The Big Picture: Time, Plates and the Sea Level
Shared Reading
Although the rocks in Torrey Pines State Natural Reserve are
partially obscured by the trees and wildflowers, you can see
some interesting geology. You should be particularly interested
because we live inside the fault zone, between the Pacific plate
and the North American plate. A fault is a break in the rocks
that make up the Earth's crust, along which rocks on either side
have moved past each other.
Geologic time is long beyond human experience. We can
give an example from the measured movement of the
mostly rigid thin plates that move over the surface of the
Earth like bits of eggshell over an egg. Torrey Pines Reserve
is near the edge of the Pacific plate, which is moving
toward the Aleutians at about two inches per year, while
the North American plate is moving south. The fault line
between these two moving plates is known as the San
Andreas Fault. If the Pacific plate continues to move
upward at the current rate of movement, we will be
dragged under the Aleutian chain in about eighty million
years and spewed up as molten rock. We have some other
faults on our ocean side, so San Diego may just get left
behind as a small island.
Most of the rocks in the Torrey Pines Reserve were deposited in or
near the ocean. However, the sea level has changed relative to the
land. During the last ice age, about 10,000 years ago, sea level was
300 feet lower than it is now (as a reference, you are about 4 ½ feet
tall!). If global warming, which is the gradual increase of the earth’s
overall temperature, melts all the ice in Antarctica and Greenland, sea
level will be an additional 300 feet higher.
San Andreas Fault
Volcanoes
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Rocks at the Reserve
All of the rock layers at Torrey Pines are sedimentary, made up of pieces of older rocks. The layers are
divided into formations that are different enough from each other to be told apart and are big enough
to be seen on a geologic map. Starting with the oldest, they are the Delmar Formation, the Torrey
Sandstone, the Lindavista Formation, and the Bay Point Formation.
The ages of the four formations are determined from the fossils found in each. The Delmar Formation
and the Torrey Sandstone are both middle Eocene (48,000,000 years old), the Lindavista Formation is
middle Pleistocene (1,000,000 years old), and the Bay Point Formation is upper Pleistocene (120,000 to
400,000 years old).
The Reserve has been cut by the sea into several marine terraces, formed where a sea cliff, with a wavecut platform is raised above sea level, resembling a step. The steps of the terraces are hard to see
because erosion has covered them with mud and cut canyons into them. The terraces have also been
cut by several faults that cut across the Reserve. The faults are easily seen in the beach cliffs and in the
road cut for Torrey Pines Road.
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Reading A
The Delmar Formation
The Delmar Formation is exposed only at the base of the sea cliffs from just south of the main reserve entrance to
just south of Flat Rock. Flat Rock is a sea stack remaining after wave erosion of the Delmar Formation. The Delmar
is mostly a greenish yellow mudstone and siltstone (like sandstone but with smaller grains). At its bottom, layers
of harder rock composed almost entirely of fossil oyster shells protrude. The shells stand out on top of the oyster
beds as the rock erodes because they are harder than the siltstone.
Layers of sandstone up to ten feet thick are also in the Delmar. All of the formation was deposited in a lagoon full
of life like our modern lagoon so fossils are abundant. Features showing deposition underwater include regular
cross-bedding.
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The Torrey Sandstone
The Torrey Sandstone crops out near the top of the sea cliffs and at many other
locations in the reserve. For instance, you will see them on the Fleming Trail, the cliffs
by the main Reserve road, and in Canyon of the Swifts. On the geologic map, it is dark
green. It made mostly of the mineral quartz and some feldspar, usually white but
often stained light brown by iron oxide from the rocks above. The rock was deposited
as a sandbar. The loose sand was cemented later by calcite from water flowing
through the sand. Few fossils are found in the sandstone because not many creatures
live in a sand bar.
From the Rim Trail in Canyon of the Swifts, we can see about 100 feet of Torrey
Sandstone in the cliff across the canyon. The small caves in the rock are called “wind
caves” although wind plays only a small role in making them. Water running over the
cliff dissolves the cement between the quartz grains, and water and wind carry the
grains away. Uneven cementing of the rock leads to the first shallow holes, then,
because the holes are shaded, the backs of the holes stay wet, more cement is
dissolved, and the holes deepen. A close-up of one cave shows the shaded back.
The sandstone in the canyon also shows dewatering cracks, the vertical white streaks that carry the brown layer
bands upward. The original sand dune was deposited in water so it was saturated, in other words, it held as much
water as it could possibly absorb. Before the sand was cemented into rock, something shook the sand so that the
grain structure in a layer collapsed. The high-pressure water squirted up through any crack, carrying loose sand
with it. Later cementing preserved these cracks as we see them.
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Reading B
The Lindavista Formation
The Lindavista Formation is the hard red rock on top of the flat areas in the
Reserve. It is green dotted on the geology map. It resists erosion more than
the Torrey Sandstone under it so it acts as a cap rock, protecting the softer
rock. Red Butte is a small remaining piece of the Lindavista that is no longer
connected.
About a million years ago, the ocean cut a marine terrace into the Torrey
Sandstone. The cobbles along the base of the Lindavista helped the cutting just
as similar cobbles on the beach today help the ocean cut into the cliffs. The
Lindavista was deposited as the ocean retreated, or moved back. It has both
marine shells and land fossils from streams. The rock is red because the
sandstone is cemented with iron oxide.
When the Lindavista erodes, marble
sized concretions, or hard rounded
rocks embedded in layers of stone in
sedimentary rocks, are visible. These
concretions are because they are
often harder than the surrounding
rock and more resistant to erosion
than the rest of the rock.
The Bay Point Formation
The Bay Point Formation is made of poorly cemented, light brown sandstone. It is
light brown on the geology map. Part of it, deposited in the ocean, has layers of
shells up to 100 feet above present ocean level. The layers above 100 feet have no
shells so are probably material washed down slopes by streams. The scenic
badlands formations are all Bay Point.
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Red Butte
Reading C
Fossils
Most of the fossils in the Reserve are found in the Delmar Formation at the bottom of Fossil log on the beach
cliffs. The oyster beds are the most obvious. The siltstone layers have other shells and casts of tubes made by
burrowing animals. Fossils show as holes in the rock with carbon in the center and sulfur staining the surrounding
rock.
In the Lindavista Formation, between the Fleming Trail and High Point, is the cast of
a tree root sticking out of the rock about two feet. A tree died and rotted away,
then the hole that was left by the root of the tree was filled by gray sand that filled
other nearby cracks in the rock. The gray filling is more resistant to erosion so it
continues to remain even after the red rock around it erodes.
In the Bay Point Formation, on the Beach Trail near the beach, is a layer of shells in
the soft rock. If you walk up the Broken Hill Trail, you will soon see the top of the shell layer. Since these
Pleistocene fossils contain about 95 % shells from modern species (an animal that was recently alive), it is difficult
to tell shells on the surface from the fossil shells have been dated at 120 thousand years old.
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Marine Terraces
We all live on a giant “staircase” in coastal Southern California. The land has been rising for over a million years as
the Pacific Plate moves northwest along the San Andreas fault. Our stairs are marine terraces cut by the ocean
each time the levels of land and sea remain the same for a few thousand years. Each step slopes gently out to sea.
You can see a terrace being cut now during any winter storm. The cobbles on the beach are thrown against the
cliffs by each wave breaker. The bottom few feet of cliff are eaten away by the impacts of the rocks. When the
resulting overhang is large enough, the whole cliff eventually falls and is washed out to sea by the waves. The
cliffs recede, or move back from their original position, so fast that even we can see the process happen in our
short lifetime.
Geologists found sixteen terraces in San Diego. In Torrey Pines Reserve,
they found the following terraces: Nestor, Guy Fleming, Parry Grove, and
Clairemont. The steps are not obvious here because they have been
covered with mud washed down by streams, cut by canyons, and faulted.
The old platforms where the ocean cut steps can be recognized by layers
Cobble layer. Rocks that cut the terraces, after
being thrown at the land by the ocean waves,
of the cobbles or rocks that were the ocean's tools. The easiest cobble
were deposited here.
layer to find is the one at the base of the Lindavista formation covering
the Clairemont terrace. In the Bay Point area, layer of cobbles mark the base of each terrace.
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Reading D
Erosion
Any land above sea level is being eroded by physical, chemical, and biological processes. Soil creeps slowly
downhill and sometimes slides rapidly. A good example of a slide in soil is just off the road near the entrance to
the Reserve. Running water is the most important erosion agent in the Torrey Pines Reserve. After a winter storm,
a plume of mud and sand stains the ocean near the Peñasquitos Lagoon. Soil and rocks in the lagoon have been
carried to sea to become new sand on our beaches.
Even the small streams crossing the Reserve erode the earth and cut deep canyons over time. Just after a rain,
they make muddy waterfalls at the beach cliffs. Rainwater also causes chemical erosion as it is slightly acidic from
the carbon dioxide pollution in the air. As a result, the rain dissolves the cementing minerals from rocks. Further
erosion is caused by loose grains of earth that have been carried away by wind or water. Plants pry apart rocks
with their roots.
Erosion also occurs plant roots pry apart the rocks or when lichens dissolve rocks with chemicals produced by the
fungus part of the plant. Finally, evidence of erosion is found where animals burrow into soft rock which then
allows water to penetrate, or get into the rock, and erode further it.
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Faults
Faults crossing the Reserve are most easily seen in the beach cliffs. The Carmel Valley Fault hits the beach about
500 yards south of Flat Rock. The Delmar Formation dips below the sand at about 100 yards south of Flat Rock but
is raised up about 26 feet into view again by the fault. In thin-bedded rocks another 100 yards south, even very
small faults which offset the rocks by only a few inches can be seen.
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Reading E
Cliffs
Spectacular cliffs are an important part of the scenery at Torrey Pines State Natural Reserve. The land must
descend somehow from the 100 yard altitude at the lodge to sea level at the beach. What is needed to produce
an almost vertical cliff rather than just a steep slope of 45 to 60 degrees?
The first requirement is for layers of rock to be strong enough and thick enough to stand up in a cliff. If we dig a
trench in weak soil, the walls will collapse, or fall apart, when it is a few feet deep. The Delmar Formation, Torrey
Sandstone, and Lindavista Formation are all strong enough but the Lindavista is not thick enough to make much of
a cliff. The more recent formations of Bay Point is not strong enough to form a very high cliff so it erodes into
what is referred to as the ‘badlands” topography with very steep slopes but no cliffs higher than 20 or 30 feet.
The second requirement is for a mechanism to cut the cliff. In several of our canyons, streams cut the rock, most
notably for the cliff in the Torrey Sandstone. Our biggest and best cliff is along the beach where the waves cut out
the bottom of the cliff. The cutting process occurs mostly at high tide during winter storms. Millions of hard
cobbles on the beach are thrown against the cliffs. The cliff top falls when too much is cut out of the earth
underneath, also referred to as the undercut.
The third requirement for a cliff to exist is that debris must be somehow removed either naturally or by human
effort. Debris comes from landslides and rock falls from the land above. In the canyons, streams remove debris
during floods. On the beach, debris is removed by storm waves during high tides.
Cliff erosion is also caused by water from rain draining into cracks behind the cliff resulting in landslides. Friction
along the cracks is reduced and landslides result. When you see water running out of cracks in a cliff, stay back!
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