Let’s start our brief story 155 million years ago, during the Jurassic Period of the Mesozoic Era, the Age of Dinosaurs.  Both the present east and west coasts of North America were being subjected to a lot of volcanic activity.  The supercontinent Pangaea was breaking apart.  The new valley, the Mid-Atlantic Rift, between Europe and North America, was filling with ocean water, to became the North Atlantic Ocean.  Along the young Mid-Atlantic Rift, black basaltic lavas were spewing out, creating the new ocean floor.  In places along the new eastern coastline, fractures parallel to the rift allowed basaltic lavas to pour onto the land.  The North American Plate, carrying the continent and the western part of the new ocean floor, was moving toward the west as it still does today.  The Pacific Ocean floor, broken into several plates, including the Farallon Plate, was being pushed under Asia and North America, melting at the edges.  Many chains of volcanoes were formed around the ocean.  Thus was born the Ring of Fire. 

Here in Southern California, San Diego County was under water off the coast of Mexico.  Volcanic islands had formed offshore (the shoreline was far inland from here so the volcanoes were east!) as the Farallon Plate was dragged eastward under the westward moving North American Plate. Some volcanoes erupted a reddish-purple magma that cooled into the rock called rhyolite.  Eroded gravels (pieces) of the rhyolite began to be carried westward in a large braided stream, and tumbled into rounded shapes called boulders, cobbles, and pebbles. These gravels can be seen on our beaches today.  The rhyolitic bedrock they came from are not in California now, but remained in Sonora Mexico when Baja and Southern California were split away from the Mexican mainland by plate-tectonic movements that ultimately resulted in opening the Gulf of California; but this is many millions of years ahead in our story.   

From western California, more magma erupted and cooled into greenish-gray andesite, which got its name from similar rock found in the Andes Mountains.  Not all of the melted rock came out onto the surface as lava flows and pyroclastic debris.  Some was trapped and cooled beneath miles of overlying rock and sediment.  It became granodiorite, a rock like granite but containing more plagioclase and less alkali feldspar. Other related rocks cooled under the surface also.  We will call them “granitics” to mean rock cooled underground (intrusive igneous rock). These masses of rock would become the Peninsular Ranges Batholith.  Look for gravels of all of these types of rocks on the County beaches.

During the Cretaceous Period, about 100 million years ago, the continuing plate collision pushed the granitic and metamorphosed sedimentary rocks (changed by the heat and pressure of burial and squeezing) up into huge mountains, which may have been as high as the Andes of western South America are today.  These rocks are exposed today in the eroded Peninsular Ranges of eastern San Diego County.  You can recognize the granitics by the huge boulders they weather into, which are visible on the surface of the hills.  

About 55 million years ago, at the start of the Eocene Epoch of the Cenozoic Era of Earth’s history, western San Diego County was still attached to coastal Mexico, and was partly inundated by the ocean.  (Dinosaurs had become extinct about 65 million years ago, probably killed off with many other species of animal and plant life by the impact of a huge asteroid near the Gulf of Mexico.)  The coastline had bays and lagoons, much like the coast of today.   Some of the sediments brought by the rivers draining the lands in the east were being deposited into the still water as a greenish-gray mud.  Oysters, clams, worms and other marine invertebrates were making their home in the shallow water.  This mud eventually was compressed into rock and exists today as the Delmar Formation.  It can be seen easily at the base of the sandstone cliff facing the ocean from La Jolla to Encinitas.  The marine life is visible as plentiful fossils.  The tidepools at Swami’s Beach are in this formation.

During most of Eocene time (55 to 33.7 million years ago) sea level rose, and the shoreline therefore moved inland, closer to the eroding mountains.  The shoreline region was “dumping ground” for the coaser sediments of gravels and sands.  Sand accumulated in tidal flats and to submarine canyons, on top of the green muds.  Much of this sandstone today is the orangish rock known as the Torrey Sandstone.  Its layers are thick, and it looks like a strong rock, but it has a system of fractures that help it to fall, sometimes disastrously, onto the beach, when it has been undercut by the waves.  Because the Delmar Formation lying beneath it is weaker, waves can cut deep notches in it, removing support from the Torrey above it.  The Torrey sandstone is characterized by caves created by weathering.  Because it frequently has round cannon-ball-size concretions (deposits of harder minerals around sand grains or fossils) in the Torrey, these make rounded holes when they fall out, many people believe they are the origin of some small caves.

Sea level rises and falls often during geologic history, and sediment accumulating environments such as bays, tidal flats, streams, and submarine fans migrate also. For example, muds solidified into sedimentary rock bodies such as the Ardath Formation, which is seen south and north of Beacon’s Beach in Leucadia. Sands compressed and cemented, into rock bodies such as the Scripps Formation.  This rock holds up the sea bluffs north of Moonlight Beach.  Because the mudstone and siltstone of the Ardath Formation are softer and contain weaker layers, waves erode it and undermine the stronger sandstone above it, as happens with the DelmMar and Torrey Formations.  Dangerous rockfalls are the result, so beaware of the bluffs on the beach.

Around 56 to 36 million years ago, chunks of the Sonoran volcanic rock, the purplish rhyolite, along with pieces of the andesite and granitic rocks were being carried to the west in flash floods in large streams coming out of the mountains in the east. The streams tumbled the rocks which became smooth cobbles and pebbles.  Much deposition took place, beginning in Lakeside and building out across the shoreline, in a large alluvial fan on top of the Torrey and Scripps sandstones.  This is called the Poway or Stadium conglomerate.

The eastward-moving Farallon Plate progressively disappeared by subduction beneath North America. Large fault zones, appeared in the Pacific floor, and cut through part of the North American continent. A famous modern example is the San Andreas fault. A slice of land began to move to northwest as the Gulf of California began opening 5.5 million years ago. California (Baja California, San Diego, Los Angeles, Santa Cruz) ripped off the North American. This slice of the plate, becoming part of the Pacific Ocean Plate, and moving away from the Mexican mainland at more than two inches a year. We are headed to Alaska, where a trench leads the Pacific Plate back into the Earth’s mantle.  At the present rate of movement, we won’t get there for tens of millions of years.  The movement of the plate, however, moves many faults creating numerous small and large earthquakes in the Californias. 

Jumping forward almost to the present, let’s look at the last 2.6 million years. This was the Pleistocene Epoch of the Cenozoic Era, and there were major climate changes.  North America slipped into the grip of the Ice Ages.  Ice sheets were sculpting the lands in the north, and valley glaciers were carving the mountains of the Sierras. The coastal area was now a flat, grassy plain with streams meandering across it their way to the ocean.  The coastline was many hundreds of feet farther west than at present, because sea level was much lower all over the world during an ice advance.  The ice sheets had trapped the precipitation formed from water evaporated from the oceans, thereby lowering sea level.  The glaciers retreat on about a 100,000 year cycle which causes sea level to rise hundreds of feet. At the coastline, the waves, much farther out than present shorelines, were cutting new terraces, which became covered by beach sands and shells, just like the beach today.

The land bounced upwards frequently because of movement along the San Andreas, Rose Canyon, and other faults, raising old beaches high above sea level.  Today, the old beaches lie elevated above the present beach also along Interstate 5 as we drive through Camp Pendleton. Old sea floors are the flat tops of the surrounding hills, and contain gigantic steps down to the present beach.  They can be seen many miles inland as the mesas of San Diego; they are old wave-cut terraces. They have a reddish cement of iron oxide (rust) between the sand grains. The youngest terraces are close to the ocean, and are the lowest in elevation.

Whenever land rises, the streams flowing across the plain cut down and make deep, large valleys.  This happened along the San Diego County coastline.  Then, as climate warmed, ice sheets melted and sea level rose, flooding the valley into an estuary, and causing the deposition of soft sediments, which would be eroded easily when sea level dropped again.  These deposits make up the Bay Point Formation of Pleistocene age, less than 1 million years old.  Look for plentiful, but broken, oyster and clam fossils looking quite modern.

Because we are so close to a tectonic plate boundary, sections of the land continue to move sideways or up or down along faults.  A small up-down fault cuts through the sediments in the road-cut on La Costa Avenue near the freeway.   It is difficult to see with the brush that has grown on the face of the cut.  Geologists think it crosses the lagoon toward the northeast.  The fault is older than the terrace sediments above it, which are not displaced by it, but appear to have been deposited after faulting