The route up Shellrock is about half trail and half bushwhack. It starts from the highway and switchbacks up to a ridge to connect to the summit. Views are offered of Dog and Wind Mountains as well as Defiance although there are no real views the last quarter of this hike to the summit. It's very steep in spots with cliffs so be careful. I've seen cougar tracks here so beware. The summit is marked by a cairn.
See the Getting There section of the main page for directions to the trailhead.
From the road, scramble straight up about 100 feet up the steep hillside to where you will pop out on the Old Columbia River Highway. This section has not been restored so it is more of a moss-covered flat ledge with small trees growing out of it in spots and some eroded sections. Head left (east) a quarter of a mile or so past 3 rock fields. At the far side (east side) of the third rock field, look up to catch the trail. This will zig zag back and forth across the rock field to its top. (This gets really old after a while and you can just head up but mind the plant life.)
At the top of the rock field it will eventually head left and contour southwest around a ridge. It will then have a few small switchbacks until it ends at the old warning/weather platform wired off with DANGER signs. From this platform, you can head straight up the northeast ridge following game trails as you are able. You'll bushwhack between the precipitous ridge and another rock field on your left. The ridge will eventually top out and you will find the rock cairn at the apex.
Total distance is somewhere between 1.3 and 1.5 miles (most sources quote 1.3). I came up with about 1800 feet of elevation gain although sources say 1100 to 1300 (but if the summit is 2093 and I-84 is at about 200 feet you can figure the distance pretty easily).
No views at the forested summit, but head west through the forest (easy & open) and drop down to the northwest. Keep going to find a small overlook atop a knife-ridge with views west down the Gorge. A belay is probably a good idea for venturing out onto the knife-ridge any significant distance.