Friday, June 21, 2013

An Eight-Day Colorado River Trip Through Grand Canyon - Part 3

The last part of this narrative involves traversing the Upper and Middle Granite Gorges. Grand Canyon has three inner gorges named Upper, Middle and Lower. These are where some of the best rapids are located as well as some of the best rocks.

A view of the Upper Granite Gorge above the Phantom Ranch area.

Near river mile 83 on the left is a small body of ultramafic rocks. These are rocks that were originally located in the earth's mantle but found their way to the surface in rare circumstances such as when a spreading center is obducted onto the edge of a continent. THis likely hasppened in the Grand Canyon region some 1,750 million years ago.

These pods of the ultramafics are found within the Vishnu Schist, a crustal rock that formed when 13 miles of overlying material covered the area. These rocks used to be sandstone and shale but where deeply buried and altered.

A close-up of the ultramafic rocks in Grand Canyon, with xenoliths of quartz from the Zoraster Granite included.

Here comes the second boat through upper Horn Creek Rapid.

The second boat running through lower Horn Creek Rapid. With such hot temperatures it was a thrill to ride the rapids. The volume we had on this trip varied between 11,000 and 17,000 cubic feet per second, an excellent level to run.

Some of the gneissic banding seen on a side hike in Trinity Canyon. The folding in these rocks shows how deeply buried and ductile they were during formation.

On the way to Bass Camp, our fourth on the trip.

The view upstream from Bass Camp with crystalline rocks next to the river, tilted Supergroup rocks in the middle, and flat-lying Paleozoic rocks on top.

Gerry is in love with deformed rocks. How do I know? It's the grin that gives it away. These beautifully massaged rocks were found in the gorge of Shinumo Creek where we enjoyed a swim beneath the waterfall shown below.

Shinumo Creek waterfall.

The reflection pool in Blacktail Canyon.

Strata in the Grand Canyon are not entirely flat-lying. Just 50 miles upstream the ledgy Tapeats Sandstone seen here at river level was 1,200 feet above the river. These strata are gently warped such that a trip down the river provides a lot of variety in scenery.

Entering the Middle Granite Gorge. The jet black rocks are metamorphic basalt lava flows.

Unconformity where the Bass Limestone above rests on the crystalline rocks of the Inner Gorge. Imagine this erosion surface formed some 1,255 million years ago. At that time the erosion surface was more flat-lying. Then limestone was laid down on top of and buried the surface. Later tilted put it at the angle seen in the photograph.

Self-portrait at Deer Creek.

Deer Creek waterfalls and gorge.

My camera battery died out after this picture was taken only half way through the canyon. Here we are pushing a large heavy boat off of the shore. This was an excellent trip with a great group of people, lots of geology, and fun times. Thanks for reading.

1 comment:

Dr. Jack Share said...

Great photos and narrative! Some of these sites I haven't seen on previous trips. Hmmmmm. Time to return? Thanks, Wayne.