The East End Beacon has a nice article about postal route markers in this area of Long Island in New York.

Following Ben Franklin’s Path Down the North Fork

August 24, 2014

If you are steeped in Southold lore, you know about the day in 1755 when Benjamin Franklin, then the postmaster for the British colonies, took a trip down the North Fork to measure the distance from the Suffolk County courthouse in Riverhead to the Oysterponds ferry in Orient, with a crew who placed granite mile markers along the path.

The mile markers were used to gauge postal rates, which in those days were determined based on the distance letters traveled.

Many of these headstone-like slabs are still here, in Southold Town (historians have surmised that the mile markers in Riverhead Town were wood, and haven’t survived the more than 250 years since Franklin’s crew had them installed).
….

Full story and images here.

See also the
Southold Historical Society, 54325 Rte 25, Southold, NY 11971.