From the Visitor Center parking lot, looking out across the valley you can see some red rocks poking up out of the vegetated slopes in the distance, above which are cliff-forming grey rocks. The red strata are the Early Jurassic (~205-180 million years old) Aztec Sandstone (the same as we saw at Red Rocks), and the gray are Permian age limestones (~240 million years old). Just as at Red Rocks, the only way to get older rocks sitting on top of younger rocks is for them to have been moved there along a fault, during regional compression. This fault is situated at the boundary between the rocky gray cliffs and the grassy slopes with red strata.
(The Aztec Sst. is our desert formation; the Permian limestones were deposited in shallow tropical seas, rather like the Caribbean today.)
Rainbow
Vista
Looking out from the Rainbow Vista pullout, you can see a variety of white, yellow and red sandstones (all Aztec Sandstone). These are all tilted down to the right. The variation in color comes from the iron (causing the red) being leached out of various layers (see photo below).
Bleached
rocks (Rainbow Vista)
Desert sandstones are commonly red, due to the passage of groundwater contianing hematite (an iron oxide indicative of arid climates). This groundwater cements the sand grains: the cement is therefore rich in hematite (as well as silica, which is dissolved "sand"). So, soon after cementation, all desert sandstones are likely to be red in color. However here (as well as at Red Rocks and Zion) some of the sandstones are red, whereas others are orange, pink, yellow, and even white. This variation occurs because later groundwaters circulating through the rocks dissolve some of the hematite, and then precipitate it in other layers (often, but not always, nearby, and almost always in layers below the source layer). It is important to note that this leaching of iron (which bleaches the rocks) does not happen bed-by-bed - that is, the leaching occurs regardless of where sedimentary layering occurs. That is seen nicely here: you can see the tilted (down to the left) surfaces, which are the slip-faces of sand dunes (i.e. the slope the sand grains roll down); the bottom of this bed (sand dune) is below the bottom of the photo, but the top of the bed (i.e. top of this dune, as preserved) is at the top of the photo where the slightly darker layer occurs, whose tilited layers are much closer to the horizontal). The boundary between bleached and non-bleached occurs within this bed, and cuts across the slip-face surfaces.
Joints
in sandstone (Rainbow Vista)
The lines in this outcrop extending from left to right are the slip faces of sand dunes: basically we have sliced the top off by erosion, and are looking at the successive positions of the sand dune through time. We call each layer a lamina.
There are also a series of parallel features trending perpendicular to this (from foreground to distance). These are joints, which are cracks in the rocks - but unlike faults, no movement has occurred (the laminae are not offset). These joints were later filled in by silica (SiO2), from groundwater.
Differential
erosion
The Navajo/Aztec Sandstone often weathers into rounded domes, as seen here. Here the sandstone walls are pockmarked with small holes where the sandstone was more prone to erosion. In these areas there was probably a slightly different chemical composition to the cement.
Mouse's
Tank
A natural hollow in the rocks where water can accumulate (a tank).
Crossbedding
The lower 2/3 shows the dune slip-faces dipping (sloping) down to the left; the upper part has almost horizontal laminae, typical of the lower section of a sand dune. These upper strata have sliced off the top of the earlier dune, resulting in an angular geometry between the two sets of layers.
The lumps scattered throughout are small concretions, cemented together more tightly than the surrounding sandstone, and therefore more resistant to erosion.
Petroglyph
wall
Petroglyphs - pecked into the desert varnish. Note the modern pictograph (white graffiti) just above and to the right of Larry's head.
Second
petroglyph wall
Lots of bighorn sheep here.
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