Canyons are inherently risky. Flash floods occur without notice on sunny days. Technical skills & surrounding topography knowledge required yet does not eliminate risk.
Exactly Why Am I doing this!?
Most excursions into the south branch of Marble Canyon will have to be split up to cover both arms of the south drainage... it would be way too difficult to include both in a single day, at least for most hikers. The main cut to the south just takes too much time to complete, is too nice to hurry, and often you will be returning by a route other than back down the drainage... so, make the special effort to enjoy the Spur cut (northern branch) with a day dedicated just for that.
The hike begins heading up the South branch of the canyon, then within 1.5 miles you make the left turn up into this near parallel canyon spur. This junction is around the 5370' elevation.
Viewed from afar (on top of the Mesa) you see a matching terminus up at the top of the canyon... a second eyebrow that looks just like the southern Amphitheatre rock formation. This canyon has a duplicate Amphitheatre at it's upper end... but, that is where the comparisons cease. This northern spur is a more enclosed, more intimate, and in many ways, more difficult drainage. It has a more dramatic choke point, a large waterfall that less than half mile up into the split may well be the end of the day for many hikers. It can be climbed on the left side, but be aware there is a point of high exposure the final ten feet of the scramble... perhaps more than you might enjoy. The alternative is to move further to the left and do a less dramatic scramble through the steep, loose scrub.
Above the falls begins an impressive steeply slanted slab section terminating in a smaller water fall that again may be either climbed or passed on the left.
For awhile after scaling that fall, I felt like I was in the wrong canyon cut... it did not appear that the drainage would make it up to the 7,000' level where the Amphitheatre would be found, but in fact, you are in the right place, and about to work your way up through one of the more interesting sections.
Keeping with that aspect of closed in and intimate, you enter a winding, narrow grotto cut, with numerous blockages that may pose some fun challenges. As an alternative, you may exit the cut and simply walk the bench that lies just above the rocks (on the right). These grottos will carry you into view of your destination: the Amphitheatre north. There is still a bit of work, and some flora to fight your way through before you reach the final rock stairstep slope... but, basically, you are there!
Now, you can retrace your way out... or, traverse over to the Marble Canyon south cut and descend through it... or, proceed up to Hershberger Peak and exit on trail 119... or descend to the Mesa.
Note: you may work your way around the bowl to the south and work your way up and out of the drainage... follow the edge of the rock cliff until an access avails itself.
This is a demanding off trail hike. If there is any question about your physical condition or your ability to deal with heights, or demanding bushwhacking, you might do better to enjoy the easier south branch.
WARNING! Hiking and outdoor related sports can be dangerous. Be responsible and prepare for the trip. Study the area you are entering and plan accordingly. Dress for the current and unexpected weather changes. Take plenty of water. Never go alone. Make an itinerary with your plan(s), route(s), destination(s) and expected return time. Give your itinerary to trusted family and/or friends.