Hermit Formation In my discussion of the Supai Group, I talked about how sometimes it’s difficult to define exactly what a geologic formation actually is. Rocks can be mixed together in small layers, rock types can grade into one another as sediment types gradually change, and so on. The relationship between the Hermit Shale and the Supai Group is a great example of that problem. The Supai group contains a number of fine-grained, red, iron-bearing shale layers that erode easily. At the top of the Supai Group sits the Esplanade sandstone, a resistant unit that is hard to erode and forms plateaus. Continuing up the canyon, we then find the Hermit formation; a fine-grained, red, iron-bearing shale that erodes easily. In other words, it looks a lot like the Supai group. So why is this a different formation and not part of the Supai group? The answer to that is erosion. The top of the Esplanade sandstone isn’t flat. There are clear remnants of deep erosion on the sandstone, including formation of channels 10s of meters deep. The Hermit Shale covers the sandstone fully, but it also gets even thicker where there are channels. There is an erosional nonconformity in-between the units, representing a substantial amount of missing time. We’ve crossed the boundary into the last Geologic era represented in the sedimentary rocks of the Grand Canyon; the Permian. These shales are about 280 million years old. That big step in time, including erosion, is enough reason to break this rock out as a different unit even though their environments were similar. They were deposited in environments much like the Supai group; sediments deposited in a quiet water, near-shore environment like a tidal flat. There are even plant fossils preserved within the rocks, which identify both the environment and the age. Like many of the shales in the Canyon, this one also erodes easily, The Esplanade sandstone below holds up a flat plateau that sticks out away from the far edges of the Canyon because the weak shale on top of it has been eroded away. The red beds of this unit also help paint the Redwall Limestone below. -JBB Image credits: https://flickr/photos/brewbooks/5180375365/ https://flickr/photos/grand_canyon_nps/4749637616 Sources: 3dparks.wr.usgs.gov/coloradoplateau/lexicon/hermit.htm https://classzone/books/earth_science/terc/content/investigations/es2906/es2906page09.cfm asset.emsofl/ONLINE%20CLASS/EarthSci/_labdata/unit7/pages/strata/hermshal.html Previous articles: https://facebook/photo.php?fbid=71718732167564 https://facebook/photo.php?fbid=717596974968016 https://facebook/photo.php?fbid=718487278212319 https://facebook/TheEarthStory/posts/718917208169326 https://facebook/TheEarthStory/posts/719035941490786 https://facebook/TheEarthStory/posts/719534524774261 https://facebook/photo.php?fbid=720485404679173 https://facebook/photo.php?fbid=720916891302691 https://facebook/TheEarthStory/posts/721282287932818 https://facebook/TheEarthStory/posts/721455997915447 https://facebook/TheEarthStory/posts/722212221173158 https://facebook/TheEarthStory/posts/722332104494503 https://facebook/TheEarthStory/posts/723288294398884 https://facebook/TheEarthStory/photos/a.352867368107647.80532.352857924775258/723925267668520/?type=1 https://facebook/TheEarthStory/photos/a.352867368107647.80532.352857924775258/724756080918772/?type=1 https://facebook/TheEarthStory/posts/724792024248511 https://facebook/TheEarthStory/posts/725410850853295 https://facebook/TheEarthStory/posts/726153457445701 https://facebook/TheEarthStory/posts/726938514033862 https://facebook/TheEarthStory/posts/727461423981571 https://facebook/TheEarthStory/posts/727462763981437 https://facebook/TheEarthStory/posts/727463347314712 https://facebook/TheEarthStory/photos/p.727464010647979/727464010647979/?type=1 https://facebook/TheEarthStory/posts/727464800647900 Dear Readers, Most of our posts are not reaching you in your news feed due to fbs filtering system. If you wish to enjoy our posts more often, use the following for information on how to go about it: tinyurl/qgwac8k.
Posted on: Mon, 04 Aug 2014 21:25:01 +0000
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