Geologic Field Guides to the Southeastern Picacho Mountains and Picacho Peak, Pinal County, Arizona

Photo from Drone video below

If you drive Interstate 10 between Tucson and Phoenix, about half way you pass between the Picacho Mountains (on the northeast side) and Picacho Peak (on the southwest side). Picacho Peak State Park is a frequent destination for picnics, rock climbing, and viewing spring wildflowers.

                                            Picacho Flowers (Photo by Jonathan DuHamel
The Arizona Geological Survey has recently made available for free download Geologic Field Guides to the Southeastern Picacho Mountains and Picacho Peak. (Link)

From the guide:

The Picacho Mountains consist largely of a compositionally diverse suite of Laramide to middle Tertiary biotite granite, muscovite granite, and heterogeneous to gneissic granite. At the southern end of the range, most of the crystalline rocks have been affected by middle Tertiary mylonitic deformation. Mylonitization is inferred to have accompanied normal faulting and ascent of the bedrock from mid-crustal depths to near the Earth’s surface. [Mylonitization is modification due to dynamic recrystallization following plastic flow.]

Ascent occurred in the footwall of a moderate to low-angle normal fault commonly known as a “detachment fault”. The crystalline rocks of the Picacho Mountains are part of the footwall of a south- to southwest-dipping detachment fault that is exposed only at the base of a small klippe of volcanic rock on a hill top in the southeastern Picacho Mountains. [A klippe is an isolated block of rock separated from the underlying rocks by a fault.]

Picacho Peak, itself, looks like the remnant of a volcano. However, it is an erosional remnant of volcanic rocks that were displaced from over the Picacho Mountains by a detachment fault.

Picacho Peak is composed of multiple andesitic lava flows interbedded with thin sequences of medium- to thin-bedded, well-sorted, medium- to coarse-grained arkosic sandstone and granule sandstone. See the guide for detailed descriptions.

Earth fissures occur in the Picacho Peak area due to groundwater withdrawal and soil compaction. (See: Earth Fissure Map of the Picacho & Friendly Corners Study Area)

For some spectacular views, see a drone flight around Picacho Peak (3 minutes):


See also:

Picacho Peak weather station – how not to measure temperature

Note to readers:

If you like murder mysteries, type the name Lonni Lees (my wife) into Amazon or Barnes & Noble sites to see her novels, a book of short stories, and reviews. For synopses and more reviews of her books see:


7 Comments on "Geologic Field Guides to the Southeastern Picacho Mountains and Picacho Peak, Pinal County, Arizona"

  1. Why not write something about the Civil War battle in that area or is it the part of history that’s to be forgotten?

    • Jerome R Petruk | November 26, 2017 at 3:05 pm |

      Because the topic was the “GEOLOGICAL” aspects of the mountains. You’d never know so watching news in the U.S., but history isn’t always a result of political, societal, or gossip forces.

      • History esp.. war can change the face of the area, and often draws people to an area just to look at it. Our State has some of the most remarkable and wonderful places to explore, and once you do you will want to take others to see them.

  2. That skirmish is marked every Spring, March usually. Watch for the 2018 date.

    • The Oracle of Tucson | November 26, 2017 at 4:08 pm |

      The Battle of Picacho Pass was the westernmost engagement of the American Civil War that took place on April 15, 1862. The action occurred around Picacho Peak, Arizona. It was fought between a Union cavalry patrol of 13 men from California and a party of Confederate pickets from Tucson.
      Depending on who you ask, Lt. James Barrett led the union forces while Sgt. Henry Holmes (later a POW) led the confederate forces.
      13 union cavalry and 10 confederate cavalry took place in the skirmish resulting in
      Casualties and losses
      3 killed (including Lt. Barrett), 3 wounded for the union forces and 3 captured, 2 wounded (disputed) for the confederates.
      90 mins later both sides disengaged.
      Every March, Picacho Peak State Park hosts a re-enactment of the battle of Picacho Pass. The re-enactments now have grown so large that many more participants tend to be involved than took part in the actual engagements, and include infantry units and artillery as well as cavalry.

      The Oracle

  3. I remember reading about it in school. Maybe fourth or fifth grade, and I think it happen sometime in April. About 120 mem in the Confederate against more than 2000 from Calif., and something about the Butterfield stagecoach station, and or route.

  4. Mr. Duhamel,
    I thoroughly enjoy the articles and commentary you post here on ADI. Intelligent, well thought out, and entertaining. Keep up the good work.

Comments are closed.