Sunday, June 21, 2009
Grand Canyon is really a canyon within a canyon. The upper portion is cut through the thick stack of relatively flat-lying Paleozoic sedimentary rocks - from the Kaibab at the rim down through 300 million years. The Tapeats Sandstone is at the base of this sequence and also forms the rim of the lower, inner portion of the Canyon.
The inner gorge is cut through not just the Tapeats but also the underlying Proterozoic (Precambrian) metamorphic rocks, in particular the dark Vishnu Schist and the pink Zoroaster Granite that threads through it. This reach of the inner gorge in the eastern portion of the Grand Canyon is the Upper Granite Gorge -- the Middle and Lower Granite Gorges are much farther downstream. From the river, we usually could only see up to the Tapeats (a mere 1000' above us), but in a few places the upper cliffs can be seen rising (many more thousands of feet) up to the South Rim. Beaches are scarce in this steep, rocky portion of the canyon. And where they are found, they are often small and narrow. The dark rocks bake in the sun and hold the heat into the evening.
Grand Canyon 2009: June 21st