Geology 101
Pima Community College
Dr. Jan Rasmussen www.janrasmussen.com
Name_________________________
jrasmussen@pima.edu Date _____________
Geology of “A” Mountain (Sentinel Peak)
“A” Mountain Field Trip Outline
This lab exercise involves taking a geologic field trip to “A” Mountain, or Sentinel Peak, on
Tucson's west side. The objective of the geologic field trip is to discover if “A” Mountain
(Sentinel Peak) is a volcano or and erosional remnant of volcanic rocks. For additional
information, go to the Arizona Geological Survey website www.azgs.az.gov and download
DGM-29 (Bedrock geologic map of Sentinel Peak (A-Mountain) and Tumamoc Hill, Pima
County, Arizona) and Open File Report 01-07, Field guide to “A” Mountain and description of
surrounding region, Pima County, Arizona.
“A” Mountain Field Trip Stops – Describe these in your report.
Stop 1 – If there is too much traffic, this stop will be omitted
Location: Mission road at base of “A” Mountain on southwest. Parking area on east side of road.
Notes: Safety meeting: Potential traffic dangers. You should take care to watch for traffic. Skip
this stop when doing the trip on your own. Road cut geology. Describe nature of rock, color,
rock type, composition, texture, environment of deposition, origin.
Rock type: Porphyritic andesite, rock name Turkey Track andesite, age 28.6 Ma (million years).
Stop 2 – Location: Parking area just after entrance to “A” Mountain Park.
Notes: Walk down to west to stream channel to look at channel, caliche in eroded stream bank.
Describe origin of caliche. Describe nature of rock, color, rock type, composition, texture,
environment of deposition, origin. What is rock type of boulders in parking area? Describe
nature of rock, color, rock type, composition, texture, environment of deposition, origin.
Was the rock deposited from area or from outside area?
Stop 3 – Location: Along side road just after “one-way” traffic starts.
Notes: On west/south side of road, note the black rock and describe it. This is Tb1 - A Mountain
basaltic andesite (27.6 Ma).
Then walk across road to outcrop of conglomerate – small pebbles of rounded cinders (vesicular
basalt) in matrix of silt, sand, ash. This Tc conglomerate is a mudflow/debris flow, matrix
supported, volcanic clasts (particles). Describe nature of rock, color, rock type, composition,
texture, environment of deposition, origin.
Stop 4 – Location: Further up road at contact of tan tuff with agglomerate.
Notes: Notice nature of contact – eroded, baked, type of material at contact, etc. Describe tan to
white tuff (Tumamoc tuff (27.4 Ma)) – note pumice fragments in tuff, black mica (biotite)
flakes, crystals – feldspar cleavage faces. Describe nature of rock, color, rock type,
composition, texture, environment of deposition, origin.
Stop 5 – Location: Slightly further up road at contact of pink to gray tuff over tan tuff.
Notes: This is also called Tumamoc tuff (Tt), with(26.4 Ma). Describe nature of contact.
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Describe nature of rock, color, rock type, composition, texture, environment of deposition,
origin.
Stop 6 – Location: At parking area at east side of “A” Mountain under the “A.”
Notes: Look at the monument. What is the white stuff? Caliche or whitewash? Find a rock
that is in place and not disturbed by man. Describe nature of rock, color, rock type,
composition, texture, environment of deposition, origin. This is Tumamoc basaltic andesite
(23.7 Ma).
Discuss surrounding mountains and rivers. Label a map (use matching to assign names to
mountains and rivers on a map. Rincon Mountains, Agua Caliente Hills, Redington Pass,
Santa Catalina Mountains to East and North; Santa Rita Mountains and Sierrita
Mountains to the South; Tortilla and Tortolita Mountains to North; Picacho Peak and
Picacho Mountains to the Northeast; Tucson Mountains to Northwest and West. Discuss
basin formation, erosion of mountains, differences between Tucson Mountains and Santa
Catalina Mountains and effect on water supply, farming.
Stop 7 – Location: At parking area around top.
Notes: Walk up to outcrop and discuss flattened vesicles, rock composition, etc. Describe
nature of rock, color, rock type, composition, texture, environment of deposition, origin.
From parking area near cars, discuss geology of Tucson Mountains – Black Mountain and
Martinez Hill near San Xavier Mission to the South, Cat Mountain Rhyolite (74 Ma) to the
west and at Gates Pass, Safford Peak (Sombrero Peak informally) as possible source of ash
for tan tuff and pink tuff. Discuss crater – quarry for foundations and rock walls in homes
built around 1920 around the University of Arizona. Discuss Basin and Range basin
forming event, complexity of rock types and structures in mountains surrounding Tucson.
Color and label the geologic map
Mountain Ranges surrounding TucsonNorth:
East:
South:
West:
Rivers in the Tucson valleyName of river at the base of A Mountain:
Location:
Direction of flow:
Inlet:
Outlet:
Name or river at the base of the Rincon Mountains:
Location:
Direction of flow:
Inlet:
Outlet:
2
Name of river at the base of the Santa Catalina Mountains:
Location:
Direction of flow:
Inlet:
Outlet:
Tucson Basin
Composition:
Depth:
Caliche
Composition:
Source
Method of transportation
Method of deposition
Extent:
“A” Mountain Rock Sequence
Map
Rock type Texture
Symbol
Special
Features
Name of
Probable
Approximate
Rock
Way of
Age
Formation Formation
Tb2
Tt
Tb1
Tc
Ttt
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Summarize the Geology of “A” Mountain
Conclusions concerning the origin of “A” Mountain as a volcano and evidences for this.
Is “A” Mountain a volcano? Or is it the erosional remnant of lava flows and ash falls?
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