From Frozen Head state park, take any of the standard routes to the 5 way junction 0.3 miles north of the summit. Then continue east past Tub Springs and 2 mi. SE down the jeep road. At a point where the road turns sharply north, there will be two abandoned coal roads sharply off to your right. The second road is well defined and turns right at a 175 degree angle (very sharp) back south, then west for 0.3 mile (level). You will pass an old abandoned coal mine with bars over it, and then an abandoned guard tower with a fence around it. Between these two is an abandoned power line. This is the start of the climb up Rat's Jaw.
Alternately, you could start from Armes Gap 3 miles north of Petros on TN 116. I have not taken this route, but I suspect that the jeep road is gated, as it is not an official park entrance. Park here and walk 1 mile up the jeep road to the aforementioned intersection to the old coal road.
The route is short and sweet, climbing from 2300 ft. to 3324 ft in 0.6 miles going NNW. Most of the climbing is in the first half. Head straight up the clearing under the old powerlines where the trees have been cleared and the sawbriers cut down to form a thick bed of thorns. There are short pitches at 45 degree slopes after crossing some of the abandoned coal roads. You will also see some abandoned coal mine shafts, the entrances not sealed because nobody ever goes here. In some places the powerlines are down and you can use them as a rope to pull yourself up. Don't worry, the power is off. This route will take you to the jeep road about 100 feet from the summit.
To descend, you can head down the jeep road 0.3 mi. north to the 5 way intersection, and either take the South Old Mac trail or the jeep road back, depending on where you started. Of course, if you are ambitious, you can also descend the way you came up :-)
You might consider long pants, gaiters, and thick gloves to protect yourself from the thorns. If you are ambitious, there are some short technical pitches (about 25 feet), or you can go around them to the right.
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