The Upper Granite Gorge begins where the run through Hance and Son of Hance rapids ends, at Mile 77.9. The main rock type of the Upper Granite Gorge (and the other two Granite Gorges) is Vishnu Schist, a hard metamorphic rock. It first appears here, at river level, and steadily rises above, as one progresses downstream. The Grand Canyon Supergroup, which sits atop it, rises with it. But the Supergroup will pinch out not far downstream. At that point, the typical profile of the inner Grand Canyon will present itself, with the horizontally-stratified Tapeats Sandstone capping the Vishnu Schist, and a broad bench, called the Tonto Platform, atop it. This is the familiar “look” of the Canyon as seen from most overlooks and in most photos.
But what about that Supergroup? What’s going on there? As previously mentioned, the Supergroup was, with a few exceptions, removed from the rock sequence through erosion. The exceptions are instances where a hunk or wedge of the Supergroup was down-dropped by activity along a fault. This lowered position preserved these hunks of the Supergroup from the episode of erosion that removed it elsewhere. Predictably, one now finds Supergroup rocks only in areas proximate to the faults responsible for them. The Butte fault was responsible for the initial exposure of the Supergroup, 12 or so miles upstream (remember the Dox Sandstone?). The Bright Angel fault, about 11 miles downstream, will provide the next encounter with the Grand Canyon Supergroup. 10 photos.