I misidentified this one - thanks to Amy for correcting me! This is North Canyon, our very first hike, where Walt wet down the rocks to make a nice slide.
Similar to this and even more beautiful was Silver Grotto...This side canyon at mile 29 had a deep muddy pool at the outlet that we swam across. The rock (Muav limestone) was polished so smooth that we had to use a rope to get up the first step. Behind that the rest were slippery but navigable up to a large open room - then only a couple of the best climbers could keep going. Kyle demonstrated how you could run around the back of that wall if you went fast enough - and if you were REALLY moving you could then run up into the next level. I didn't try it. Coming down was easier - slide on your bottom.