Cob is a balance between sand, clay and a binder material, usually straw or horse hair. It often had cow or horse manure added too, although I believe these are best left to ferment a little before mixing. Given the high fiber content of these manures, they aid the role of the binder.
There are ideal proportions of sand and clay in the soil that you are looking for. Typically a clay soil will need a little extra sand, although this is not always the case. Performing a
jar test is a good way to determine the makeup of your soil.
If you don't have clay in your soil (i.e. it is all sand or silt) then it will probably be prohibitively expensive for you to use cob.
Finally, you'll want to use subsoil (
not topsoil, which contains organic matter). If you dig down, you'll notice the difference clearly. This may not be a problem for you - I don't know anything about "High Desert" and maybe there is little or thin topsoil.
HTH