I had just bought a packraft and wanted to show my Dad Black Rocks in Ruby/Horsethief Canyon but only had a day to spare. Mountain biking at Rabbit Valley a few weekends before I came up with this half baked route:
Shuttle a car or bike to McDonald TH in Rabbit Valley and start hiking from the OHV “training area” up Rabbit Valley 3
up the hill till you reach a saddle. From hear the route descends steeply down an unnamed canyon (aka “Prodigal Son Canyon”) to the river. There is a faint track leaving the trail left along the saddle, follow this till you are looking straight down the canyon. Descend down the scree filed till reach the top of the sandstone where a short ~6’ 4th class moves gets you to the first cottonwood. Note: although dropping immediately into the drainage seems easiest there is one dry fall at the bottom of the first section that seemed too spicy for my dad, maybe 4th class. From here follow the canyon down negotiating a few short dry falls until you are at the top of the intermediate bench before the canyon deepens and turns left. Here you intersect a trail used by OHV’s coming east (too far off established trails) along the bench. Down canyon is a ~12’ dry fall which I thought might have been the end of my day. Luckily following the faint trail right and then dropping left down the first steep wash you cross allows you to get to the canyon bottom again. From here two more dry falls can be bypassed on the right before the canyon releases you on the floodplain. Cross the river and launch at the cottonwoods. After Black Rocks you can’t miss the mouth of McDonald Canyon. Take out on the upstream side above the RR bridge on a steep bank and you’ll immediately encounter the McDonald trail. Follow it back to your shuttle vehicle stopping to view petroglyphs along the way.
It’s a great trip but I’m not sure I’d recommend bringing your 60+ YO father, especially when its a first decent.