Since I live in the Area you’re talking about I’ve always wondered why so many of the trees are like you described. I’m originally from Ohio so I figured it was something about the stones in NJ as I’ve never heard of “Coppiced”.
In fact, I have a “5 tree” Oak on my property that is probably 6 feet at the base that splits into 5 18”-24” trunks and I’ve always wondered how it got that way.
Ron Dupont, a local author and owner at the HL general store wrote a book on the History of Vernon. http://www.amazon.com/Vernon-Township-NJ-Images-America/dp/0738511102 It has pictures showing the area completely denuded of trees. What a difference it is compared to today.
The difference today is amazing; one of the prerequisites for operating forges/furnaces by iron mines was inexhaustible wood supply, and they meant it. The properties purchased by Peter Hasenclever, the Parrott brothers (who invented & produced the gun of the same name for the Civil War), and others were massive because of that requirement.
On the NY side of the border you’ll see many trees like that as well. I envy your proximity to those areas, I drive 45 minutes north to get to the closest ones.