Afoot and Afield: Orange County. Jerry Schad
Читать онлайн книгу.
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, passes along the eastern base of the Santa Ana Mountains. Horizontal and vertical movements along this fault over the past 5 million years have shifted the Santa Ana Mountains northwest relative to the adjacent landforms to the east, and have raised and tilted the whole mountain block into its characteristic west-sloping orientation. Sudden earth movements along any of these faults have been and will again be responsible for most of Southern California’s devastating earthquakes.
The geologic history of Orange County is a fascinating one, and the diversity of landforms and rocks in Orange County and the Santa Ana Mountains is enough to pique the curiosity of most any amateur geologist. Refer to Appendix 2 for sources of additional information.
Native Gardens
About 800 different kinds of wild flowering plants are found within Orange County’s 782 square miles, a remarkably large number considering its diminutive size among California counties.
There are two reasons for this abundance of plant species. One reason has to do with physical factors: topography, geology, soils, and climate. Countywide, the diversity of physical factors and the complex interrelationships among these factors have led to the existence of many kinds of biological habitats.
The second reason is Orange County’s location between two groups of flora: a southern, drought-tolerant group, most clearly represented by various forms of cacti; and a northern group, represented by moisture-loving plants typical of California’s northern and central Coast Ranges. As the climate changed, varying from cool and wet to warm and dry over the past million years or so, species from one group and then the other invaded the county. Once established, many of these species persisted in protected niches even as the climate turned unfavorable for them. Some survived unchanged; others evolved into unique forms. Some are present only in very specific habitats.
The bulk of Orange County’s undeveloped land can be grouped into three general classes, which botanists often call plant communities or plant associations. In a broader sense, they are biological communities because they include animals as well as plants. These plant communities are briefly described below.
The sage scrub (or coastal sage scrub) community lies mostly below 2,000 feet in elevation and extends east from the coastline to the foothills and lower spurs of the Santa Ana Mountains. The dominant species are small shrubs, typically California sagebrush, black sage, white sage, and wild buckwheat. Two larger shrubs often found here are laurel sumac and lemonade berry, which like poison oak are members of the sumac family. Interspersed among the somewhat loosely distributed shrubs is a variety of grasses and wildflowers, green and colorful during the rainy season but dry and withered during the summer and early-fall drought.
The chaparral community is commonly found above 2,000 feet in the Santa Ana Mountains, where it cloaks the slopes like a thick-pile carpet. The dominant plants are chamise, scrub oak, manzanita, mountain mahogany, toyon, and various forms of ceanothus (“wild lilac”). These are tough, intricately branched shrubs with deep root systems that ensure their survival during the long, hot summers. Chaparral is sometimes referred to as an “elfin forest,” a literal description of a mature stand. Without the benefit of a trail, travel through mature chaparral, which is typically 10–15 feet high, is almost impossible. Sage scrub and chaparral vegetation tend to intermix readily throughout the Santa Anas, the chaparral preferring shadier, north-facing slopes, and the sage scrub preferring hot, dry, south-facing slopes.
The southern oak woodland community is found in scattered locations throughout the county, from the bigger coastal canyons in the San Joaquin Hills to moist flats and canyons throughout the Santa Ana Mountains. Within the Orange County area, the indicator tree is the live oak, but sycamores may also be abundant. In the Chino Hills, native walnut trees form a major component of the southern oak woodland community. Beneath the trees, various chaparral and sage scrub plants often form a sparse understory.
Aside from these three widespread natural communities, much of the nonurbanized land in and around Orange County is given over to agriculture and grazing. Areas characterized by heavy grazing have grassy flats and bald slopes called potreros (pastures) supporting mostly nonnative grasses and herbs, such as wild oats, filaree, mustard, and fennel.
To a small extent, several other natural communities are found in Orange County: rocky shore, coastal strand, coastal salt marsh, freshwater marsh, riparian woodland, and coniferous forest.
The riparian (streamside) woodland community, existing in discontinuous strips along some of the bigger watercourses, is perhaps the most biologically valuable. Not only is this kind of environment essential for the continued survival of many kinds of birds and animals,