“A million years is a short time — the shortest worth messing with for most problems. You begin tuning your mind to a time scale that is the planet's time scale. For me, it is almost unconscious now and is a kind of companionship with the earth.” ― John McPhee

Understanding deep history, how geology and climate have changed over time, can be key to understanding patterns of biodiversity across the southern California deserts and sky islands. In the Paleozoic Era, 570-350 million years ago the western edge of North America was covered in a shallow sea. There were no polar ice caps and so the oceans were much higher. This was the time when trilobites ruled the seas. Over those millions of years sediments and the bodies of those early creatures rained down on the floor of shallow seas, creating deep layers of carbon and calcium-rich sediments, sediments that were compressed and formed into limestone and dolomite rock masses

Midway through this time, from the late Paleozoic through to the late Mesozoic Eras, 400-65 million years ago, the single supercontinent Pangea began to break apart ultimately forming the continents we recognize today. That rending of a single supercontinent into separate land masses occurred due to the creation of mid-ocean ridges, spreading zones where “new ground” emerges from below the Earth’s mantle and pushes older ground away from spreading ridges as slowly moving plates. Where those moving plates encounter more stable land masses they can be pushed below the stable land (subduct). Those subduction zones are areas of high volcanic activity. The continental crust is thicker but lighter than the oceanic crust, so as the oceanic crust bumps into the continental crust it subducts, sinking to depths of 50 to 100 miles where it is so hot that the oceanic crust releases fluids trapped inside. The fluids melt the surrounding rock. The molten rock then migrates upwards erupting at the surface forming volcanoes.

What has been called the “ring of fire," the volcanoes that form the outer land margins of the Pacific Ocean in both western North America and eastern Asia, are testimony to this geologic process. Along the west coast of North America, the still-active volcanoes that form the Cascade Range from Mount Shasta through Oregon, Washington, and British Columbia are part of that ring of fire. The oceanic plate that subducted below our west coast is called the Farallon Plate. Around the late Mesozoic, about 60-65 million years ago, the Farallon Plate had completely subducted in the region extending from what is now Baja California up to Northern California. Without any subduction the California portion of the ring of fire ceased being active. The volcanoes stopped erupting and began the process of eroding away.

The molten rock still trapped below those senescent volcanoes slowly cooled and solidified forming granite. Continental uplifting increased the erosion of what were left of the volcanoes, exposing their granite cores, an ongoing process that has been occurring throughout the Cenozoic Era, from 65 million years ago to the present. Those granite cores form the Peninsular, Transverse, and Sierra Nevada Mountain ranges, the ranges that produce the rain shadow that creates our deserts. As those granite cores were exposed and uplifted, some chunks of that Paleozoic Ocean floor came along for the ride. There are large exposures of limestone on the desert side of the Transverse Range, readily observable driving on Hwy 18, near Lucerne Valley, rising out of the Mojave Desert on the way to Big Bear.  Smaller pockets of limestone and dolomite can be found on the desert sides of the Peninsular Range; all are remnants of that ancient ocean. Limestone and dolomite are deficient in certain minerals - iron, potassium, phosphorus, and manganese - that are important for plant growth. Whereas forests, shrublands and desert vegetation clothe the eastern slopes of the Transverse Range, few plants could grow on the nutrient-poor pockets of limestone. That left an open space, an open niche, where those plants that could overcome the mineral deficiencies of carbonate soils could evolve and thrive. There are a handful of plants endemic to those limestone soils and found nowhere else on our planet – adding to the rich biodiversity of our desert region. Those plants include: Cushenbury buckwheat, Parish’s fleabane, Cushenbury milkvetch, and Cushenbury bladderpod.

Shifting to a more recent time, the Pleistocene, the oscillating ice ages over the past two million years also had a strong influence on creating the high biodiversity of our desert region. As those mile-thick expanses of ice repeatedly covered and uncovered the northern half of North America, species that could move fast enough to escape the slow but relentless advances of ice found refuge in the southern California deserts. When the ice and cold retreated, those northern adapted montane and alpine plant species either followed their preferred climate back north or remained in the granite blocks of the Peninsular and Transverse Mountain Ranges where many of them, isolated from others of their kind, evolved into species unique to our region.

During the Pleistocene glacial maxima, glaciers reached down through the Sierra Nevada peaks, finding their most southerly footing on Mount San Gorgonio at the desert end of the Transverse Range. During the cooler and wetter climate that characterized those glacial maxima, a large lake formed, filling what is now the mostly dry Baldwin Lake and much of the Big Bear Basin (the current Big Bear Lake is a human-made reservoir). As lakes do, the lake accumulated fine clay eroded from the surrounding landscape, clays that coated the lake bottom. Not the calcium-carbon rich sediments of the Paleozoic Era, but eroded clay and rocks from the surrounding iron-rich granitic mountain slopes. With on-going uplift of the mountains along with the end of the Pleistocene glaciation, the lake evaporated, exposing heavy clay soils mixed with fist-sized and smaller quartzite rocks. Frost-heaving ice crystals during the winter has pushed many of the rocks to the surface creating a near-continuous rock pavement. Unlike the coarse gravely soils on surrounding the hillsides that were not created beneath a lake, these heavy clay soils dry out during the summer, so much so that they cannot support perennial trees and shrubs. That leaves open ground, red-stained by the high iron content, that like the limestone soils, creates an open niche. Today that treeless open ground is called a pebble plain, and like the limestone soils, there are plants endemic to the pebble plains that are found nowhere else. They include the Kennedy’s buckwheat, Bear Valley sandwort, and ash grey paintbrush.

Identifying patterns of biodiversity is one thing. Explaining those patterns is something more.

Nullius in verba

Go outside, tip your hat to a chuckwalla (and a cactus), think like a mountain, and be safe.