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GeoHistory of the Valley of the Sun
and the Superstition Mountains

This capsule version complements the map-- The Rocks of the Valley of the Sun.
Please see below for printed version.

   Many of the rock formations visible around the Valley of the Sun are very, very old. They date from nearly 2 billion years ago, from what geologists call the era.
   Prior to that, our planet was a fiery, violent, almost lifeless ball of rock, finally having cooled enough from its formation to form thin crustal layers.
   Surface rocks developed from a great basin of sediment -- there was liquid water and oceans by then -- and life was just getting started. Small, simple organisms lived and died in hostile seas.

   At various times, until around 1.4 billion years ago, these rock formations buckled up, molten rock from deeper in the earth intruded them, and then they cooled slowly.  Mountain chains and valleys had formed, now part of a continent all of its own.
   Among other places,  rocks which form the remains of the old land mass are to be seen at Squaw Peak, Black Mountain, the McDowell Mountains, the Sierra Estrella, and the majority of the rock forming Camelback Mountain.
   Most of its rocks and landscape -- a thickness of over eight miles -- then eroded away to flatness!

   What we now call the Pacific Ocean came and went across the remnants, and subsequent highlands did not rise again until dinosaurs roamed a different landscape, 1200 million years later and on through the , , and  periods.
   The land crumpled again, and the , as we know them, began their birth to the east about 75 million years ago, causing the future Arizona region to experience the forces of strong geological unrest for the next 25 million years!
   Some of the world's biggest copper deposits are in Arizona because of all this activity.

(continued below)

Moss on Granite

Moss on Precambrian granite

   Twenty million years of quiet times then passed, before more mountain masses began their violent growth to the east of the present day Phoenix area, now during the  period.
   In places they became so steep that mud and rock and even catastrophic landslides rushed from their slopes.
   The evidence of these is the fragment-laden reddish rocks of Papago Park, Red Mountain, and the west end of Camelback Mountain.
   Great volcanic  soon exploded from the crust of the earth. Their thick, layered ash deposits form part of the Usery Mountains and the famed Superstition Mountains, which pierce the skyline east of Phoenix.

   For reasons not yet well understood, large domes of young molten rock in turn pushed upwards through the surface rocks, throughout the new continent from northern Mexico to southern Canada.
   A number of these dome structures are prominent in Arizona. One of the classic ones, studied by geologists from around the world, is South Mountain at the end of Central Avenue!
   Geologists call them
"", and what caused their arrangement is still a mystery.
   All of these previously mentioned formations have now set the stage for a few final geologic episodes -- those that give the Valley its distinctive look.

(continued below)

   Driven by giant convection cells in the  of the Earth below, the crust around us began to rip apart in a roughly east-west direction about 20 to 15 million years ago.
   In this torn fabric, from Montana to Mexico, great blocks of crustal rock started to settle downward and form vast valleys -- the Valley of the Sun is one.
   Among the mountains left to stand between them are what we see today as the Phoenix Mountains, the Sierra Estrella, and the McDowell Mountains.
   Geologists call this immense zone the .
   In Arizona, the edge of the great continental rift is the , north and east of Phoenix.
   Beyond it lay the still relatively undisturbed and flat-lying rocks of the  , visible to all in the magnificence of the Grand Canyon.

   Also, during this time, deep fractures let loose dark, fluid  lava flows we now see along the Black Canyon Freeway (I-17) and capping some of the mesas north of Phoenix.
   faulting continues today, though its effects are much diminished now in the Phoenix area, making for low earthquake potential.
   Its activity is stronger, however, to our north in Nevada and Utah.
   Finally, the valleys began to fill with layer upon layer of sand, silt, gravel, and salt beds, giving them the flat expansive nature we see today.
   In places, near Luke Air Force Base, for example, these deposits total about 2 miles in depth.
   And underneath Glendale sits a body of nearly pure salt thought to be about 15 cubic miles in volume!

   Phoenix, Scottsdale, Mesa, and most of the other cities of the Valley are sitting on thousands of feet of  fill.  The mountains and  around us are the mere tips of what is underneath.
   Think of digging straight down for a mile or two, through an ocean of sand and gravel, before you hit bedrock!

Return to Map:
"The Rocks of
the Valley of the Sun"

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Golden Barrel Cacti on Basalt boulders in the Valley of the Sun.

Golden Barrel cacti on basalt boulders

   You can download and print a more geologically detailed version of this map and its accompanying color-coded GeoHistory in PDF format (.7MB). 
   If you don't have Adobe Acrobat Reader (with which to view it), you can download it for free from

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