[ssba-hide]I’d like to be an expert on Grand Canyon geology but I’m not. Whenever I’m descending through the Redwall Limestone I’m watching the wall of rock to pick out the boundary between the Redwall above and the Muav Limestone below, but I’m rarely certain. The two formations usually merge into one massive cliff, and only lower down will the distinctive color and shape of the Muav become apparent. This despite the fact that there is an unconformity, a geological time gap, of 65 million years between between the Muav and the Redwall.And most times I’m not certain about the boundary between the Muav and the Bright Angel Shale below it either. But it is quite clear here on the South Kaibab Trail, which, when it reaches the base of the Muav, tracks right along the boundary for a time before starting to descend into the Bright Angel. You can see here the characteristic layered, rounded blocky look of the Muav. Here‘s another spot on the North Bass Trail where the shape and character of the rock makes it clear that you are in the Muav. This spot is at the base of a steep descent that passes though both the Redwall and Muav layers, as is the case on most canyon trails. Get far enough west or northeast up towards Marble Canyon in the Grand Canyon and the Muav Limestone becomes the floor of the entire canyon, the other usual lower formations having disappeared underground. That is the case here along the Nankoweap Trail in Nankoweap Canyon. You reach the base of the long descent of the Nankoweap Trail at Nankoweap Creek, and you then follow Nankoweap Creek downstream for over three miles through Nankoweap Canyon to reach the Colorado River, strolling along on Muav Limestone the entire way. When you hike in the Grand Canyon, absolutely the most important thing to you is water. And in that the Muav Limestone is your friend. Rainwater that falls on the top layer of the canyon, the Kaibab Limestone, just disappears, seeping down into the ground. And it keeps on going down, passing though layer after layer of permeable rock: rock that water can pass though. There are some small springs that appear at the base of the Coconino Sandstone, since water has a hard time passing through the Hermit Shale below it, but the really big springs in the canyon flow out from near the base of the Muav Limestone, just above the Bright Angel Shale which blocks water from descending further. Here we see one of the largest springs in the park, Roaring Springs, flowing out from the Muav, as seen from the North Kaibab Trail. All the millions of visitors to the South Rim, some 20 miles away, are drinking water that has poured out from Roaring Springs, via a pipeline built in the 1960’s to transport it there.
To read more detailed geological information about the Muav Limestone, visit the United States Geological Service page for the Muav Limestone.