Since a bark mill was sometimes part of a saw mill operation and sometimes part of a tannery operation they were not often marked on maps in the 1800's, instead the title "saw mill" or "tannery" was shown on the map. There were however bark mills that stood alone by them selves.
The mill was turned by a mule (frequently a blind mule as all it had to do all day was walk in a circle). Usually boys operated it by thrown bark in the mill than shoveling up and packing the powdered bark into barrels. In turn the barrels were shipped to a tannery, (local or by ship to Baltimore) where the powder was watered down in vats and allowed to sit for several weeks to absorb the tannin in the bark and then the liquid was drawn off to other vats where hides were soaked in the solution it for six months to a year.
Interestingly in China in the 1800 and 1900's a blind man was used to push or pull the shaft on the grinder, as it employed the person and he worked cheap.