On this day we reached the hidden depths of the canyon that reveal a sequence of rocks three times as thick as the Paleozoic section. These rocks record another 550 million years of history in the time prior to the Paleozoic Era. The rocks are known collectively as the Grand Canyon Supergroup and they range in age from 1.25 billion years to 700 million years, part of the time period we call the Proterozoic Eon.
How does one fit 12,000 feet of sediments and lava flows into the bottom 1,000 feet of the Grand Canyon? The rocks are tilted! In the picture below, the Tapeats Sandstone can be seen as a horizontal layer at the top of the cliff. Beneath the Tapeats are tilted layers of black Cardenas Lavas and reddish Dox Formation. The eroded surface that forms the boundary between the two sequences is called an angular unconformity. The next question might be "how did they get tilted?".
Moments later we had an answer: the rocks of the Grand Canyon Supergroup were tilted by major faults that formed when the North American continent was breaking away from Australia and Antarctica in latest Proterozoic time. We passed one of these major fault lines, the Butte Fault, near mile 66. The sedimentary layers have been dragged to a nearly vertical orientation (below).
We saw one of the truly rare evidences of human activity from our vantage point on the river. In the picture below one can barely make out the Desert View Watchtower (just left of center on the rim). The tower is three or four stories high, and yet was barely visible among the high cliffs. The tower is one of the main tourist centers on the South Rim of Grand Canyon National Park. I thought of how many times I had stood at Desert View looking down and wondering what it would be like to float down the river. This week it was finally happening, after a wait of forty years!
Despite the heat, I hiked the whole loop. It was hard to think of this barren area as a possible "home", but it was probable that the inhabitants moved higher up in the canyon in the hottest times of year (and conversely this would be far more comfortable a home in winter).
Ron in the other raft was pointing towards the North Rim. I looked up and saw for just a moment the arch of Angel's Window, a popular viewpoint on the Walhalla Plateau.