See the context of this sign.

Foundation For Life

The capacity of any landscape to produce
vegetation is the basis upon which all life depends.
Rocky slopes, gravelly soil, mild winters and
bi-seasonal rainfalls encourage greener and more
varied growths here than in other deserts.

Sedimentary rocks, laid down more than
300 million years ago, erode easily from the
surrounding mountains. Freezing cold, followed
by extreme heat, expands then contracts the
soft rock. Heavy summer rains drive the rocks,
sand and silt through arroyos (gullies) to form
sloping bajadas.

The gravelly top soil covers a crust subsurface
of caliche, containing calcium leached from
limestone, which slows water absorption into
the soil. Desert plants with shallow root systems
are able to absorb adequate moisture before the
soil dries.

Alghough appearing inhospitable, the desert's
rocky bajadas provide shelter and concealment
for tender seedlings and shade for lizards, snakes
and mice.

Don't miss the rest of our virtual tour of Saguaro National Park in 687 images.