Sunrise
It was actually very rewarding, getting up so early...
Sand
dunes
This is definitely the time of day to go dune-watching! Note figure for scale on dune crest. Vegetation is confined to the low ground between dunes, where the water table is closest to the surface.
Off
to class!
Dunes are basically asymmetric, with a shallow slope on one side and a deep slope on the other. Wind blows sand grains up the steep side, and they cascade down the shallow side. In this way, sand dunes march over the landscape, eroding their backsides and the eroded material falls down the front.
Class
The side of the dune in shadow is the upwind direction; the dune is migrating to the right. The crest of the dune is knife-sharp. The wind that blows the grains and moves the dunes also forms smaller ripples on the surface of the dune, which can be seen nicely right behind Howie.
Dune
crest
Here you can see the sharp crest of the dune, and the ripples on the back side (right).
Ripples
The general form of ripples are that they are long and straight-crested. They have the same 3-D asymmetric shape as the larger dunes. However, they bifurcate (split in two) on occasion. The crests are evenly spaced, indicating constant wind speed and direction.
Superimposed
ripples
Here there are big ripples, with smaller ripples superimposed on them. The big ripples are clearly asymmetric, with shallow gentle backsides (in shadow) and steep narrow sides (in light). The smaller dunes are not as long as those in the previous photo, because they are superimposed on the bigger ripples and therefore can only have a limited length. The two scales of ripples indicates two different wind speeds, although in the same direction (because the ripple crests are all parallel).
Sand
crescents
The angle of repose for dry sand is about 32° - a pile of sand wil not have slopes greater than this. If anything interrupts this slope, making it steeper locally (for example, a footprint), the sand will flow down to re-attain the 32° angle. With footprints (or any other depression), this forms crescents below the indentation. In this photo, downslope is to the right.
Playa
An ephemeral lake or pond. Any rain will collect in the low depressions in between sand dunes, which is also where the groundwater table is closest to the surface. any fine-grained material will also collect down here. The muddy water will dry out, and mudcracks will develop. The light colored areas are regions that have dried out.
This vegetation is the creosote bush, which gives the Mojave Desert it's characteristic scent!
Mudcracks
Large and small mudcracks, indicating two rates of drying out. The mechanism is the same that we saw at Badwater, except that there is no salt pushing up from below - hence, the mud polygons do not push up over each other at the edges.
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