Part of the Pawtuckaway State Park system of trails, the South Ridge Trail has some real bonuses to it, and some brutally difficult terrain.
About 3/4 of the way up Mountain Trail
, just a bit before you hit Tower Road (if you hit Tower Road, you missed the trail), you'll come to the sign for South Ridge Trail. Head right and prepare yourself.
The trail starts off fairly tame, but once you get about 1/4 of a mile in (maybe less), it takes a turn for the brutal. The trail becomes extremely steep, and not only this, the rocks just get bigger, and bigger, and bigger, to the point where basically, not only are you hiking your bike, but it's quite difficult even to hike with your bike. Be especially careful on the rocks you DO attempt to ride; if they are wet they will be VERY slippery.
About halfway through the trail, you'll be rewarded with a nice view from atop a big rolling rock. Just after this, look to the right, and you'll see the fire tower. The trail is a little harder to decipher at this point, but basically after you see the fire tower on your right, you'll head down to your left, down a couple rock stairs and then the trail will become apparent. Also keep your head up, as the trail is marked by white reflectors on trees, and pretty uniformly, so if you pay attention to these you won't get lost.
The down is not as treacherous as the up, but there are still some hike-able moments, including a tiny rock gap where you'll have to hoist your bike up. Take in the massive rock splendor here - it's quite the view.
The rest is a mix of relatively steep, semi-technical downhill. Watch for a bridge on your right about 3/4 of the way through - this will be Shaw Trail
coming in. Finish out on some steep switchbacks down to Round Pond Rd.