Ancient Oceans of San Bernardino County

Earth’s water oceans have been around for at least 3.8 billion years and may even have been around since shortly after the planet’s formation. The ocean was the incubator of life, with the oldest known fossils representing 3.5-billion-year-old marine bacterial colonies called stromatolites. The oldest rocks found in San Bernardino County are much younger, albeit still the oldest in California! These ancient stones are now highly metamorphosed from the heat and pressure of being buried deep but were originally deposited in shallow seas 1.7 billion years ago. Outcrops of these rocks can be found in mountain ranges throughout the county, including the San Bernardino and San Gabriel ranges. The oldest fossils in the county are 1.3-billion-year-old stromatolites from Death Valley National Park.

Approximately 548 million years ago an event called the Cambrian Explosion occurred, in which multicellular life massively diversified. For 400 million years after that event, southern California remained entirely underwater except for some island arcs offshore. Marine environments preserved in the rock range from reef-analogous masses of coral-lookalike bryozoans and sponges, to ‘forests’ of crinoids, to open waters inhabited by early fish. Ammonites and trilobites are highly recognizable denizens of San Bernardino County from this time. The water was relatively warm and shallow – a consequence of the preserved environment being upon the continental shelf. The deep-water environments of the oceanic abyss are deposited atop dense oceanic plates, which normally subduct below continents and are destroyed.

Rock outcrops containing fossils from this stretch of time are mostly exposed within the county’s modern mountain ranges. How did they get up there if they were once at the bottom of the sea? Essentially, long after being deeply buried, these rocks were thrust upwards by tectonic activity. The uplifted highlands they were ensconced within were then slowly stripped of younger rock layers by erosion, leaving mountains behind and revealing these fossils for the first time in hundreds of millions of years!

It wasn’t until approximately 154 million years ago, during the late Jurassic period, that the west coast of North America extended to what is now the border between southern California and Nevada. No non-avian dinosaur bones have ever been found in San Bernardino County, mostly because the county was largely underwater while these creatures walked the earth. However, carnivorous dinosaurs did leave footprints behind in coastal sand dunes! More plentiful are shallow oceanic sedimentary rocks preserving the shells of marine invertebrates, including ammonites with elaborate shells. The most common oceanic rocks from the age of the dinosaurs in San Bernardino, however, are those that formed due to undersea and island volcanoes.

As time went on, the California coast slowly crept westward. The Alberhill Clay in Riverside County dates to the Paleocene epoch (66 – 56 mya), shortly after the extinction of non-avian dinosaurs 66 million years ago. This rock unit contains fossils of plant leaves while alternating between marine and terrestrial deposits, representing how rising and falling sea levels resisted or accentuated the expansion of the continent. Marine rocks of similar age elsewhere in California contain shark teeth.

Portions of southwestern San Bernadino County remained underwater up until the end of the Miocene epoch 5 million years ago. The rocks forming the Puente Hills and portions of Cajon Pass are remnants of this final chapter in the story of San Bernardino County’s ancient ocean. The coastal sea at the time was alive with dolphins, whales, seals, sea lions, sharks, sea turtles, fish, and a wide variety of invertebrates such as bivalves, marine snails, etc. While similar animals still live off the California coast today, their ancient equivalents were not the same as their modern counterparts. For example, the largest sharks, like †Otodus megalodon, were much bigger than modern equivalents while many of the whales were smaller.

What’s notable, however, is the distinct differences in extinct animals present north and south of the San Andreas Fault. This is because since the fault formed 30 million years ago, it has been causing the land north of the fault to slowly move southeast while the land south of the fault moves to the northwest. The SBCM’s specimen of the cajon dolphin (†Allodelphis woodburnei) is an excellent showcase of this, having lived and died in colder waters near San Luis Obispo 200 miles northwest of where its remains were found in Cajon Pass. This displacement becomes less pronounced the younger the rock unit, since more recent deposits have had less time to move along the fault.

One interesting aspect of the Miocene marine deposits in the Puente Hills is that they include claystone deposited in deep and calm waters, unlike the shallower environments more often preserved elsewhere. What’s more, these units contain the remains of fish that lived in the mesopelagic “twilight” zone of the ocean. Bioluminescent lanternfish and long viperfish with gaping jaws are among the deep-sea creatures whose remains are found in the Puente Hills. The calm waters these fish were laid to rest in contribute to excellent preservation, with many of the fish skeletons being complete or nearly so. Terrestrial plant leaves are also preserved in addition to these denizens of the deep, swept out to sea before sinking to the bottom. Their presence shows that the environment, while deep, was also still relatively close to shore.

The youngest Miocene rocks preserved in the Puente Hills have a different fossil assemblage, consisting of fishes, sharks, clams, and snails hailing from shallower water. Local valleys cut into the rock are sometimes filled with 1.6 million- to 11,700-year-old Pleistocene sediments containing terrestrial species such as mammoths, indicating the end of the ocean in San Bernardino County. Rocks in Los Angeles County trace the progression of the coastline westward until it arrives at its present-day position.