Well, all - or essentially all - of soils in PA are were originally forest lands.  (The only exceptions might be the areas in the Southeast corner of the state next to the Delaware River, and the northwest corner of the state next to Lake Erie.

Just because all the soils were originally forested regions does not mean you can lump them all into one group and treat them all the same.  The Penn State Agronomy Guide shows 12 different geological regions in the state, and the soils are based on the underlying geological base.   

In general, we talk about limestone soils, and shale soils as being the predominate agricultural soils; also have sandstone based soils.  However, a lot of times these are currently in forested areas.  

I find shale soils can have higher CEC on a soil test, but that does not mean they are are more productive.   And soil O.M. levels have more to do with management than they have to do with geological base.