Sheep Mountain, Wyoming – a classic plunging anticlineThis is Sheep Mountain, a feature found in the

Sheep Mountain, Wyoming – a classic plunging anticlineThis is Sheep Mountain, a feature found in the Bighorn Basin of northern Wyoming, in-between the Big Horn Mountains to the East and the Absaroka Mountains to the west. This feature is a classic example of a plunging anticline, one of the major types of folds found in sedimentary rocks of mountain ranges.An anticline is a fold where layers that were once flat-lying have been bent and are now concave downwards. They are distinguished because the center of the folds usually exposes older units, while younger units are found at the edges. The classic map pattern seen here appears because the fold isn’t perfectly flat, instead it tilts off to one direction. You can demonstrate this style using a book or stack of paper; bend the stack of paper so that it is concave (open) downwards. Then, tilt it to one direction – you’ll have a V shape or a U-shape when you look down at the edge, with the lowermost pages at the center and the uppermost pages at the edge. That’s the same setup as a plunging anticline, just with erosion having lopped off part of the center to make the units more exposed.The rocks at the outer edge of Sheep Mountain are Cretaceous in age, formed as the Rocky Mountains were growing and as a large seaway flooded the central part of North America. The rocks get progressively older as you move towards the center of the feature; there are Jurassic aged rocks and easily observed Triassic aged rocks as in this part of the country the Triassic rocks are usually red, just as here.The central units of this anticline are the oldest units seen at this spot. They include the Permian-aged Phosphoria Formation, which has layers of chert in it that make it harder to erode, as well as some older limestones and sandstones. Those harder to erode units make the central part of this anticline stand up more strongly against erosion than the layers at the edge.The youngest rocks in this fold are Cretaceous in age, and the deformation that built the modern day mountains in this area is also Cretaceous in age. As mountains grow, they erode rapidly and dump sediment into basins nearby. Eventually, the growing mountains often overrun those sediments, folding and faulting the same sediments that were once shed off the growing mountains.The modern-day Bighorn river has cut its way across this mountain, forming a cross-sectional view along its canyon. Note that it goes just around the tip of the more resistant Phosphoria formation, staying in the easier to erode reddish rocks where it can.-JBBImage credit: Bernhard Edmaier (with permission, prints available at his page)https://www.bernhard-edmaier.de/en/images-of-the-earth/?filter=44Reference: https://bit.ly/2Y17eJw -- source link

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