The Pleistocene epoch, often referred to as the Ice Age, spanned approximately 2.6 million to 11,700 years ago. This geological timeframe witnessed significant global changes, including dramatic shifts in climate and sea levels. California during this era was profoundly different from the present, shaped by natural forces and inhabited by creatures that no longer roam its landscapes.
California’s Ice Age Landscape
During the Pleistocene, California experienced a cooler and wetter climate than it does today. Glaciers, though not covering the entire state, impacted high-elevation areas like the Sierra Nevada, where ice fields blanketed the range down to elevations of 6,000 to 8,000 feet. Valley glaciers extended even lower, reaching as far as 1,900 feet in some western slope canyons.
Increased precipitation led to a rise in freshwater lakes across the state. Large basins in the Great Basin and southeastern California, including ancient versions of Mono and Owens lakes, were filled, and river levels throughout California were higher. The coastline also shifted, with sea levels dropping, exposing land now submerged, such as the area that forms San Francisco Bay.
The vegetation of Pleistocene California adapted to this climate. Pine forests dominated slopes in Southern California, interspersed with mixed evergreen forests, oak woodlands, and chaparral. This created a unique blend of plant species, with northern and southern ranges overlapping.
Giants That Roamed California
Pleistocene California was home to a variety of megafauna. Among the most iconic were Columbian mammoths, herbivores weighing six to eight tons and standing up to 16 feet tall, characterized by their large, curving tusks.
Predators like the Saber-toothed Cat (Smilodon fatalis) had distinctive, blade-like canine teeth up to 7 inches long. These powerful carnivores likely used ambush tactics to hunt their prey. Dire wolves (Canis dirus) hunted in packs, coexisting with other large carnivores such as American lions and short-faced bears.
Other large herbivores included giant ground sloths, such as the Shasta Ground Sloth. Ancient camels and horses also roamed California’s plains.
First Human Footprints
The arrival of the first humans in California during the Pleistocene is debated. While archaeological evidence points to human presence in North America around 15,000 years ago, some findings suggest an earlier arrival.
One intriguing discovery is the Cerutti Mastodon site near San Diego. Researchers found mastodon bones with deliberate breakage, along with hammerstones and stone anvils. Dating of these bones suggests a burial date of approximately 130,700 years ago.
This evidence would push back the timeline for human presence in the Americas. Although no human remains were found, the bone breakage patterns suggest an early human species processed mastodon bones for marrow or tools. This finding underscores the evolving understanding of early human migration routes.
Windows into the Past: Fossil Sites
California’s paleontological record offers glimpses into its Pleistocene past. The La Brea Tar Pits in Los Angeles is one of the most renowned and productive Ice Age fossil sites globally. Here, asphalt has seeped to the surface for at least 50,000 years, forming sticky pools that trapped and preserved millions of fossils.
The La Brea Tar Pits contain over 660 species, including large vertebrates like saber-toothed cats and dire wolves, as well as microfossils of plants, insects, and mollusks. This allows scientists to reconstruct the environment, revealing that the Los Angeles area was cooler and moister between 40,000 and 8,000 years ago. Ongoing excavations continue to uncover new specimens.
Beyond La Brea, other fossil discoveries include sites like Diamond Valley Lake, which has yielded remains of Imperial and Columbian mammoths. Ancient lakebeds and cave deposits across the state also reveal Pleistocene life. These sites provide data for studying the evolution, ecology, and extinction of California’s Ice Age fauna and flora.
The Great Extinction Event
The end of the Pleistocene epoch, around 11,700 years ago, saw a mass extinction event where most of California’s megafauna disappeared. This included saber-toothed cats, dire wolves, American lions, ground sloths, extinct horses, bison, and camels. The causes of this widespread extinction are complex and involve a combination of factors.
Theories suggest both climate change and human impact played roles. As the climate warmed and dried, the landscape became more susceptible to fires. Research indicates an increase in wildfires around 13,500 years ago, peaking between 13,200 and 12,900 years ago.
These large-scale fires, combined with habitat loss due to a warming climate and human hunting, pressured megafauna populations. While human populations were likely low, fire provided a means for them to have a broad impact on the landscape. The interplay of these factors led to a transformation of California’s ecosystems, as the large Ice Age animals disappeared.