you can ask them. My guess would be 24", but if your just digging post holes, the deeper the better..
Look here:
http://www.soundfootings.com/pdf/US_Map_Frost_DepthAVG.pdf
I'd be just as concerned about lateral movement in your soil type.
You are between 18-24". So I would go with the later. I also sent you a PM earlier.