11.7: Quaternary Events

Recent Times

The Quaternary refers to rocks 2.58 Ma to today. Almost all of the Quaternary consists of the Pleistocene Epoch, with the exception of the last 11.7 ka, which is termed the Holocene Epoch. The Holocene can be understood in human terms as the time after the last Ice Age, corresponding to when agriculture, cities, laws, warfare, and reality TV shows developed. A proposed new division, the “Anthropocene,” would demarcate human influence on the globe; however, there is to date no official agreement about how to delineate the Anthropocene from the Holocene.

The last part of our story of the development of the Coast Ranges involves something very cool and something not quite so good.

Formation of San Francisco Bay

At the beginning of this chapter, we talked about dividing the northern and southern sections of the Coast Ranges by San Francisco Bay, which is one of the few deep-water ports in the western US, and became the launching point for most of the gold prospectors who flooded into California after the gold discovery in 1848.

Today on weekdays hundreds of thousands of commuters cross numerous bridges over the waters of San Francisco Bay, perhaps looking wistfully across its undulating surface as they sit frozen in universal gridlocked traffic. The vast bay looks as if it’s always been there. The reality is that San Francisco Bay only recently assumed its current shape, and the earliest peoples in the region saw a landscape completely unrecognizable today.

During the last Ice Age, vast quantities of evaporated water stayed stuck on land in the form of glacial ice. Tremendous ice sheets covered most of what is today Canada, and in a few places crept south as far as Olympia, Washington and New York City, which is why Central Park has such distinct glaciated features (but that’s a story for another book). Because so much water was locked up in glacial ice but evaporation continued, sea level dropped precipitously during the last Ice Age: over 100 meters lower than its present level.

About eighteen thousand years ago, the climate began to come out of the Ice Age and all this land-based ice began to melt, surging back into the oceans steadily through streams and rivers, and on occasion, as in the Great Missoula Floods, bursting downhill towards the oceans in titanic surges of water. By about eight thousand years ago, sea level was close to where it is today (or at least, where it was a few decades ago).

What does this mean for San Francisco Bay? As astounding as it is to contemplate, for all except the last few thousand years, you could have walked across a broad, relatively flat valley festooned with redwood trees and ferns, from San Francisco to Berkeley to San Jose to Marin. There simply was no San Francisco Bay. At the lowest points, a significant river drained toward the ocean, and had you followed it, you could have walked several days on landscapes today underwater until you reached the Farallon Islands (Figure \(\PageIndex{1}\)), now far out to sea and barely visible on a clear day.

Figure \(\PageIndex{1}\): Satellite image of San Francisco Bay Area showing major faults (red lines), geographic features, and shoreline positions after the last glacial maximum. From 18,000-11,500 years ago, you could have walked to the Farallon Islands. This work by the USGS is in the public domain.

When humans first came into North America and the San Francisco region, at least by fourteen thousand years ago (though there are credible recent claims for dates thousands of years earlier), they experienced a landscape that would today be utterly unrecognizable. They no doubt left vestiges of their lives–shell mounds, discarded worked stones, burial sites. We don’t find any of these today from fourteen thousand years ago; most of the archeological sites on land within the current San Francisco Bay date from a mere two thousand years ago. But there was a California we can hardly even imagine–bursting with all manner of life. And that’s where the tragedy comes in.

Video \(\PageIndex{1}\): Virtual field trip: Geologic history of the San Francisco Bay

The following video describes the formation of San Francisco Bay.

A Menagerie of Extinction

An old ad campaign for the dairy company Berkeley Farms played on the seeming contradiction of farming within the densely urbanized environment near UC Berkeley by asking, “Cows in Berkeley? Mooo!” For those peoples who first explored this ancient California–from the damp forests of the north to the salty, wave-kissed coasts to the parched deserts of the south–the question might have been, “Saber-toothed cats in Berkeley? Run for your lives!”

Smilodon fatalis, the saber-toothed cat, and our official state fossil, existed in California at the same time as the earliest human migrants, forming a rather disconcerting welcoming committee. (It should be noted that Smilodon was not a tiger, as they are sometimes mistakenly named, and not closely related to modern big cats.) Though there is no direct evidence of Smilodon predation on humans, and humans range on the smaller side of typical Smilodon prey, it is reasonable to think that these fierce carnivores greeted the arrival of groups of vulnerable two-legged meat as an agreeable variation on their usual fare.

Figure \(\PageIndex{1}\): The Saber-toothed cat (Smilodon fatalis) which lived in North America until 13,000 years ago. Smilodon fatalis derives from the Greek word "blade" "chisel" or "tooth." This work by Charles R. Knight is in the public domain.

Saber-toothed cats were not the only large predator encountered by early humans. The short-faced bear (Arctodus simus) would have been a uniquely terrifying animal to encounter, standing at over 3 meters and weighing in over 600 kg. An averaged sized human would have looked directly at their eye level as the bears approached.

Many other species flourished in Pleistocene California: dire wolves, camels, bison, horses, sloths. Fossils of all these were excavated and are currently displayed at the La Brea Tar Pits museum in Los Angeles.

Perhaps most magnificently of all, the Columbian mammoth (Mammuthus columbi) strolled the plains of the Golden State. These massive proboscideans clocked in at over 4 meters at the shoulder, far larger than modern elephants, and they exhibited elaborately curved tusks much longer than any living relative. While many people have heard of Wooly mammoths, we didn’t have them here, as they existed at much higher latitudes than California. In our state, Columbian mammoths roamed in great herds, perhaps led in a matriarchal group, as today’s elephants exist. They used to be quite common; a Columbian mammoth fossil has even been discovered on the campus grounds of a California community college, Las Positas College in Livermore.

Figure \(\PageIndex{1}\): Illustration of the Columbian mammoth (Mammuthus columbi) which roamed California during the Pleistocene. This work by the NPS is in the public domain.

So why don’t we have elephants and camels and bison and giant elephant-like animals in California today? When one examines the end dates for most of the animals described above, the number 12,000 years ago keeps coming up. That seemed to be an abrupt break in the type of species living in California.

This decline can certainly be attributed to climatic changes. If conditions change an animal’s food source, they tend to go extinct. But because of the overlapping timing of human settlement in this continent, there is another factor possible here: we killed them. Elsewhere in the United States, there exists clear evidence of human butchering of mammoths. It is not unreasonable to think that clever, motivated humans found ways to hunt the animals listed above. Would that have been enough to drive all these California species extinct in such a short time? Perhaps we should ask the passenger pigeon (Ectopistes migratorius), which used to darken American skies in the billions in the nineteenth century, now remembered only by a few stuffed museum specimens.

A Note

At this point, we must acknowledge that the sections above have been brief and by necessity have left out a great deal of detail. Especially missing from the above sections is a full sense of the scope of unsettled issues and controversial questions. Much is unknown, or poorly known, and this provides fertile ground for geologic researchers to investigate further, and to disagree with each other using evolving evidence. Certainly a great deal of what is conjectured above will, in time, be shown to not be aligned with new evidence and will be revised.

Introductory science students might see such large gaps in knowledge as a sign of scientific weakness, but such a view profoundly misunderstands what science is. Science is not a list of facts to be memorized. Rather, science is better understood as a series of critical questions about issues not yet resolved. To recall Plutarch, a mind should be seen not as a vessel to be filled, but as a fire to be kindled.

In science nothing is ever proven. Rather, incorrect ideas can be shown to be incompatible with evidence; in other words, science is a systematic process to falsify what does not fit existing data. With new and evolving instruments and techniques, data can change, and therefore so can the conclusions. Having significant parts of the story of California geology unknown or not yet resolved is a sign of strength, not a failing. If we already knew all the answers, it would be boring.