Take the Lamarck Lakes trail from the North Lake trailhead parking area. Follow the trail up to Lamarck Col (approx. 5.5 miles). Once at the col, drop down the western side of the col about 50 feet and then head south along the ridgeline leading to Mount Tom Ross; the highpoint along the ridgeline. Continue to keep to the western side of the ridgeline, staying closest to the top of the ridgeline to avoid loose rock lower down. As one gets closer to the North Ridge of Mount Tom Ross, the difficulties increase and the profile of the ridgeline narrows, making for a more exciting and challenging scramble. There are several spots where exposure is encountered but the climbing is never any more challenging than 3rd class. Near the end of the ridgeline, downclimb a small gendarme on its west side and down to the base of the North Ridge. Traverse along the base, heading south for about 100 feet until a small gully is located on the western side of the North Ridge. This is the start to the North Ridge.
*NOTE - As a word of technical advice and common sense, do yourself a favor.....reverse the approach along the north-south ridgeline back to Lamarck Col when returning from the summit rather than be tempted to drop down into Darwin Canyon from the North Ridge. You'll save yourself the hassle from "crawling" out of the canyon and back up to Lamarck Col.....a very thankless chore on steep scree and sand....in addition to the loose rock found below the ridgeline!
The North Ridge
Once at the base of the North Ridge, locate the small yet distinguishable rock gully (a clean looking gully that follows excellent granite) as described in 'The Approach' section. Climb up clean features (fun and challenging 3rd class cracks, ledges, and flakes), keeping somewhat close to the ridge as a reference and following the path of least resistance. Continue up and right as the ridgeline "bends" up to the summit. Aim for a large set of blocks 200 feet below the summit. Pass through these blocks and up to an open slab. Easy friction up this open slab leads past blocks directly below the summit. Scramble up these blocks and up to the summit. The summit is an angled block with a cool overhang on its southern side. Great photo ops with views looking into Sabrina Basin and Hell Diver Lakes. Currently, there is NO summit register on the summit.