Geology of Cabrillo National Monument
This set of notes on the Geology of Cabrillo was left on the computer of one the monument’s most exemplary volunteers, John Pierce Harris–Pierce to all of us. He was a naturalist, an adventurer, a lawyer, a wildlife photographer, a professor of geography and photography, and a volunteer for Cabrillo National Monument. “Pierce spoke with such passion about natural resources that it was infectious,” said Terry DiMattio. He logged more than 1,000 volunteer hours in various capacities, ranging from the annual Cabrillo Festival to whale-watching weekends, to teaching our new volunteers about tidepool geology. Pierce died Sept. 2, 2006.
“Hi I found this on my computer, likley I wrote it years ago before Dr. Abbotts last book, cheers P
ps I am not a geologest just a Wyoming trained naturest who can read really fast, all the information below was found by walking around and reading the references I have listed. cheers P”
Some Geologic Terms
- Anticline: a fold that arches upward
- Syncline: a fold the sags downward
- Fault: a crack in rock where the rocks have moved in relation to each other
- Dip: the angle of a rock layer in relation to the horizontal surface
- Strike: Compass direction of a rock layer as it intersects with the horizontal
Clastic rock: a rock made up of broken fragments of other rocks
- Conglomerate: rounded clasts more than 2 mm diameter
- Breccia: sharp edged clasts
- Sandstone: clasts of 1/16 mm to 2 mm
- Shale: consolidated mud, less than 1/256 mm fragments
- Silt: 1/257 mm to 1/16 mm
- Limestone: made of calcium carbonate (CaCO3)
- Skeletal: made from fragments of shells, etc.
- Oolitic: made of spherical concretions of CaCO3
Metamorphic rocks: rocks that have been physically altered by heat or pressure
Volcanic rocks: rocks that originate from volcanic activity
Sedimentary rocks: rocks that are layered from the consolidation of sediments
Igneous rocks: rocks that solidified from molten material
- Geology of the San Diego Metropolitan Area, California, Bulletin 200, Calif. Division of Mines and Geology, Sacramento CA 1975
- Geologic Hazards in San Diego, P.L. Abbott, San Diego Museum of Natural History, San Diego, 1977
- Geology of San Diego County, Burns Ed, Sunbelt Publications, El Cajon, CA 1997
- Pages of Stone, #2 The Mountaineers, Seattle, 1986
- Assembling California, McPhee, Noonday Press, NY 1993
Sea Cliffs, Beaches and Coastal Valleys of San Diego County, F.P. Shepard, U.C. Press, Berkeley, 1984
Point Loma is a block faulted peninsula oriented north south, and an anticline oriented east west with about five miles of the westward portion eroded away by the sea. The east side of the Point is composed of sediments (limestone and sandstone and some conglomerates) striking north south and dipping 10 degrees east.
Cabrillo is an anticline (west side is believed to be beneath the sea about 4 miles out) with the western end eroded away leaving a series of marine terraces which have further eroded leaving a relatively steep slope from the lighthouse down to the sea at the tidepools.
The rock at the tidepool area is predominately poorly consolidated marine conglomerates: sandstone, siltstone, mudstones and limestones. They are believed to be deposited in the shallow seas adjoining the mainland during late Cretaceous (about 90 million years ago).
Visible from the tidepool area are four geologic formations:
The Point Loma Formation is a greenish marine sandstone interbedded with fossiliefrous shale.  This formation including the portion under water is reported to be at least 120m thick and dates from 104 million years ago plus or minus 10 million years. Only about 6m of this formation is visible from the tidepools and dates from about 90 million years ago.
The Cabrillo Formation sits directly on top of the Point Loma formation and is composed primarily of brown conglomerate marine sandstone and reportedly over 160m thick. About 120m of this formation is intermittently visible from the tidepools.
The Linda Vista Formation (about 2 million years ago) is the highest point of Cabrillo National Monument and is a relatively hard reddish brown (hematite cemented) sandstone.
Also visible from the tidepools are large lenses of cobbles and conglomerates in the road cuts. The top of the Cabrillo Formation (about 75 million years ago) is approximately the end of the Cretaceous period and the beginning of the Tertiary. Unfortunately for us, about 13 million years of the upper Cretaceous and lower Tertiary periods are missing at Cabrillo National Monument.
Perched precariously on top of the Point Loma Formation on the trail just above the tidepools are several large boulders. These boulders are one of the wonderful mysteries of the Cabrillo National Monument geology. These boulders are metamorphic conglomerate volcanic in origin and are foreign to our area, the nearest known volcanoes of this type are from the Santiago Peak area near Lake Hodges about 60km north east of us. How did these boulders get here? Some writers have suggested they were transported by a large river or landslides but no geologic evidence of this is to be found. The late Professor Shepard of Scripps Institution of Oceanography suggested they may represent submarine landslides in the area. Were they Glacial deposits? Not likely as they show no marks of glaciers and there was no known glacial activity in our area during that geologic period.
Between the Point Loma Formation and the Cabrillo Formation are some slope wash components of the recent (130 thousand years ago) Bay Point Formation; the lower part of this formation is marine and the upper part terrestrial.
Fossils are any traces of ancient life. A fossil may be an impression like a foot print or a petrified bit of bone or an entire animal or plant where the tissue has been replaced by minerals.
Local Period Possible Fossils
Point Loma Formation Upper Cretaceous Foraminifera, Molluscs (Mosasaurs?*)
Cabrillo Formation Upper Cretaceous Foraminifera, Molluscs (Mosasaurs?)
End of Cretaceous at Cabrillo — Dinosaurs are no more!
Linda Vista Formation Lower Pleistocene Molluscs
Bay Point Formation Middle Pleistocene Molluscs, Foraminifera, Ostracods
* On display at the San Diego Museum of Natural History is a large Ammonite collected from the Point Loma Sediments, some Ammonites are reported to have teeth marks of Mosasuras, a 10m (33 foot) giant sea going reptile of the late Cretaceous. 
Sea Stacks, Arches and Caves
The towers of rock separated from the shore are sea stacks, these are left when an arch collapses and leaves the seaward side isolated from the shore. Sea stacks, arches and caves are caused by differential erosion, where softer rock material is eroded by wind and sea more rapidly than harder material.
Approximately15 nautical miles south of the tidepools are three islands located within the territorial waters of the Republic of Mexico. These islands are believed to be composed of a mixture of Cretaceous and Miocene rocks of volcanic origin. There is a small colony of Northern Elephant Seals, Mirounga angustirostris, on the islands which may be observed from off shore. There is a small Mexican Army garrison on the south island and landing on the islands is not permitted on any of the islands. There is no fresh water on the islands.
Basic principals of modern geology
- Unconformity, a break or interruption in the normal sequence
- Uniformity, same processes have been operating over the history of the earth
- Superposition, oldest rocks & sediments at the bottom, youngest at the top
- Original horizontally, sediments are laid down flat
- Crosscutting relationships, Erosion and cracks that cut through sediments and rocks are younger
- Fossils, life spans of species from evolving to extinction differ
- Faunal succession, often the sequence of life forms found in fossils helps in dating, some sequences can be correlated with other places in the world or in a small area and succeed each other in a distinct recognizable order. Fossils are any evidence of the prior existence of a plant or animal found in rocks.
Radiometric age dating
Marie Curie noticed some rocks exposed her photographic film. Many rocks and minerals decay radioactively from parent to daughter. Parent > Daughter U235 > stable lead 207 U 235in >713,000,000 years becomes 1/2 stable lead 207 thus if you find some U235 that is 1/2 lead 207 then it is 713,000,000 years old, if 1/4 lead then 356,500,000 and so on ad infinitum. All living things take in Carbon 14 in the same proportions and cease accumulating C 14 upon death, therefore it is possible to date many plants and animals by analyzing the % C 14 present up to about 50,000,000 years. Carbon 14>in 5730 years becomes 1/2 stable carbon, thus if you find some burnt wood that is 1/2 C14 and 1/2 C it is 5730 years old + or – 500 years (error) if 1/4 C then 11,460 years old, if 1/8 then 22,920, if 1/16 then 45,840 after this samples are a bit small for testing…. There are other materials such as Argon that are also useful in radiometric dating. All this is done by grinding up the rocks, separating the parts chemically, heating the minerals to be tested to 1000 to 2000 degrees Celsius to ionize, then accelerate with 10,000 volt accelerator in a magnetic field to separate the heavy from the light ions and feed into your computer to analyze in % parent and daughter. A mastery of these basic principals of geology and radiometric dating are necessary for geography, archaeology and many other sciences.
 Geology of San Diego County, D. Burns Ed, p 58-59
 Geology of the San Diego Metropolitan Area, California, Bulletin 200, Calif. Division of Mines and Geology, Sacramento CA 1975 p 15
 Bulletin 200, p 15
 Geology of San Diego County, D. Burns Ed., p 58-59
 Sea Cliffs, Beaches and Coastal Valleys of San Diego County, F.P. Shepard, U.C. Press, Berkeley 1984 p 155
 Sea Cliffs, Beaches and Coastal Valleys of San Diego County, F.P. Shepard, p 155
 Sea Cliffs, Beaches and Coastal Valleys of San Diego County, F.P. Shepard, p 155
 Bulletin 200, p 29; Geology of San Diego County, D. Burns Ed, p 77
 Geology of San Diego County, D. Burns Ed, p 57
Last revised 29-Jul-13