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.
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.