areas along the Mediterranean Sea.
The inland zone, consisting of the Santa Ana Mountains and foothills, experiences somewhat more extreme daily and seasonal temperatures because it is less influenced by onshore flows of marine air. The higher summits of the Santa Ana Mountains, for example, have average temperatures in the 50s to 20s in winter, and the 80s to 50s in summer. Precipitation averages about 30 inches annually in the higher Santa Anas, which is just enough to support natural pockets of coniferous and broadleaf trees, such as pines, oaks, and maples. Almost every year, some fraction of the precipitation arrives in the form of snow, which briefly blankets the mountain slopes down to an elevation of about 3,000 feet.
Despite its reputation for a gentle climate, Orange County is occasionally subject to hot, dry winds called “Santa Ana winds” (after Santa Ana Canyon, just north of the Santa Ana Mountains). These winds occur when an air mass moves southwest from a high-pressure area over the interior United States out toward Southern California. As the air flows downward toward the coast, it compresses and becomes warm and dry. Low passes in the mountains or river valleys that act as wind gaps (such as Santa Ana Canyon) funnel these desertlike winds toward the coast. During stronger Santa Anas, common in late summer and fall, Orange County basks under warm, blue skies swept clear of every trace of pollution (except possibly smoke from wildfires). Temperatures along the coast can then reach record-high levels; the city of Orange, for example, once recorded a temperature of 119 degrees during a Santa Ana.
Rainfall in Orange County is as erratic as it is slight. On the coastal plain, the annual precipitation has ranged from merely 4 inches to a record of 32 inches. Up to 5 inches have fallen on the coastal plain in a single day, and in the Santa Ana Mountains, one storm dumped 9 inches in a single night.
By and large, Orange County’s balmy, dry climate is remarkably well suited to year-round outdoor activity. Nevertheless, high temperatures, scarcity of water, and occasionally smoggy air during summer and early fall make that particular period less desirable for hiking in the inland foothills and mountains. During the other seven or eight months of the year, the weather is often ideal.
Reading the Rocks
Of California’s many geomorphic (natural) provinces, Orange County claims parts of only two: the Los Angeles Basin and the Peninsular Ranges. The bulk of the county’s urbanized area lies in the Los Angeles Basin, while the mostly undeveloped Santa Ana Mountains and the semideveloped San Joaquin Hills belong to the Peninsular Ranges province.
The Los Angeles Basin province extends from the base of the San Gabriel and Santa Monica Mountains (part of the Transverse Ranges province) to the north to the base of the Santa Ana Mountains and the San Joaquin Hills on the south. In a geologic context, it can be pictured as a huge, deeply folded basin filled to a depth of up to 7 miles by some volcanic material and land-laid sediments, but mostly by sediments of marine origin—sand and mud deposited on the ocean bottom from 80 million years ago to as recently as 1 million years ago.
The Los Angeles Basin area has experienced uplift during the past 1–2 million years, and as this took place, the surface of the basin accumulated a layer of terrestrial sediment shed from the surrounding hills and mountains. The basin, in fact, would still be filling up with sediment were it not for the flood-control dams and channeled riverbeds that have largely replaced the original meandering Santa Ana, San Gabriel, and Los Angeles Rivers.
Cloud-walking, Laguna Bowl Road (see Chapter 4)
Hikers following this guidebook will discover many interesting and sometimes colorful exposures of marine sedimentary rocks in places like the Chino Hills, San Joaquin Hills, and foothills of the Santa Ana Mountains. These sediments, uplifted by a variety of geologic processes, are continuations of the formations that lie deep underground in the center of the Los Angeles Basin.
The marine sediments you will see—sandstone, siltstone, shale, and conglomerate—tend to be rather soft and easily eroded. Along the south coast, where the San Joaquin Hills meet the sea, several wave-cut “marine terraces” are identifiable on the coastal headlands. They exhibit a record of changing sea levels and gradual uplift over the past 1–2 million years. In many places, the terraces themselves are deeply cut by drainage channels—the coastal canyons—which themselves are quite recent features.
The Santa Ana Mountains, along with the San Jacinto Mountains, lie at the northwest tip of the extensive Peninsular Ranges province, stretching south toward the tip of Baja California (the province, in fact, derives its name from Baja’s peninsular shape). Each range in this province possesses a core of granitic (granitelike) rock, overlain in many places by a veneer of older metamorphic rocks. Many of the Peninsular Ranges, including the Santa Anas, are raised and tilted fault blocks, typically with steep east escarpments and more gradually inclined western slopes.
As you travel through the Santa Ana Mountains (and their distinctly named subdivisions, the Elsinore and the Santa Margarita Mountains), you’ll begin to piece together their geologic history. Starting at, say, Caspers Wilderness Park in the foothills and moving up toward the crest of the Santa Anas, you first pass among light-colored marine sedimentary rock formations that were pushed up and tilted by the rise of the mountains to the east. Next comes metamorphic rock of two basic kinds—metasedimentary and metavolcanic. These brown- or gray-colored rocks, roughly 200–150 million years old, are metamorphosed (changed by heat and pressure) forms of marine sedimentary and volcanic rock that were plastered against the core of the North American continent some tens of millions of years ago. These rocks were riding on one or more of the Earth’s tectonic plates, which collided with and were subducted (forced under) the western edge of the continent.
Near the crest of the Santa Ana Mountains, you find light-colored granitic rocks. Here’s the reason: About 100 million years ago, when the subduction process was in full swing, much of the material on the edge of the plate being subducted melted underground and accumulated in the form of vast pools of magma. Because this magma was less dense than the surrounding materials, it rose toward the surface. Some escaped through volcanoes, but most of it remained underground long enough to slowly cool and crystallize, forming coarse-grained “plutonic” (generally granitic) rocks. Erosion then nibbled away at the overlying metamorphic rocks, finally exposing—typically in high places—the granitic rocks.
In the southern Santa Anas and throughout most of California’s share of the Peninsular Ranges, this granitic rock is now well exposed. In the northern Santa Anas, however, its distribution is less extensive. The highest peaks in the range, Santiago and Modjeska Peaks (together called Old Saddleback), are still covered by older, overlying metamorphic rocks.
The granitic rocks are still rising, more than offsetting the leveling effects of erosion. Thus, although the rocks of the Santa Ana Mountains range between old and ancient, the origin of the range itself as a structural unit is quite recent.
Of more than casual interest to Orange Countians is the fact that for at least the past 10 million years, the Peninsular Ranges province (along with the present Los Angeles Basin and a wedge of coastal central California) has been drifting northwest relative to the rest of the North American continent. In a global view, this motion is seen as a lateral sliding (or rather a repeated lurching) at the interface between the largely oceanic Pacific Plate and the largely continental North American Plate. The average rate of movement in the early 21st century is about 2 inches per year—enough, if it continues, to put Orange County abreast of San Francisco 12 million years from now.
The famous San Andreas Fault (which runs about 40 miles northeast of Orange County) is the principal division between the two plates. But earth movement can also take place along splinter faults south and west of the San Andreas. One such splinter fault, the Elsinore Fault,
