Hi All, The Historical and Museum Commission of PA has a very nice map showing the development of the counties from the beginning. I think it was only $3. It looks like about the lower third of Beaver County was in Cumberland Co. in about 1750, then in Westmoreland Co. in 1773. It further looks like all of Beaver County was in Allegheny Co. in 1788 - 1800. I have made inquiry as to townships in 1790 in Allegheny Co., but have not had a response as yet. Good hunting. Iral Nelson Richland, WA