We tear ourselves away from Elves Chasm and drift downstream towards Blacktail Canyon, another favorite. In Stephen Aisle, at Mile 117, the Monument Fold fault intersects the river, and brings the Tapeats Sandstone to river level.
Blacktail is a slot canyon cut into the Tapeats Sandstone, but with exposures of the schist also present. The interface of the Tapeats and schist is particularly dramatic, and constitutes one of the most readily available and vivid examples of the Great Unconformity encountered along the river corridor. From Wikipedia: “An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of the younger …”. This particular example was named the “Great Unconformity” because of the considerable period of time that is missing between the two layers. The lower surface of the Tapeats is 1.2 to 1.6 billion years younger than the upper surface of the schist. The upper surface of the schist is called an “erosional surface”, which means that rock was removed by erosion from this deposit, leaving, in this case, a particularly flat surface. In this instance, a mountain range was completely leveled, after which the Tapeats Sandstone was deposited atop it along the edge of a shallow inland sea.
Note: This and prior posts on the subject of running the Colorado River in the Grand Canyon are excerpted from my iBook: “The Grand, the Colorado River in the Grand Canyon, a Photo Journey and Visual Guide, 2nd. edition”. The book costs $5.99 and is available from the iTunes Store, using this link: https://itunes.apple.com/us/book/the-grand/id672492447?ls=1