Creosote bush protected!!  I don't think so, not around here anyway.  It is
the common large bush in the Chihuahuan Desert just south of where I live.
It has been slowly moving northward but cold winters tend to push it back.
The name, by the way, comes from the creosote-like smell you get from the
leaves.  It is very, very common in the southern portion of the state and
often attempts are made to clear it off of range lands to improve grazing.
The circles talked about are not common and are tough to see even when you
do find one.  Most often you see creosote bush evenly spaced on the side of
a hill.  The spacing is pretty much determined by a fight for available
water.  The other similar sized bush in the area is mesquite.  To relate
this to shells, there is a shell that lives on mesquite bushes or trees --
but I have yet to hear of one on a creosote bush.  I'll have to check on
that.

Tom Eichhorst in New Mexico, USA