Wednesday, December 31, 2014

What a Geologist Sees - Part 19

Glacier National Park is one of the places that I hope to visit, someday.

[For the moment, a photo taken by a friend will have to suffice until I can get there to take my own.]

The feature pictured at right is Chief Mountain, which consists of Proterozoic limestones sitting atop Cretaceous shale and sandstones. This is totally "out-of-whack" with "what should be". Under normal conditions, the Concept of Superposition applies, wherein in a series of layered rocks (usually sedimentary, but could apply to some volcanics), the oldest layers are at the bottom and the youngest are at the top. The Proterozoic is much older than the Cretaceous.

The presence of a "thrust fault" is the reason for the existing condition. During periods of tectonic deformation, where there is lateral compression, sometimes sheets of rock will break loose and get "shoved on top of" younger layers. The lateral breakage is usually along a shale layer. In this case, the thrust fault is referred to as the Lewis Thrust Fault or the Lewis Overthrust. Erosion after the period of faulting isolated Chief Mt. from the remainder of the thrust sheet, thus we call Chief Mt. a "klippe", which is a type of "erosional outlier", i.e., it is laterally separated from similar rocks.

The mesas and buttes of Monument Valley are also erosional outliers, but they have no underlying thrust fault, so they are not klippes.

There are plenty of thrust faults in the Appalachians, including the Georgia Piedmont, though they are obscured by soil and vegetation cover. There is a thrust fault (name forgotten) exposed in an outcrop on the west side of I-285, north of the intersection with I-20 and near the Chattahoochee River, but the traffic makes it difficult to pull over and get a good photo.

[I will fill in some more details later...]

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