Jan. 7: Mosaic Canyon (Death Valley)

Jan. 7th: Death Valley National Park

Mosaic Canyon

view out of canyon

Death Valley from mouth of
Mosaic Canyon

The white areas on the valley floor are salt deposits, precipitated from briny waters. Such ephemeral (temporary) salty lakes are called "playas".

	Mouth of canyon There is no water presently flowing in Mosaic Canyon, The stream bed consists of sand (1/16 - 2 mm) and gravel (>2 mm) sized particles, typically angular - they have not been bouncing about in a river for long enough to have their edges knocked off (compare the Ramapo River, which has nicely rounded stream cobbles). Evidently the stream here has not flowed very frequently, as vegetation is established in the streambed. The grey "cliff" in the middle ground consists of older stream deposits. Note the horizontal banding - sedimentary rocks are, in nearly all environments, deposited as flat layers on flat surfaces. The rocks in the background are the "bedrock" rocks forming the mountains themesleves. Here you can also see layers, but they have been tilted (they dip down to the left). Some of these are sedimentary rocks, and some are sedimentary rocks that have been changed by pressure and temperature into metamorphic rocks such as marble.
	Recent stream deposits A close-up of the stream deposits. These are called "alluvium". Note the very large boulder in the middle of the photo near the stream bed (there is a pen propped up below it for scale). This would have required a major flash-flood to transport it! You can also see that different layers are predominated by different size particles - some layers are much "coarser-grained" (i.e. have larger particles) than others.
	Lower canyon: marble and alluvium The bedrock of the canyon here is marble, which is metamorphosed limestone (limestone is usually formed in the ocean, by precipitation of CaCO₃ from seawater, and often contains fossils). The walls of the canyon, from the floor to appx. 2 m high (6 ft), are covered with lithified alluvium: alluvium (stream deposits) that have been turned to stone, by cementing with minerals precipitated by groundwater. These are fairly recent (a few 1000 years). Directly opposite Howie, at about knee level, you can see that the coarse-grained alluvium sits on finer grained gray alluvium. The contact between the two is sharp (i.e. very distinct), but not flat: here, the river that carried the coarse alluvium scoured the existing stream bed (i.e. the gray deposits), and created the topography you see on that boundary. These alluvial deposits are just plastered on the sides of the canyon, deposited by earlier flash floods, and do not extend "into" the canyon walls.
	Upper canyon This photo was taken from the bend in the canyon. The canyon below this follows a fault - a fracture where the rock on either side moved relative to each other, probably incrementally over several thousand years. The bedrock in the foreground is still marble. The sunlit area in the center/background is the open area right before the stream enters the canyon.
	Plateau View looking into the mouth of the canyon. The light-coloured surfaces at the canyon entrance are tilted marbles. Kaitlin is overwhelmed by the geology...
	Fault The diagonal feature is a fault. The rocks on the left slid down relative to the rocks on the right. This is one of many faults in the upper part of the canyon. Faults form at fairly shallow levels in the Earth's crsut, as a result of tension (pulling apart, as shown here), compression (squeezing together, like the Keystone Thrust at Red Rocks), or shear (blocks slide past each other in a horizontal plane). When rocks are under stress they either break, forming a fault, or fold, if conditions are hotter (i.e. deeper levels in the crust).
	Fault breccia This is limestone/marble bedrock that is on one side of a fault. The rock fractured when the fault motion occurred, forming a rock type called a "fault breccia" ("breccia" is a rock consisting of angular fragments >2mm in diameter). If you look closely, you can see that the fragments are simply pulled away from each other. This rock type gives Mosaic Canyon its' name.
	Small Faults In the center are a pair of small faults - follow the black bands from left to right, and you can see the displacement. Not all faults are large!
	Folds The beds below the scale bar are not simply horizontal deposits. If you look closely at the bed immediately below the scale bar, you can see the layers in it are folded, in a "V" shape (with the open end of the V to the left). These folds indicate deformation probably at deeper crustal levels than the faults - which suggests multiple deformation events, when the rocks of Mosiac Canyon were at different levels in the crust.
	Alluvial deposits The rocks above Matt are layered, like all well-behaved sedimentary rocks - but the layers are nearly vertical! This is a chunk of alluvial material that slid down the canyon wall and got cemented in it's present position, probably by a later flash flood deposit. The material at Matt's shoulder-level (and below) are also alluvium, although containing much larger chunks (indicating fairly high water velocity).
	Students sitting part-way up the cliff, at the top of the canyon. (L-R: Janell, Julie, Sean, Crisdan, Larry).
	The "island" This landform is made up of appx.-horizontal alluvium (stream deposits) - later floods eroded away the material all around it, leaving this as a little island in the stream bed. (L-R: Sean, Anthony, Francesca)
	Tilted strata These are the rocks shown in the photo of the "Plateau" (see above) - specifically, the well-lit ones on the left side. These were originally horizontal, but were tilted by tectonic forces. Matt is sitting about half-way up the main cliff.
	Angular clast This rock probably fell from one of the nearby cliffs, but has also been moved by flash floods. Students (Anthony, Francesca) for scale.
	Moms Debra and Zenia in the middle of the streambed.
	And we're still here? Maria and Chris.

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