We have to fess up. Up until now, we have been telling you that the Kaibab Limestone was the top layer that had to be eroded away first when the Colorado River started flowing about 4.8 million years ago. That is probably not the case, actually. There are strata that were on top of the Kaibab Limestone back then. They all eroded away over the last 4.8 million years. But it has just been simpler to ignore this issue until now. There are a few places where these rocks have been preserved, underneath lava flows. The hard basalt rock of a lava flow erodes much slower than even the Kaibab Limestone. So a basalt lava flow can protect the rock underneath. Please Fast Travel to one of these spots at 36.0553 (latitude) and -111.7730 (longitude). That green coloring is the selection by the U.S. Geological Survey to portray the Moenkopi Formation. The Moenkopi is composed mostly of weakly cemented siltstone, some compressed mud (shale) and some weakly cemented sandstone. It erodes quickly. So it took a basalt flow to cover it up. QUESTION: What rock type is underneath the Moenkopi? And do you think the Moenkopi formation covered the entire Grand Canyon region before it was eroded? Don't put down this answer. But PLEASE STAY AT THIS LOCATION TO ANSWER THE NEXT QUESTION Kaibab Limestone. Yes, the Moenkopi Formation covered the entire Grand Canyon region before it was eroded.
