You live in the same general area of the country as I do so I can tell you one thing....after x amount of years, things end up deep!
Take a look at the post I made of an old school in the middle of the town where I live.
http://forum.treasurenet.com/index.php/topic,401510.msg2860607.html#msg2860607
Built 1921 and the brick sidewalk...in the middle of a milltown, is under 5 inches of dirt. So figure approximately 90 years to add 5" of dirt onto brick, let alone some place out in the woods where leaves and such go unraked, decompose, erosion, etc.....
There is a suprising number of research tools online I use that would benifit you. "Google Earth" is great when you use it with the overlay feature with pics from "Penn Pilot". Google Earth also has the historic bar where they have images from other years so you can see changes over the last 20 years or so.
Sanborn maps are available in most libraries. From the late 1800's to early 1900's, they were old fire insurance maps and extremely detailed, showing every building in detail, businesses, whatever was in the vicinity of buildings....like I said Very detailed. When Sanborn was free on the internet, I downloaded every single map available for Western Pa. Now there's no online access free to the public except through some libraries and universities. Neither Pitt not Penn State have online access nor the Carnegie Library. If you have a particular area in mind, I may be able to sent you a Sanborn map of the area in PDF format.
And just a note...it took me days to download the maps and I am glad I did. I have them backed up on 3 different drives just in case....I find them that valuable.
Most counties have websites and there's tons of information you can weed through just by googling a town. It's all about time and patience.
I also hit 2nd hand bookstores and buy the history books that were written in the late 1800's, early 1900's. Much more information in them than the watered down versions of today.
Carnegie Library has an inter-library exchange progrm with other libraries. The main branch in Oakland has a Pennsylvania room just bursting at the seams with historical data. But don't bother with the book on playgrounds. It is obviously well worn from the many detectorists that have looked through it....lol
Depending on where you hunt, you may consider getting a bigger coil to give you that extra inch or 2 of depth and maybe even a smaller coil to hit that old historic park polluted with trash.
Good luck in your research...Western Pa is indeed full of great detecting sites.
Al
Just one more thing...if you aren't a charter member on this site, you may want to consider it. The search feature is great and there are tons of good links to sites in the Pa. and maps forum.