The Colorado River in the Grand Canyon – entering the Upper Granite Gorge


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.

A sunny (and low water) view of Hance Rapid, before the enlargement of the delta, which extended out to Muffin Rock

A sunny (and low water) view of Hance Rapid, before the enlargement of the delta, which extended out to Muffin Rock

After the flood, an X marks Muffin Rock. Photo by Britt Runyon, Sept. 2012

After the flood, an X marks Muffin Rock. Notice the steep angle of rise of the rock layers downstream. Photo by Britt Runyon, Sept. 2012

Below Son of Hance, the Hotauta Conglomerate

Below Son of Hance, the Hotauta Conglomerate

The first appearance of the Vishnu Schist, with layers of the Supergroup above

The first appearance of the Vishnu Schist, with layers of the Supergroup above

A view back to Son of Hance and Hance rapids

A view back to Son of Hance and Hance rapids

Vishnu Schist mixed with granite, with a diabase sill and Bass Limestone above

Vishnu Schist mixed with granite, with a diabase sill and Bass Limestone above

Upstream view from a little further downstream

Upstream view from the mouth of Mineral Canyon

Fluted schist

Fluted schist

Fluted schist. The schist takes different colors, depending on the source rock (schist is a metamorphic product of other rocks)

Fluted schist. The schist takes different colors, depending on the source rock (schist is a metamorphic product of other rocks)

Sockdolager Rapid awaits, just downstream at Mile 79.1

Sockdolager Rapid, just downstream at Mile 79.1. Notice (at the top of the photo) the pinching out of the Supergroup layers.

 

 

About believesteve

I am a photographer and have published a book of photography and accompanying text on running the Colorado River in the Grand Canyon. The first (print) edition is out of print, but a second edition is available as an iBook (eBook) through the iTunes bookstore. All Grand Canyon, river and nature lovers will enjoy my book: https://itunes.apple.com/us/book/the-grand/id672492447?ls=1
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