The approach is from Mowich Lake and Spray Park via a fine hike (Wonderland Trail) through woods, up grassy slopes on a climber's track, and along the crumbling rock of Ptarmigan Ridge. The final scramble feels pretty exposed (you can watch the rocks you inevitably dislodge bounce all the way down to the Carbon Glacier on the E or the Mowich Glacier on the W), but the views from the high (10,300') camp are spectacular. The brave sleep on this perilous perch where the Ptarmigan Ridge abuts the main volcanic cone, directly below the Liberty Cap icefall...
Approach Mowich Lake via 165. If you are not going to do a loop, you will probably want to drop a car at Sunrise (approaching from the N via 410).
Be forewarned that the road to Mowich Lake is often closed (as late as mid-July some years). In such cases, or if you do not have a shuttle car, an alternative approach (thanks to mtnartman for this addition!) is possible from White river using the Liberty ridge approach to the edge of the Carbon glacier on Curtis ridge. From there drop to and cross the Carbon glacier below the obvious seracs to the west.
Choose a path that is clear of the Carbon's seracs and ascend the steep slopes where it merges with the Russell glacier. Be sure to stay N of the ice cliffs where the Russell calves onto the Carbon. This brings you onto the Russell well above Echo & Observation rocks. Once on the Russell follow the normal approach. This may sound convoluted, but the route is quite plain when you see it and works well for single-car folks who plan to descend via the Emmons.
Ptarmigan is a ridge that extends northwest from the summit of Rainier, suggesting the way back to Seattle. From the high camp, the route traverses the upper Mowich Glacier, climbs to Liberty Cap Glacier on steep snow and ice (and a bit of rock), traverses up to and around Liberty Cap and finally ascends to the summit. Descend via the easiest route, most often the well-flagged Emmons Glacier to a Camp Schurman/Sunrise exit.
More detail (thanks to mtnartman): From the high camp descend and cross a bergshrund and avalanche slopes. Continue traversing the slope to below the lowest bit of rock before turning a corner. Note: If conditions are right you can save some decent by climbing back up and crossing the 'shrund above the lowest rock. We found this to be no problem. Once on the wall above the shrund make a rising traverse back to the east (toward ice cliffs) avoid rock bands. You will come to an obvious passage through the rock bands then a more direct traverse again to the E. This will bring you out to an indistinct rib. From here cross into the gully that leads directly up to the toe of the largest rock buttress of the upper ridge.
The original route traverses left under the buttress but it seems most parties now opt for the right variation. A short climb from the toe (right var.) brings you to the bottom of the final ice slope before the exit chute. This is usually blue ice and several hundred feet. Climb this to a rock rib. Cross the rib and make one more traverse on snow or ice and the exit chute becomes obvious. It is the first major break in the cliffs of the ridge. Climb the chute easy 5 to the glacial slopes above. Climb out on the glacier and gain about 1000' ft. before cutting back right to the ridge crest through crevasse patterns. Follow the top of the ridge on easy glacier slopes to the summit of Liberty Cap.
Go light if you're going to carry all equipment over! In addition to bivy gear, you will want harnesses, rope, crampons, ice axe, 3-10 ice screws (for the steeper ice and possible crevasse negotiations), a few snow anchors (especially for steep snow fields above Mowich Glacier, and a very light rock rack for crossing the rock bands below the Liberty Cap Glacier (maybe a couple medium cams and a smattering of nuts).
Caption: Northwest slopes of Mount Rainier as viewed from Ptarmigan Ridge. Photo taken over benchmark at 7,802 feet elevation, southwest of Observation Rock, showing a good view of Mount Rainier's summit - Columbia Crest (14,410 feet). North Mowich Glacier is in the foreground. (PHOTO CREDIT: Lyn Topinka, September 1983) Ptarmigan Ridge is the crumbly grey pile of rock on the left. The high camp is located where the grey ridge terminates in snow. We traversed the upper Mowich to the roughly-triangular snow field (with the shadow line bisecting it) that leads from the upper right hand side of the Mowich up towards the Liberty Cap Glacier via rock bands. Once on the Glacier, it's easier terrain up and over to the summit...
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