Mt Lyell (3,997m) is the highest peak in Yosemite National Park.
Most people hike it via Lyell Canyon from Tuolumne Meadows.
Mt Lyell from Tuolumne MeadowsThe trailhead is an adventure in itself because... of the Yosemite bureaucracy. While driving east on 120, pass the Toulumne Meadows campground and turn right into the place marked as Wilderness Permit/ John Muir/ Pacific Crest/ Lodge. If you are camping at the hikers' campground, turn right again where it says "Wilderness Permits". Park, walk beyong the bear lockers and turn right into the trail that leads to the walk-in campground. (The well-marked campground off 120 is for tourists who make reservations: whether it is full or not, head straight for the hikers' campground that, being less advertised and involving a little hike, has almost always available sites, and is not checked by nazi-style guards anyway, i.e. you can pretty much camp anywhere nearby). When you are ready to start the hike, drive the road to the lodge past the ranger station until a sign sends you left to a parking lot for hikers. The parking lot at the trailhead is reserved for the elite tourists of the lodge. Hikers are considered inferior beings and forced to a distant parking lot so the rich tourists are not exposed to smelly hikers. Then walk down the hikers' parking lot and walk 200 meters to the lodge's parking lot. The John Muir trailhead is right at the beginning. (Note: rangers are truly awful here, avoid all contacts. I saw rangers at mignight checking all parking lots with floodlights to make sure no hiker sleeps in the car, which, of course, would be the most environmentally-friendly thing to do, but would deprive the rangers of the fees that they need to pay for their own salaries).
After winding a bit southwest through the forest, the John Muir trail is basically a flat, monotonous and sandy hike along the Lyell creek. For more than two hours you see the very same landscape. The Lyell Canyon is not terribly exciting. So if you are a slow hiker or a backpacker, this may not be an interesting hike. The first major junction (10kms in) is at Potter Point (the white mountain to your right) and that is the trail to go to Ireland Lake. After crossing the creek, you continue in the same meadow for about an hour. Then finally some action: a series of switchbacks take you to a double bridge and to the left side of the Lyell creek, where more switchbacks take you to the end of the canyon, where Donahue Pass (center), My Lyell (right) and Mt McClure (even further right) are clearly visible. Here the John Muir trail crosses the creek again (the trail to the left has not apparent purpose). This is a popular place for John Muir hikers to camp. The landscape is finally alpine. Cross to the western side of the creek by walking on stones (in august it is really easy) and continue on the John Muir trail that first heads northwest. At some point the trail turns sharply left and seems to loop back (and down) in the direction you came from. If you continued, you would cross the creek again to go to Donahue Pass. This is the point, instead, where you want to get off the trail and head straight northwest up the canyon. There is a ridge over your head. If you walk to its right, you'll end in the McClure basin, that contains two pretty lakes. Head straight up after the second lake (there is an obvious ramp right at the end of the lake before the waterfall, although that will take you to two more lakes that are a bit far from the glacier, or you can scramble up the granite ledges straight towards the glacier).
If instead, after leaving the John Muir trail, you elect to stay to the left of that ridge (i.e., 100-200 meters to the right of the creek) you'll end up in the Lyell basin, which is much easier to cross but longer. You will be walking mostly on granite slabs and grassy terrain.
Whichever way you arrive, at some point you should see on your left you Mt Lyell and straight in front of you McClure.
Head for the rocky hill below McClure and climb it (easy class-2 climbing) into the glacier itself. Most likely at this point there will be snow even in august. The slope is not such that you need crampons if you only traverse the glacier. You should be able to traverse left to right to a reverse arch of rocks that connect McClure to Lyell. From here you can reach Lyell's westermost ridge. (If you have crampons, you may go up vertical towards the center of Mt Lyell and then climb rocks to the summit plateau, but it is much scarier).
The westermost ridge is less scary than it looks. It is all class-2 climbing except a couple of spots where you have to move left in order to avoid two big gaps. This ridge leads to a sharp spike. When you are almost at the top of that spike, you should start seeing the summit plateau. Move left and at this point you will not need to use your hands anymore. Just continue left to the false summit. The real summit is right behind it.
As the condor flies, it is 3.5km of cross-country hiking from the John Muir trail to the summit (easily twice that much in real hiking because you can't follow a straight line).
The total distance is approximately 23kms each way, but of course it depends which route one follows up the glacier.
Pictures of this hike
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Mt Lyell from Silver LakeAnother way to reach Lyell is from the southeast. Drive east to Tioga Pass and then Lee Vining and turn south on highway 395. Then take the June Loop road (highway 158) to Silver Lake. Take the Rush Creek trail to Agnew Lake, Gem Lake and Waugh Lake. Then follow the Rush Creek to the Marie Lakes. The Lower Marie Lake (3309m) is 18km from the trailhead. From the western side of the Lower Marie Lake hike towards the Middle and Upper Marie Lakes. Upper Marie Lake is located just north of Rodgers Peak. Head west and stay to the right of the peak that towers over the lake. Find a class-2 moraine that leads to the Lyell-Rodgers ridge and follow the ridge toward Lyell's east ridge (this is a famously exposed knife edge). The trick is now to drop down and traverse the northern glacier to reach the class-3 spine to the summit.