Mile 35.5 - There used to be 100 ferries operating on the Potomac, this is the last one and it's still pretty busy. The cars line up on what looks like a boat ramp and fill up the ferry. The ferry follows a wire cable to the other side (lower right). You can see how narrow the crossing is. It runs continuously, year round, from 5 a.m. to 11 p.m. $4 one way or $6 round trip.
For up-to-date conditions, call them at 301-349-5200. Flooding and ice can close the ferry.
You can download this PDF for explicit driving directions from I-270 & Route 28. It's about 1.7 MB. Or click on the "GoogleDirections" link at the top of this page and let Google give you customized driving instructions from wherever you live.
You can read about the history of the Ferry at Poolesville.com. Between the American Legion Bridge at Lock 12 to Point of Rocks, this is the only other Potomac River crossing, even though the Maryland/Virginia/DC area is a major population center. I wish there was a bridge here.
If you're wondering who Gen. Jubal A, Early was, you can visit http://www.jubalearly.org. He was a civil war general of which it's been said, "Honest and outspoken, honorable and uncompromising, Jubal A. Early epitomized much that was the Southern Confederacy. His self-reliance, courage, sagacity, and devotion to the cause brought confidence then just as it fosters reverence now."
This bridge, built in 1876, supported traffic over the canal to the ferry.
You won't find a better spot to stop for refreshments until you reach Barrons, at the 76 mile marker. A word of caution though. There are picnic tables, rowboats, canoes and a boat ramp but they're all private and you should pay here before you use them!
If you look closely at the topmost window, you'll see flood markers. In 1972, the entire store, up to the window, was under water.