[NOTE: The following originally appeared at the lead review of the SAN DIEGO UNION BOOK REVIEW in 2006.]
Covering much of San Diego’s east county and parts of Imperial and Riverside counties from the mighty Santa Rosa Mountains to the Mexican border, the 650,000 acre Anza-Borrego Desert State Park (ABDSP) is today one of the most arid, inhospitable and geologically active regions in North America. Across and around Anza-Borrego’s vast cactus-studded desolation spreads a bizarre, moonscaped labyrinth born of tectonic processes so enormous that they seem to defy any human comprehension. Symptoms of this ongoing massive geological upheaval in the area include its highly varied typography, geothermal activity, hot springs, and numerous large scale earthquakes (over 20 quakes measuring 6.0 or higher on the Richter Scale have occurred in and around the region in the past 100 years alone!). Within the park boundaries, this tectonic activity has created major fault zones, multi-hued badlands, upthrusting mountains, subsiding basins, and on-shore spreading zones that unite in a geological performance played out few places on earth—and no place else in North America.
Other than the howls of coyotes and the occasional sound of tires moving across lonely highways, this landscape is today mostly silent. But listen more carefully: within Anza-Borrego’s badlands lie some 6000 meters of exposed, fossil-bearing sediments (by way of contrast, the Grand Canyon’s exposures are only 1/3 as great); these deposits, which have accumulated during the past 7 millions years or so, have a grand and utterly improbable story to tell about a period when Anza-Borrego was not a desert but a verdant environment of rivers and floodplains, lakes, forest, and savannah; for several million years this region supported a bestiary of exotic flora and fauna that reads like a “Who’s Who” of unique North American creatures, including the largest known mammoths, saber toothed cats, iguanas, jaguars, giant tortoises and short-faced bears, dwarf pronghorns, flamingos, ground sloths, zebras, camels, and the largest flying bird of the northern hemisphere (“Aiolornis incredibilis”), with a 16 foot wingspan.
This story—told in the secret language of leaf impressions, bone and shell fragments, teeth, and animal tracks—is magnificently recounted in George T. Jefferson and Lowell Lindsay’s landmark new study, “Fossil Treasures of the Anza Borrego Desert.” Unfolding as a series of 20 chapters written by paleontologists and geologists from across the U.S., and supplemented by several hundred color photographs, exquisite illustrations, and 2-page paleolandscapes that help bring the past to life throughout, “Fossil Treasures” offers both specialists and ordinary readers an embarrassments of riches
Clustered near the beginning and end of the volume are several chapters that set the stage for the paleontological drama that follows by summarizing the region’s geology and climate, its history of fossil research, and providing background about the nature of fossil collecting, stratigraphic dating, intercontinental connections, and other topics that help explain how and why these fossils came to exist where they did.
The remaining chapters explore the fossilized remains of the creatures themselves that have been uncovered from over 250 localities in the Park. These fossils include more than 550 types of plants and animals—most of them extinct and some only known from remains recovered in the Park—ranging from preserved microscopic plant pollen and algal spores to mammoth and baleen whale bones.
These fossils are discussed in their natural order, beginning with the invertebrate fossils (corals, mollusks, crustaceans, and echinoderms) of the ancient Imperial Sea (the proto-Gulf of California, which 7 million years ago reached as far north as the San Gregorio Pass near Palm Springs), and continuing on through the lower vertebrates (fish, amphibians, and reptiles), birds, small and large carnivores (wolves, bears, saber toothed cats), and concluding with the region’s most famous fossils—those of the “megaforms” such as camels, horses, and mammoths.
The “back story” chapters make for some of the most interesting reading in the entire volume. Along the way readers are treated to dozens of subplots on such topics as how the Grand Canyon sediments wound up in Anza-Borrego, the evolution of horns and teeth, the role that camels played in the American southwest during the 19th century, the role that changes in the earth’s polarity and ash from volcanic super-explosions several hundred miles away have played in dating Borrego’s fossils, and continental connections that allowed two-way migrations of terrestrial animals across the Isthmus of Panama starting some 3.5 million years ago, as well as across the Bering Land Bridge during the Ice Ages.
During the Pliocene and Pleistocene Epochs of geologic time, when ground sloths, camels, mammoths, and other large, bizarre, and now-extinct animals roamed Anza-Borrego, the environment was very different from the desert setting we see today. Climatic and geologic conditions prevailing then sometimes permitted the remains of these animals to be deposited where they lay, relatively protected from the elements, under mud or sand, in places that allowed them to be partially transformed into fossils over the millennia. All the protein and other soluble organic materials contained in living bone decayed and dissolved away, leaving only its mineral matrix, a substance quite brittle all by itself. The fragile bits of vertebrae, skulls, teeth, limb bones, and jawbones found in Anza-Borrego are generally more delicate than the rock strata in which they are embedded; they usually have not been hardened, as have fossils from many other settings, by exposure to minerals dissolved in the groundwater.
Larry McCaffery’s most recent book is EXPELLED FROM EDEN: A WILLIAM T. VOLLMANN READER (Thunder’s Mouth Press). A Professor of English at SDSU, he currently lives in Anza Borrego’s magnificent desolation with his wife, Sinda Gregory.