Looking again at the photo I just posted, I noticed something for the first time -- the falls comes out of a "hanging valley." The canyon above the falls has been glaciated -- sort of a U shape profile. But it looks like the tributary glacier from that canyon flowed into the main glacier, which filled the valley and flowed from left to right (north to south). So that may be why the falls plunge over a sheer edge, rather than flowing down a steep canyon.
And if that is right, then the groove in the bottom of the tributary canyon is post-glacial -- the creek cut down through the rock in the center of the canyon. You can also see the layers in the cliff near the falls. The rock in this mountain range is mostly sedimentary or metamorphic -- softer than the igneous granite of the Sierras, for example.