Boy Scout Camp
I wondered when the Boy Scout Reservation would get mentioned. They actually have a mountain biking program on the reservation during the summer. Hopefully, the secret won't get out now!
Here is a map on how to get there-
http://www.bsa-brmc.org/campmap.htm
Its been a few years since I rode there. I spent several summers in High School and College working as a counselor in various roles for the Boy Scouts and took my mountain bike along the last few summers. After I graduated from college I went back a few times for some biking before moving away from the area. The last time I was there (1994) the trails had been severely damaged by storms and we did HOURS of bushwhacking and hike-a-bike.
The Camp Powhatan Side-
If you do visit you should stop by the Ranger's house before entering the reservation. It is on the left. The last time I was there the Ranger was Charlie Stultz. He can give you the latest on trail conditions and perhaps a map.
As a matter of courtesy I would suggest making a donation to the scouting program when you stop at the Ranger's house and think that $5 to $10 per person is appropriate. Most state parks charge more than this and don't depend on donations for their existence.
I would suggest you park your car at the first bridge across Mack's creek and ride up the valley on a wide gravel road. A couple miles on take a left onto a fire road called Greenwood. This will involve fording a wide creek that can be very deep and swift depending on recent rains. Once you get to the top of the initial climb up Greenwood there are a number of single-track and widetrack options to the left and right. Many can be done as loops taking you back to Greenwood.
If you ride Greenwood all the way to its terminus you end up at the top of the Mack' s Creek watershed which you can then follow back to your start.
Greenwood is actually sort of the backbone of the trail system. It cuts through the reservation and is pretty well maintained. There are some nice long climbs and as I remember lots of water bars for catching air on the downhills.
The Camp Ottari Side-
The riding is not as easy to access from this side. You can ride up North Mountain on a ridge top trail which was nice but steep single track the last time I was there. Once on the ridge you will intersect a few trails that will allow you to string together some nice riding. Another option is to ride up Little Laurel (many stream crossings) on an old road grade. Once you are at the top of the valley you will have to do some climbing to link to other trails (i.e. off of Greenwood).
I was just cruising the boards here looking for the latest on SW VA single track as I'll be back in the area visiting family at the end of the month. I think I'll try to make it over to the Res for some mountain biking now!
Rob