Welcome to Mesa College Geology Courses

                                  By Don Barrie, geology professor

The above photo shows the tributary of Tecolote Canyon immediately south and west of the Mesa College campus.  Once used by the Kumeyaay Indians as a place to find food and shelter, Tecolote Canyon is one of many coastal canyons in the greater San Diego area.  Thousands of years of sidewall erosion have exposed a diverse array of rock types in the canyon walls and bottom.  For Mesa College students, the canyon is an easily accessible geologic laboratory where several of San Diego’s prominent sedimentary rock units can be observed and studied.

The above photo shows the approximate geologic boundaries (contacts) between three prominent sedimentary rock units exposed in the sidewall of Tecolote Canyon immediately south of campus. 

The Stadium Conglomerate and the underlying Friars Formation have been dated as Eocene in age (approx. 40-45 million years old), whereas the Linda Vista Formation is much younger (approx. 1-1.5 million years old). 

A time gap in the geologic record (roughly 40 million years!) separates the Linda Vista Formation from the much older Stadium Conglomerate and Friars formation.  After millions of years of erosion had worn away the upper surface of the Stadium Conglomerate, only then was the Linda Vista Formation deposited on top.  Geologists call such an erosional time gap an unconformity.  Following deposition of the Linda Vista Formation, the entire area experienced tectonic uplift, creating the Linda Vista terrace upon which Mesa College is built.

The above photo shows a close-up of the Stadium Conglomerate, which consists of volcanic cobbles in a sandstone matrix.  Such volcanic cobbles occur all over San Diego and are known to geologists as Poway clasts.  According to San Diego State University geology professor, Pat Abbott, these cobbles were deposited on the surface of an ancient alluvial fan that built out into the Pacific Ocean as westward flowing streams carried eroded bedrock fragments from a source area far to the southeast.

Interestingly, reddish cobbles like the one above, called rhyolite, can be matched to intact volcanic bedrock exposures in Sonora, Mexico, over 250 miles to the southeast.  Did the ancient stream that deposited these cobbles flow all the way from Sonora to San Diego?  No. Instead, the very ground itself has moved!  As Baja California tore itself away from mainland Mexico about 5.5 million years ago, the Baja peninsula slid northwestward, along with San Diego, as the Gulf of California opened up to the east.  This severed the Poway clast stream cobbles from their source area in Sonora.  Quite literally, San Diego was once connected to mainland Mexico!

Here's how Dr. Abbott describes it:

"One way to see for yourself the effects of this opening (of the Gulf of California) is to use scissors on a map; cut along the San Andreas fault and then slide the western piece to the south, removing the Gulf of California and realigning Baja California with Mainland Mexico; this places San Diego adjacent to the present day state of Sonora in northwestern Mexico" (From Abbott, 1999, The Rise and Fall of San Diego, Sunbelt Publications, p. 94).

Check out this wonderful Quicktime animation (click on large or small format)  by professor Tanya Atwater that shows the northwestward movement of Baja and dispersal of Poway clasts.