Not sure if it's a post oak, but it could be. I really like finding very old trees like that one. It helps put old boundaries in perspective, but usually the trees are long gone. I was figuring on not finding any trees when going over the deed but I have found the majority of them. And they are all massive.
I believe it is a white oak which has a growth factor of 5 X 54" = 270 years old, good call John.
Also a large tree like that would have one hell of a tap root that would remain for many many years after the tree is gone and could be held as a corner if the tree was called for.
International Society of Arboriculture has developed a formula for calculating age from simple measurements.
1
Determine the species of oak in your backyard. Oaks generally fall into three categories or clades: red, white and golden oaks. Look in tree identification resources online or in your local library.
2
Measure the circumference of the oak's trunk. Wrap the tape measure around the trunk at 54 inches above the ground.
3
Calculate the diameter of the oak. Use this formula: circumference (in inches) divided by pi, approximated at 3.14. For example, if your oak's trunk measures 314 inches around, its diameter would be 314 divided by 3.14, or 100 inches.
4
Determine the growth factor for your oak. If it is a Shumard or pin oak, the growth factor is three; red oak, four; white oak, five; and shingle oak, six. If you have a different variety of oak or were unable to identify the oak, obtain a rough age estimate by using four.
5
Multiply the oak's diameter in inches times the growth factor to get the approximate age of your oak. For example, a Shumard oak with a trunk diameter of 20 inches is about 60 years old.