Beach cliffs
 
 

Geology of Torrey Pines State Natural Reserve

By Don Grine, Geophysicist Emeritus

Rocks at the Reserve | Delmar Formation | Torrey Sandstone
Lindavista Formation | Bay Point Formation | Fossils | Marine Terraces
Erosion | Faults | Cliffs

Trail guide with GPS coordinates  (pdf download)

The Big Picture: Time, Plates, and Sea level

Although the rocks in Torrey Pines State Natural Reserve are partially obscured by the trees and wildflowers, you can see some interesting geology. The rocks are always in full bloom. You should be particularly interested because we live inside the fault zone between the Pacific plate and the North American plate.
Painting of Broken Hill & flowers
by J. J. Grine
Painting of Broken Hill & flowers

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, moving toward the Aleutians at about two inches per year. If the movement continues, we will be dragged under the Aleutian chain in about eighty million years and spewed up as molten rock. We have some faults on our ocean side so we may get left behind a bit as a small island.

Most of the rocks in the Reserve were deposited in or near the ocean. 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 now. The lagoon to our north was carved 300 feet deeper than its present mud floor, then filled with mud when the ice melted and the sea rose. If global warming melts all the ice in Antarctica and Greenland, sea level will be 300 feet higher, lapping at the Lodge.

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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 absolute year ages come from radioactive dating of igneous rocks found elsewhere near sediments with the same fossils. The radioactive clocks start when crystals are formed in the cooling molten rock.

The Reserve has been cut by the sea into several marine terraces. 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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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.

 

Torrey sandstone overlying Delmar Formation
Torrey sandstone overlying Delmar Formation

 

Eocene oyster bed
Eocene oyster bed

 

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. Each wave deposits a layer of sand on the lee side of a sand bar, inclined at the angle of the end of the bar. Then, a new surge of water cuts off the top of the bar. Note that the cross-bedding in our picture is in opposite directions in the top two layers. The water flow had to be in opposite directions at the times the layers were deposited.

 

Crossbedding in Delmar Formation
Crossbedding in Delmar Formation

 

If enough animals burrowed in the sand and mud, the cross-bedding was destroyed and the rock is “bioturbated.”

 

Fossil worm burrows in Delmar formation
Fossil worm burrows in Delmar formation

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Torrey Sandstone

 

The Torrey Sandstone crops out near the top of the sea cliffs and at many other locations in the reserve, for instance on the Fleming Trail, the cliffs by the main Reserve road, and in Canyon of the Swifts. The “type section” where the rock was first described is on the Torrey Pines grade which runs through the reserve. On the geologic map, it is dark green. It is mostly quartz with 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.

Geological Map of Torrey Pines State Reserve
Geological Map of
Torrey Pines State Natural Reserve

 

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.

 

Wind caves in Torrey sandstone
Wind caves in Torrey sandstone

 

A concretion is seen in the back of the shaded cave. The concretions are caused by deposition of calcite and ironoxide cements from solutions running through the sandstone. Rainwater dissolves the cements from the sandstone and the rocks above it during wet times and deposits them during dry times. Deposits grow on earlier deposits so the concretions often start at a point and grow in nearly spherical layers. If one starts along a fossil twig, for instance, it can grow into a concretion that looks like a pipe.

 

Wind cave with concretion inside
Wind cave with concretion inside

 

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. Before the sand was cemented into rock, something shook the sand so that the grain structure in a layer collapsed and water in the layer carried the load of sand above it. The high-pressure water squirted up through any crack, carrying loose sand with it. Later cementing preserved these cracks as we see them.

 

Dewatering cracks in Torrey sandstone
Dewatering cracks in Torrey sandstone

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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 had connected with the layer that the Lodge sits on. The road cut between the Fleming Trail and High Point shows the Lindavista over the Torrey Sandstone.
Red Butte - Lindavista Formation
Red Butte - Lindavista Formation

 

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 present cliffs. The Lindavista was deposited as the ocean retreated. It has both marine shells and land fossils from streams. The rock is red because the sandstone is cemented with iron oxide.

 

Lindavista overlying Torrey Sandstone
Lindavista overlying Torrey Sandstone

When the Lindavista erodes, marble sized concretions are left on top of the rock because they are more resistant to erosion than the rest of the rock. The concretions were formed by cycles of solution and deposition like the larger concretions in the Torrey Sandstone.

Concretion marbles from Lindavista Formation
Concretion marbles from
Lindavista Formation

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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.

Badlands of the Bay Point Formation
Badlands of the Bay Point Formation

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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. Fossil logs show as holes in the rock with carbon in the center and sulfur staining the surrounding rock.

 

Fossil log
Fossil log

 

Fossil worm tubes in Delmar Formation
Fossil worm tubes in Delmar Formation

 

In the Lindavista Formation, in the roadcut between the Fleming Trail and High Point, is the cast of a tree root sticking out of the rock about two feet. It looks much like the dead roots of Torrey pines near it. A tree died and rotted away, then the root hole was filled by the same gray sand that filled nearby cracks in the rock. The gray filling is more resistant to erosion so remains after the red rock erodes.

 

Cast of tree root in Torrey Sandstone
Cast of tree root in Torrey Sandstone

 

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, it is difficult to tell shells on the surface from those in an Indian midden of modern shells. These shells have been dated at 120 thousand years old.

 

Fossil shell layers
Fossil shell layers

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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 it moves northwest along the San Andreas fault system. The ocean has meanwhile risen and fallen as the amount of water frozen in our polar regions during ice ages changes. The motion is not steady for either the ocean or land. Our stairs are marine terraces cut by the ocean each time the relative levels of land and sea remain still for 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 breaker. The bottom few feet of cliff are eaten away by the impacts. When the resulting overhang is large enough, the whole cliff falls and the debris is broken and washed out to sea by the waves. The cliffs recede so fast that even we, during our short lives, can see the process.

 

Cobble layer at base of Lindavista Formation
Cobble layer at base of
Lindavista Formation

 

Geologists found sixteen terraces in San Diego by detailed mapping and elevation measurements of terrace shoreline sand abrasion platforms. 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 abrasion platforms where the ocean cut steps can be recognized by layers of the cobbles that were the ocean's tools. The cobbles are now at the base of the sediments that buried the steps. The easiest cobble layer to find is the one at the base of the Lindavista formation covering the Clairemont terrace. In the Bay Point, layer of cobbles mark the base of each terrace.

 

 

Cobble layer
Cobble layer

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Erosion

 

 
Any land above sea level is being eroded by physical, chemical, and biological processes. Soils creep slowly downhill and sometimes slide 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 Reserve. After a winter storm, a plume of mud and sand stains the ocean out from Peñasquitos Lagoon. Soil and rocks in the lagoon have been carried to sea to become new sand on our beaches. Dams on our rivers have reduced the sand washed down into the ocean and our beaches are starving for sand.

Erosion - mud slide
Erosion - mud slide

Mud plume nearshore at Torrey Pines State Beach
Mud plume nearshore at
Torrey Pines State Beach

 

Even the small streams crossing the Reserve cut deep canyons in time. Just after a rain, they make muddy waterfalls at the beach cliffs. Rainwater also becomes slightly acidic by dissolving carbon dioxide from the air, then dissolves cementing minerals from rocks. Loose grains can be carried away by wind or water. Plants pry apart rocks with their roots.

Lichens dissolve rocks with chemicals produced by the fungus part of the lichen. Animals burrow in soft rock to allow water to penetrate and erode.

 

Canyon of the Swifts
Canyon of the Swifts

Beach waterfall
Beach waterfall

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Faults

Faults crossing the Reserve are most easily seen in the beach cliffs and in the Torrey Pines grade on old Route 101. 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.

 

Carmel Valley Fault from beach south of Flatrock
Carmel Valley Fault from
beach south of Flatrock

 

Close-up of small fault
Close-up of small fault

 

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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. The view of the Reserve from Del Mar shows a gradual steepening of the slope to the beach to about 45 degrees halfway down. Then the land plunges vertically to the sand. What is needed to produce an almost vertical cliff rather than just a steep slope of 45 to 60 degrees?

Beach cliffs
Beach cliffs

 

The first requirement is for layers of rock strong enough and thick enough to stand up in a cliff. If we dig a trench in weak soil, the walls will collapse when it is a few feet deep. Even in strong soil, OSHA requires shoring of trench walls to prevent collapse on workmen. 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 Bay Point Formation is not strong enough to form a very high cliff so it erodes into our ‘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. The mechanism was man's machinery in our road cuts by the Fleming trail and on North Torrey Pines road. In several of our canyons, streams cut the rock, most notably for the cliff in the Torrey Sandstone opposite the Rim trail. 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. The hard cobbles on the beach are thrown against the cliff by the million. The cliff top falls when the undercut is too great. If you look up from a recent rock fall, you will usually see an arch in the cliff face. The arch shape is self-supporting even if the rock above it is cracked.

The third requirement for a cliff to exist is that debris must be prevented from accumulating enough to bury it. Debris comes from rock falls from the cliffs and from soils washed down from the slopes above the cliffs. A hard, nearly level cap rock at the top of a cliff will reduce the wash debris and protect the softer rock below. The Lindavista Formation is the cap rock for our road cuts. The old beach cliffs at various elevations in the Reserve have been buried so well that they are hard to see at all. In our road cuts, debris removal is by trucks. 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 or such human uses as lawn watering draining into cracks behind the cliff. Friction along the cracks is reduced and landslides result. When you see water running out of cracks in a cliff, stay back.


Address questions on TPSNR Geology to Don Grine
grine@roadrunner.com

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Aerial photos of Torrey Pines State Beach
 

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