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INTRODUCTION TO ENCINITAS GEOLOGY
By Kathryn Thorbjarnarson, Ph.D.
Dept. of Geological Sciences
San Diego State University
GENERAL GEOLOGY
ROCK TYPES
Sedimentary Rocks – rock formed from loose particles of rock or
sediment (clay, silt, sand, gravel). Sedimentary rocks form by
the sediment being compacted and cemented together. This
process usually occurs after the sediments have been buried.
Geologic forces can uplift the buried rocks to the surface and/or
streams will erode canyons through the sedimentary rocks
(example: Grand Canyon). Sedimentary rocks can also form by
minerals precipitating out of solution (go from dissolved in water
to solid mineral).
Metamorphic Rocks – rock formed by changing (metamorphosing) an
existing rock using high pressure or high temperatures (without
melting and forming molten rock or magma).
Igneous Rocks – rocks formed by the cooling of molten rock or
magma. The magma can cool deep below the surface of the
Earth or can cool at the surface after coming out of a volcano.
Buried igneous rocks such as granite can be uplifted to the
surface from tectonic forces and exposed when the rocks
above them are eroded away.
ENCINITAS GEOLOGY
Metamorphic and Igneous Rocks – Eastern Encinitas has these types
of rocks in Olivenhain (granitic rocks and Santiago Peak
volcanics). The greenish Santiago Peak rocks are part of a
large mountain chain that existed over 100 million years ago.
You can see large cobbles of these igneous and metamorphic
rocks on the beaches which were carried to the coast by a large
river 40 million years ago. In the summer, these cobbles are
generally covered by sand. In the winter, storm waves move
the sand and allow you to see the cobbles.
These cobbles are a mix of metamorphic and igneous rock types.
The easiest to identify are purple or pink and are rhyolite, an igneous
rock which came out of a volcano. The tumbling of the rocks
smoothed them into roundish shapes.
Try to find a rhyolite cobble next time you are at the beach!
Sedimentary Rocks – the rocks seen in western Encinitas are
sedimentary rocks. As the mountains to the east eroded, the
sediments were deposited in lagoons or beaches. Over time
these sediments were buried and lithified (turned into
sedimentary rock) over 50 million years ago.
Geologists studying these rocks give them names.
The Del Mar Formation is a mixture of a greenish claystone
(formed from clay) and grayish sandstone (formed from sand).
Because of the properties of the rock and the fossils found
within the rock, geologists know this rock formed 50 to 52
million years ago from sediments deposited in a lagoon. You
can see this rock at the bottom of the sea cliff on the south end
of Moonlight Beach. A better place to see this rock and all the
oyster fossils it contains is at the tide pools along the beach
between Solana Beach and Cardiff.
Del Mar Formation at Swamis
Oyster fossils within the Del
Mar Formation at Swamis
The Torrey Sandstone is a white to dark brown sandstone. In many
areas in Encinitas is looks very white or blonde colored. This
rock formed from a sandy beach deposit 50 to 55 million years
ago. You can see the Torrey Sandstone on the main part of the
sea cliff at Moonlight Beach. You can also see it along cliff
faces while driving inland in Encinitas. Another famous location
for the Torrey Sandstone is the slopes surrounding the
playgrounds of Park Dale Lane School!
South Moonlight Beach – Torrey sandstone is the light colored rock
Views of Torrey sandstone along Encinitas Boulevard
Torrey Sandstone at Park Dale Lane playground.
You might have a question at this point. If these sedimentary
rocks were formed near the ocean and were buried over 50 million
years ago, how come they are at the surface today and high above
the level of the sea?
Good question!
The Earth’s upper surface is made up of many large pieces
called tectonic plates. These tectonic plates move in differing
directions relative to each other. In southern California, we are on the
edge of a tectonic plate that is moving side by side relative to the
plate next to us. All this moving around also causes upward
movement of the crust and brings rocks from deep below up to the
land surface. This movement happens over millions of years and
usually can’t be felt except in an earthquake! All the material that
used to be on top has been eroded away and moved somewhere
else as sediment and probably to create new sedimentary rocks!
That explains how the old, buried sedimentary rocks were
brought to the surface and above sea level. But if we look at the
profile of Encinitas (and most of San Diego), we notice something
else.
Side or profile view of topography (change in land surface elevation)
From the beach in the west to the top of the hill on Encinitas Blvd inland.
The Torrey Sandstone looks like it has been carved into a
staircase pattern as it was lifted up above sea level. On top of
each “step” (you can see two steps in the figure above) are light
brown sandstones (near the coast) and reddish brown
sandstones (at top of hills inland). As the rock was lifted up, it
sometimes stopped for a long period of time. At sea level, the
ocean waves were able to carve a flat surface into the rock
creating the top of a “step” also called a mesa. The
combination of the pattern of rock lifting up, pausing, lifting up
and stopping again created this stairstep pattern. Each “step”
is a former beach zone with its flat surface and has younger
rocks deposited on top of the step after it was lifted away from
the sea.
The oldest “step” and sandstone is the highest inland one
and is 1 to 1.5 million years old. The reddish color is very easy to
recognize and is caused by minerals containing iron. If any of you
shop at the Target shopping center, this sandstone (known as the
Linda Vista Formation) can be easily seen from the parking lot. It
is the reddish brown rock ledge seen at top of the horizon behind
the shops.
Linda Vista Formation – a reddish brown sandstone – seen at the top of the cliffs
Located behind the shops in the Target shopping center.
The younger beach sandstone is at the top of the sea
cliffs and is a medium brown color. This sandstone is called the Bay
Point Formation and is only 80,000 to 130,000 years old. This rock
has not been cemented very well and is easily eroded. As you walk
along the beach, you may notice many houses and patios very close
to the edge or actually hanging over the edge of the cliff.
Sea Cliffs south of Moonlight Beach. The Bay Point – a medium brown
Sandstone is at the top of the cliff. The white Torrey Sandstone is below.