I also dug an old mill stone, but mine was behind an 1849 mill. I believe my stone was a drive belt stone as in a pulley system due to the grooves on the outside of the stone. Yours may also have been a drive pulley for the old leather straps they used for belts. That was very common to use...
What is a quahogger?? LOL Nice Finds - Some rivers and streams are good now with the water so low- found some nice bottles and old wood wagon wheels that's been under water/mud on a stream bank for ??? yrs. The flood of 2010 uncovered some good stuff too.