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Re: [pct-l] Stories from the edge

Yes, I've noticed the dark hue to the earth around that part of Arizona
myself, but never really analyzed it in much detail.  I spent some time last
year in Tucson and noticed that as one traveled north on I-10 to I-17 thru
Phoenix, the soil became progressively more rust colored with a darkish
contrast.  I believe desert "varnish" is basically an oxidation process -
rock or soil coming in contact with the elements, both air and water.  I've
seen it in a few different areas, which feature different rock types.  It's
probably best known for the dramatic vertical streaks it creates in
sandstone country, on big walls in places like Zion or West Fork of Oak Creek
Canyon in AZ.  But I've also seen it in basalt rock.  You'll find petroglyphs
in New Mexico are abundant in lava bed regions, such as Petroglyph NM near
Albuquerque and Three Rivers near Alamagordo.  Here, the Natives have chipped
through the desert varnish to create their images out of the underlying rock
hues.  In some cases, the petroglyphs are beginning to fade, due to the
continuing processes that create the varnish.  So, it's ongoing, and
apparantly desert varnish can form in a number of geologic regions.
As for why it seems to be a desert, or semi-arid occurrence, I don't know.
Actually, none of this is textbook info, so someone schooled in geology, 
please speak up!
Didn't notice any shells near Salton Sea, but that does sound interesting.
I believe that prior to the Imperial Valley having come under irrigation,
the Salton depression was a regular spillover area for the Colorado River
and its then-changeable (and vigorous) route to the Sea of Cortez.  I wonder
if currents or perhaps seabirds could be to blame.
Happy sleuthing,
- Blister>
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