This is not a huge loop ride wise but as far as quickly getting that high back country feel quickly and a 5+ mile downhill that rivals Toads in my IMHO this ride rocks. I don't have the exact stats in front of me, but we spent just over 2 hours in ride time and there was around 2300 feet of vert and apox 13 miles of pure single track love. Since we had two cars we did a short shuttle up hwy 88, leaving one car at Oyster point (there is also limited parking available at horse canyon trailhead) and the other at the Thunder mountain trail head. Get the Mokelumne Wilderness map which covers this area better than any other map. Near the top of Horse Canyon you have the option for a short out and back to Scout Carson Lake.
Once we hit Horse Canyon from Thunder Mountain we rode to the top for an out and back for the top 2.1 mile section of Horse Canyon.
This can be ridden as a loop off road, ride a short ways up the Horse Canyon trail and go left. The trail climbs steaply at first but mellows out. You then cross 88 and ride up the old highway bed to Martin Meadows. The single track from Martin Meadows to the Thunder Mountain trail head is really nice, runs along some cliffs with great views. Check out the little spur trail to Castle point for a killer 360 degree view. Unfortuataly I do not have any pictures of this segment. This will add a couple more miles and more vert to your ride.
The Thunder Mountain Trail climbs fairly quickly through the lower forested area and the views start to get amazing.
A new trail before you get to Glove Rock should be available to connect to Kirkwood soon.
Riding up to the top of Horse Canyon, there will be a few short hike-a-bike sections to get to the top though if your a mere mortal.
Spur trail out to Scout Carson Lake.
Lunch at the top of Horse Canyon at the Wilderness boundry
Horse Canyon is a multi use trail, do not be surprised to see a motorcycle, chances are you will not see one though.
The lower part of Horse canyon is fast bermed single track, that will leave a huge smile on your face and bummed that it is over once your back at the car.