Driving Through the Most Dangerous Plate Boundary in the World: In the Pleistocene, a Different Kind of Danger

An Egret and Tule Elk at the San Luis National Wildlife Refuge near Los Banos

The Great Valley began as a shallow sea (a forearc basin) between the Mesozoic subduction zone and the Ancestral Sierra Nevada volcanic arc. As noted in the previous post, the sea filled with thousands of feet of sediments, and as the subduction zone transitioned to a transform boundary, the sea gave way to land, and the one of the most fertile valleys in existence emerged. The Great Valley has become one of the most important agricultural regions on the planet. 95% of the original landscape has been altered to grow food and feed. If the land isn't covered by crops, it's covered by pavement and cities.

The Cosumnes River Preserve north of Stockton

As I said in the previous post, the agricultural development isn't necessarily a bad thing. With an increasing population of mouths to feed in the world, it would be silly not to utilize the richest soils on the planet. But we do live in a highly interconnected ecosystem, and we need to preserve what we can of the richness and diversity of life in our world. A few wetlands have been protected from development to allow the survival of some of the migratory birds that overwinter in our valley, like the Sandhill Crane, the Ross's Goose, the Snow Goose, and many others. Some of the rivers flowing through the valley are still allowed to reach the sea, preserving salmon and other aquatic species. There is a great deal of conflict about where to define the limits of water and land use, especially in this horrific drought year.

The Cosumnes River north of Stockton

This past year I spent as a new birder has been a revelation as I have sought out those small corners and edges of the valley that preserve something of the original ecological richness. I was stunned to find that flocks of tens of thousands of cranes and geese spend their winters just a few miles from my home near Modesto.

Snow Geese and Ross's Geese at the Merced National Wildlife Refuge

Tule Elk, among the largest members of the deer family (aside from the Moose), used to live in the valley by the millions as well. One subspecies was down to a single breeding pair in the late 1800s, but they were protected by a rancher, and several thousand now survive in some widely separated refuges. The wolf was driven to extinction, as was the terrifying California Grizzly Bear. Other species retreated into the Coast Ranges and Sierra Nevada foothills.

The Tule Elk herd at the San Luis National Wildlife Refuge

This blog series has mostly been about the "most dangerous plate boundary" in the world, referring to the geologic hazards inherent in living near an active subduction zone: earthquakes, tsunamis, and volcanic eruptions. But a different kind of danger lurked in the Great Valley when humans first arrived: the so-called "megafauna": an ecosystem of large mammals that mostly went extinct just 10,000 or so years ago.

Wooly Mammoth at the Paige Museum

One of the most imposing creatures would have been the mammoths. These elephant relatives were widespread across Asia, Europe and North America, and passed into oblivion only a few thousand years ago. They were gone from the continents by 10,000 years ago, but a small population (both in stature and in numbers) survived on Wrangell Island in eastern Russia until 4,300 years ago.

There were giant ground sloths (above), and camels. Lots of camels, so much so that they are one of the two most common animals found in the Fairmead paleontological site near the Great Valley town of Madera. The Fairmead excavations have been the most important source of fossil material on the valley floor (the fossil species are also well known from the La Brea Tar Pits in Los Angeles.

The other common fossil is the horse. Most people think horses came to America with the Spaniards in the 1500s, but they actually evolved in North America and spread to other parts of the world before going extinct in their ancestral home around 10,000 years ago. There were also species of deer and pronghorn, and the buffalo roamed the valley as well.

Though any of these creatures could have injured humans in self-defense, it was their predators that would have made life in the valley terrifying. There were huge Sabertooth Cats. There were Jaguars and American Lions (pretty much the same as African Lions).

The California Grizzly Bear was dangerous enough, but even larger bears lived in the valley as well. The Short-faced Bear was a good 50% larger than a Grizzly, and may have been the largest terrestrial mammal predator ever. They were five feet high at the shoulder, and stood 11-12 feet tall if they chose to. Terrifying indeed.

They were smaller than the cats or bears, but the Dire Wolves hunted in packs, and that made them perhaps even more dangerous than the others. They were 25% larger than today's wolves.

The Great Valley would have been a dangerous place for humans when they arrived 13,000 years ago or earlier. It's possible that the megafauna is extinct today because humans had tools for hunting and defense, but the connection is not yet clear. Climate change could have played a role too, or disease, or any other number of possible explanations. In any case, I feel a sense of loss when I wander through the few remaining pieces of original habitat imagining the creatures that used to live here. A sense of loss, but at least I am not so fearful of being maimed or killed.

Are you interested in seeing more of these creatures? If you are ever traveling in the Great Valley for any reason, make the time to stop at the Fossil Discovery Center in the Madera area. The very famous La Brea Tar Pits are the other great source of information on the extinct megafauna. The Paige Museum at the tar pits is an excellent place to visit when you are in Los Angeles.