Geology of Sonora Pass
Summer 2020


Earth Science 35SP - Field Geology (1.5 units)

Instructor: Jeff Tolhurst, Ph.D.
Office: Online
Office Hours:
By appointment
Phone: (209) 588-5235 office (+ msg)
Course Credit: 1.5 semester hours
Prerequisites: None
Day and Time: Premeeting on Tuesday, July 21, from 6:00-7:30 pm (online, log onto Canvas)
Room: Online and in the Field
Geology by Frank H. T. Rhodes (optional), published by St. Martin's Press, 175 Fifth Ave., New York, NY, 10010,

Field Trip Forms



Geocaching/Earthcaching on, or near, Sonora Pass, Highway 108:

1. Sonora Pass Volcanics and New Plate Boundary
2. Columns of the Giants Fault Zone and Source
3. Donnell Vista Granitoid
4. Sword Lake Debris Flow
5. Trail of the Gargoyles
6. Pinecrest Lake Glacial Moraines and Stream Capture
7. Little Sweden Overlook
8. Columbia Marble
9. Dragoon Gulch and the Don Pedro Terrane
10. Table Mountain Mima Mounds
11. Peoria Mountain and the Peņon Blanco Formation
12. The Red Hills and Serpentinite
13. Other (Note: There are other EarthCaches that you can submit for credit, not authored by CCEarthCaches. Be sure to email the responses to CCEarthCaches, however, for credit.)

Facebook: Columbia College Geoscience Program Page (ask to join)


Geology of the Sierra Nevada


Online Resources:
1. Garry Hayes:
"You Just Can't Take Sonora Pass for Granite": (Geotripper Article)

2. Garry Hayes/Noah Hughes: "Geological Excursions in the Sonora Pass Region": (Geotripper Article)
3. Dr. Cathy Busby
Professor of Tectonics, Sedimentology, and Volcanology: (Selected Publications)
4. Callan Bentley:
"Mafic Volcanics atop felsic intrusives: Sonora Pass, California" (AGU Blogosphere Article)

Suggested Reading:
1. "Geologic Trips: Sierra Nevada" by Ted Konigsmark, published by GeoPress, P.O. Box 964, Gualala, CA 95455.  This book is also available through Mountain Bookstore or from Bored Feet Press, P.O. Box 1832, Mendocino, CA 95460; (888) 336-6199, (Link).

2. "Geology of the Sierra Nevada" by Mary Hill, published by the University of California Press, Berkeley, CA, 94704, (Link).

3. "Geology" by Frank H. T. Rhodes, published by St. Martin's Press 175 Fifth Ave., New York, NY, 10010, (Link).

4. "Rough-Hewn Land" by Keith Meldahl, published by University of California Press, 2013. (Link)


Given information on the geological history and processes occurring near the Sonora Pass area, you will be able to explain/describe: 1) the main tectonic processes that have formed and continue to shape the mountains; 2) describe the geologic history of the region; and 3) explain the main geological processes observed in the area.


The class will meet two times: 1) online pre-meeting (Tuesday, July 21st from 6-7:30 pm online in Canvas); for the self-guided trip (Friday-Sunday, July 24th-26th). The pre-trip meeting will cover information on what to bring, what to expect, and what to turn in for credit. We will also cover some information on the geological background of the Sonora Pass area to prepare us for the trip. A trip itinerary will be given. The self-guided trip will have Zoom meetings at 8 am each day, then you will head out into the field to find and log EarthCaches for each day. The summary paper will be due by the end of class.


Tuesday, July 21st:
Log onto and sign into Canvas for the pre-meeting orientation. You will also log onto and create an account. You will be shown a list of EarthCaches in, and around, Tuolumne County. Read each Earthcache and be prepared to answer the questions posed for each before you leave for the field each day.

Friday July 24th
8 am - 5 pm: You will navigate to, and accurately log, approximately 3-4 Earthcaches.

Saturday, July 25th: 8 am - 5 pm: You will navigate to, and accurately log, approximately 3-4 Earthcaches.

Sunday, July 26th
8 am - 5 pm: You will navigate to, and accurately log, approximately 3-4 Earthcaches.

Monday, July 27th: Summary paper due by 11:59 pm.

What to Bring:

  • clothing for potentially very hot and cool weather
  • food - pack a lunch for the day
  • sturdy hiking/walking shoes
  • sunscreen/chapstick
  • sunglasses
  • a hat
  • money in case you need to stop at at store
  • a field journal/notebook with extra pens/pencils/colored pencils
  • a smartphone, or tablet, or laptop with internet connectivity to log Earthcaches (some sites won't have internet, so log onto the EarthCaches for the day beforehand)
  • other

What to Expect:

Expect warm as well as cold/possibly wet, rainy (afternoon thunderstorm) conditions along the Highway 108 corridor. Temperatures may range from about the mid to low 60's to the mid 90's during the day this time of year. Who knows what will happen - be prepared for changing conditions since it will be difficult to say what the weather will be like.

What to Turn in for Credit:

In order to assess your learning, I'll ask you to log each Earthcache you find on the trip. A final summary of what you learned on the trip about the Geology of the Sierra Nevada will be required for full credit. All work will be turned in online. To summarize, turn in the following:

  1. A description of your Background with a photograph of yourself, entered on Canvas.

  2. An accurate log for 12 (or more) Earthcaches found in Calaveras and Tuolumne Counties and a photo of yourself at each Earthcache site, documenting your find.

  3. A 3-4 page summary of what you learned including: 1) the geologic history of the Sonora Pass and Sierra Nevada region; 2) igneous processes; 3) metamorphic processes; 4) sedimentary processes; and 5) tectonic processes (subduction/compression, rifting/extension, faulting, and folding). Basically explain how the Sonora Pass and Sierra Nevada mountains formed. (Note: Use 1" margins, Arial or Time New Roman 12 point font, and double spaced text and submit online through Canvas in one of the following formats (MS Word; Google Docs; Adobe PDF).

  4. A final class survey.

  5. Field trip liability/release/authorization forms before you leave.


Grading Scheme:

A = Satisfactory completion of 1-5 above; 12 or more Earthcaches logged accurately; strong demonstration of understanding of geological concepts demonstrated in the summary paper. 

B = Satisfactory completion of 1-5 above; 9-11 Earthcaches logged, some inaccuracies; moderate comprehension of geological concepts observed in the summary paper.

C = Satisfactory completion of 1-5 above; 5-8 Earthcaches logged, some inaccuracies; poor comprehension of geological concepts observed in summary paper.

D = Unsatisfactory completion of 1-5 above; 4 or fewer Earthcaches logged; no summary paper turned in.

F = No attendance or left trip early and did not turn any work in.

Also, appropriate conduct and behavior expected of a college-level geology student will be factored into your grade. (Click here to read the Student Code of Conduct from the Columbia College Catalog).

That having been said, I hope we all have a good trip! Feel free to contact me at the phone number, or email address, above if you have any questions before or after we leave.