20. Seneca Tract

The Seneca Tract is a property owned by the Northern Virginia Regional Park Authority.  It has not been formally developed into a park as yet, but there is a great network of trails and carriage paths. The parking area and trailhead is at the end of Seneca Road, marked "P" on the map below. You can hike down to the Potomac River, where you will find a 1-mile channel that is the remains of George Washington's Patowmack Canal. (In 1790, it was one of the first operational canals routing boats around Seneca Breaks, a set of rapids where the Potomac River drops 10 feet in one-and-a-quarter miles.) The red trail is the Potomac Heritage Trail that extends from Loudon County, through the Seneca Tract, to Riverbend Park, and further south to Great Falls where it connects with the Fairfax Cross County Trail. 

Directions: To get to the Seneca Tract from Tyson's Corner (and thus, from most of the DC area), take Leesburg Pike (VA 7) seven miles west from the Dulles Toll Road and turn right at VA 193, Old Georgetown Pike (there is a light at the intersection). Make an immediate left onto Seneca Road (VA 602) and go 4 miles north to the parking lot at the end of the road. For a more scenic, but slower, route, you can follow Old Georgetown Pike (193) west from the Beltway, and turn right onto Seneca Rd (VA 602) just before VA 193 ends by merging with VA 7. 

The bluff in the upper-right hand part of the map, on the Maryland side of the river (marked Chesapeake & Ohion Cana National Historical Park), is actually Blockhouse Point Conservation Park, which is an equally beautiful area for hiking.

Bonus: you can put-in a canoe at Violette's Lock Road (turn left off River Road, one mile west of the upper parking area for Blockhouse Point) , paddle across the Potomac River just above Seneca Breaks and enter the Patowmack Canal channel that starts at this point and hugs the Virginia bank in the Seneca Tract. Paddle back over to the Maryland side at the bottom of the 1-mile channel, pull your canoe up into the C&O Canal at the base of Blockhouse Point, and paddle back to your starting point on the flat water of the canal.