So I am up on the northern end of Fairfax and I don't have too many problems with depth on the at pro. My biggest issue is the high iron content and little pockets I get here and there. I also found when I push out more to Remington on some friends properties the soil has so much iron you can barely detect.
*I should add, while I have found things in clay well beyond 8", most of my civil war finds in our clay here are close to surface. But I am also working land that has not been developed.