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By John J. Mack

A simplified summary of Catalina’s geology - Although the flora and fauna of Catalina Island justifiably get much more attention, the rocks that lie under your feet are equally memorable. Geologically speaking, Catalina Island is quite young. Its oldest geologic formations are a little more than 100 million years old.

While that sounds like an immense span of time, it is barely 2 percent of the entire age of the Earth. Catalina Island's youngest formations were formed in the past 5 to 20 million years, and the Island itself has been above the surface of the ocean for just the last few million years or so. But this span of time still encompasses the end of the Cretaceous Period (the end of the age of the dinosaurs) and the rise of the age of mammals in the Tertiary Period.

The basic story of how the island was formed, and the rocks you see around you were deposited, was assessed by Stephen Rowland in a 1984 summary paper: "...the geology of Santa Catalina Island consists of a Mesozoic metamorphic basement complex intruded and overlain by Miocene igneous rocks...[with] a few scattered deposits of Tertiary sedimentary rocks..."  What this somewhat puzzling sentence is saying is that there are basically three broadly identified and locally complex types of rock that make up Catalina Island.

The first type is the Catalina schist, which is exposed throughout the western and central parts of the Island.  This is the "Mesozoic metamorphic basement complex" described above. The second type is the quartz diorite of the Catalina Pluton, exposed on the East End and reaching Middle Canyon. This is the "intruded Miocene igneous" rock.  And finally, there are the various andesite rocks that form an exposure that nearly bisects the center of the Island from the channel shore to the windward side from White's Landing to Mount Banning.  These are the "overlain Miocene-aged volcanic rocks" that Rowland describes.

It is important to note that true sedimentary rock is uncommon on the island except for a few deposits of marine limestone on Mount Banning and Twin Peaks, some diatomaceous limestone between Empire Landing and the Isthmus, and some ancient beach deposits above Little Harbor and at a few other locations.

With a little attention to detail and knowledge of where the rocks outcrop, each of the three main types is easy to learn.

Dig a Little Deeper

Edited by Jerry Roberts

Issue #29 / All About Geology


Catalina’s Dominant Rocks
Exploitable Resources
Fact or Fiction? Gold on Catalina?
Did You Know … Catalina Rocks Breakwaters  

This geologic map of Catalina Island highlights the major rock types across the island: the Catalina Schist (green), Catalina Pluton (blue) and the various andecite rocks (pink), along with a variety of rocks found in lesser amounts.

Missed recent issues?
Issue #1 / All About Bison

Issue #2 / All About Birds

Issue #3 / All About Plants
Issue #4 / All About Eagles
Issue #5 / All About Ravens & Crows
Issue #6 / All About Natives & Invasives  
Issue #7 / All About Rain
Issue #8 / All About Bison Roundup
Issue #9 / All About Foxes
Issue #10 / All About Weeds
Issue #11 / All About Eagle Hatchlings
Issue #12 / All About Snakes
Issue #13 / All About Diurnal Raptors
Issue #14 / All About Diurnal Raptors II
Issue #15 / All About Giants & Dwarves
Issue #16 / All About Fire Ecology
Issue #17 / All About Mule Deer
Issue #18 / All About Feral Cats
Issue #19 / All About Acorn Woodpeckers
Issue #20 / All About Tachi the Fox
Issue #21 / All About Observing Nature
Issue #22 / All About 2013 Bald Eagle Update
Issue #23 / All About Invasive Plants
Issue #24 / All About Poisonous Plants
Issue #25 / All About the Value of Nature
Issue #26 / All About Edible Invasives
Issue #27 / All About Plants in Summer
Issue #28 / All About Marine Ecosystems



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