Pacific Crest Trail: Northern California. Jeffrey P. Schaffer

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      Cinder Cone lying 3.5 miles northeast of Lower Twin Lake, Section N

      Returning to the idealized geologic section, we see both a lateral and a terminal moraine on the east side of the crest, these usually being massive deposits left by a former glacier. (However, some lateral moraines are thin, merely a veneer atop an underlying bedrock ridge.) If glaciers do not erode, then why are moraines so large? Rockfall is the answer. It can occur at any time, but it is especially prevalent in late winter and early spring (due to cycles of freeze and thaw that pry off slabs and blocks). During and after a major earthquake, a tremendous amount of rockfall occurs, as noted in the 1980 Mammoth Lakes earthquake swarm, which was centered near the town along the east base of the range. Rockfall was greatest along and east of the crest, and so perhaps it is good that the PCT lies a few miles west of it. The greatest amount of local rockfall along the PCT route was from the ragged southeast face of Peak 11787, north of Purple Lake in the southern California section (Map H16). What glaciers do best is haul out a lot of rockfall, from which moraines are constructed and with which rivers are choked. Over the last two million years there were 2–4 dozen cycles of major glacier growth and retreat, and the glaciers transported a lot of rockfall. At the head of each canyon, where physical weathering was extremely pronounced, there usually developed a steep-walled half-bowl called a cirque. Before glaciation these already existed in a less dramatic form, as can be seen in the unglaciated lands west of Rockhouse Basin (Section G).

      In the idealized geologic section, the last significant change was the eruption of lava to produce a cinder cone, which partly overlapped the terminal moraine, thereby indicating that it is younger. A basalt flow emanated from the cinder cone during or immediately after its formation. A carbon-14 date on wood buried by the flow would verify the youthfulness of the flow. Weathering and erosion are oh-so-slowly attacking the range today, at a rate much slower than in its tropical past, but nevertheless they are seeking to reduce the landscape to sea level. This will not occur. Future PCT hikers in the distant geologic future can expect a higher range, for eventually the Coast Ranges of central California should be thrust across the Great Central Valley and onto the Sierra Nevada, the crust-crust compression generating a new round of mountain building.

      For now, PCT hikers can study the existing landscape. When you encounter a contact between two rocks along the trail, you might ask yourself: Which rock is younger? Which older? Has faulting, folding, or metamorphism occurred? Is there a gap in the geologic record?

Biology

      One’s first guess about hiking the Pacific Crest Trail—a high adventure rich in magnificent alpine scenery and sweeping panoramas—turns out to be incorrect along some parts of the trail. The real-life trail hike will sometimes seem to consist of enduring many repetitious miles of hot, dusty tread, battling hordes of mosquitoes, or slogging up seemingly endless switchbacks. If you find yourself bogged down in such unpleasant impressions, it may be because you haven’t developed an appreciation of the natural history of this remarkable route. As there is a great variety of minerals, rocks, landscapes and climates along the PCT, so also is there a great variety of plants and animals.

      Even if you don’t know much about basic ecology, you can’t help noticing that the natural scene along the Pacific Crest Trail changes with elevation. The most obvious changes are in the trees, just because trees are the most obvious—the largest—organisms. Furthermore, they don’t move around, hike, or migrate in their lifetimes, as do animals. When you pay close attention, you notice that not only the trees but the shrubs and wildflowers also change with elevation. Then you begin to find latitudinal differences in the animal populations. In other words, there are different life zones.

      Life zones

      In 1894 C. Hart Merriam divided North America into seven broad ecosystems, which he called “life zones.” These zones were originally based primarily on temperature, though today they are based on the distribution of plants and animals. The zones correspond roughly with latitude, from the Tropical Zone, which stretches from Florida across Mexico, to the Arctic Zone, which includes the polar regions. Between these two are found, south to north, the Lower Sonoran, Upper Sonoran, Transition, Canadian and Hudsonian zones. All but the Tropical Zone are encountered along the California sections of the PCT.

      Just as temperature decreases as you move toward the earth’s poles, so too does it decrease as you climb upward—between 3° and 5.5°F for every 1000-foot elevation gain. Thus, if you were to climb from broad San Gorgonio Pass for 10,000 feet up to the summit of San Gorgonio Mountain, you would pass through all the same zones that you would if you walked from southern California north all the way to Alaska. It turns out that 1000 feet of elevation are about equivalent to 170 miles of latitude. Although the California PCT is about 1600 miles long, the net northward gain in latitude is only about 650 miles—you have to hike 2.5 route-miles to get one mile north. This 650-mile change in latitude should bring about the same temperature change as climbing 3800 feet up a mountain. On the PCT you enter Oregon at a 6000-foot elevation, finding yourself in a dense, Canadian Zone pine-and-fir forest. Doing your arithmetic, you would expect to find an equally dense fir forest at the Mexican border 3800 feet higher—at a 9800-foot elevation. Unfortunately, no such elevation exists along the border to test this prediction. However, if we head 85 miles north from the border to the Mt. San Jacinto environs, and subtract 500 feet in elevation to compensate for this new latitude, what do we find at the 9300-foot elevation? You guessed it, a Canadian Zone pine-and-fir forest. Ah, but nature is not quite that simple, for the two forests are unmistakably different.