In article et,
"Dave" wrote:

My subsequent question
is where topsoil comes from to begin with in a semi-arid region primarily
consisting primarily of limestone outcroppings and calcium carbonate fill
(caliche)?

Uh, calcium carbonate is limestone. The miracle isn't the top soil but
the sub-soil. Evidently, things were not always as sever as the are
today. What did they say in Jurassic Park? Life will find a way.
--
Billy
http://angryarab.blogspot.com/