The North Couloir provides the easiest access to Mt. Sill from the north. It primarily consists of a 500-foot class 3 ice axe/crampons snow climb, followed by some easy class 4 rock climbing.
There are two approaches one can take to reach the base of the climb. The shortest, but far more tedious approach goes up the South Fork of Big Pine Creek Trail from the TH to Willow Lake. Follow the secondary trail near Willow Lake west towards Elinore Lake. You can head for Glacier Notch (the saddle between Mts. Sill and Gayley) from Elinore Lake or by continuing west before reaching the lake, but be warned that both routes involve a great deal of boulder hopping. The slopes on this side (class 2) are easier than the alternative, and you can avoid crossing the Palisade Glacier. Since you'll need crampons for the route anyway, this last point isn't much of a savings.
The longer, but easier approach heads up the North Fork of Big Pine Creek Trail past Third Lake and on to Sam Mack Meadow. A fairly good use trail continues on the east side of the meadow for anothe mile or so to the toe of the morraine left by an older, larger version of the Palisade Glacier. A generous amount of cairns mark the route around the left side of the morraine. Climbing the morraine itself is a big mistake - though it might look shorter on a map, it is a huge pile of moveable rubble that will slow you considerably. Follow the cairns around to, and up the drainage on the north side of Mt. Galey over ever-ascending granite slabs. There are good campsites available amongst the slabs just before reaching the lateral morraine.
You'll know you've reached the lateral morraine when you run out of nice slabs and can view the Palisade Glacier. Head south, following along the lateral morraine around the west side of Mt. Gayley. You will likely need crampons/axe on the Palisade Glacier for several hundred yards to reach the bottom of the chutes on the west side of Glacier Notch. Do not get off the glacier too early or you will find yourself on sandy, steep slabs. The easiest route follows up from the highest point where the glacier reaches on this side.
The chutes up to Glacier Notch are notoriously loose, and it is almost impossible to climb without knocking some rock down. Be aware of other parties that may be above or below you. The chutes are class 3, but the loose rock makes some parties nervous. Some use a rope in this section, but it is not really necessary.
From Glacier Notch, traverse the boulder field to the base of the permanent L-shaped snowfield. Put on your crampons. Earlier in the summer you can climb entirely on snow to the narrow saddle between Mt. Sill and Apex Peak, the small peaklet to the northwest. Later in summer, the top of the snowfield melts out and you will find yourself on disagreeably loose talus. If you dislike climbing the snow and/or loose talus, you can find class 3 rock to climb on the right side of the route under Apex Peak.
At the saddle, leave your axe/crampons unless you trust not your fellow climbers. Climb up to the left about 50 feet to gain access to an exposed class 2-3 ledge system that takes you traversing across Sill's West Face. Some parties choose to use a rope here due to exposure, but the climbing is not technically hard. If you choose to use a rope, please use courtesy in regards to others on the route. If another party is behind you, or ahead of you on the way back down, let them pass before creating a bottleneck on the ledge with your rope.
On the far south end of the ledge is the class 4 climbing to reach the Southwest Ridge above. This section is about 40-50 feet in length, and fairly easy for a class 4 rating. The holds are generous and the exposure moderate. There are at least two routes that can be followed here (see picture below), helpful if you are waiting on another party.
Once on the Southwest Ridge (class 2-3), follow it to the summit. Close to the summit you will find large blocks of granite comprising the uppermost region. These make for enjoyable class 3 climbing with minimal exposure.
Ice axe and crampons are recommended for all seasons. It is highly unlikey you won't
need them (or wish you'd brought them). Bring a rope if uncomfortable on class 4 terrain. A few cams, slings, and a short rope are more than sufficient for the 50-foot section.
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