Thirty million years ago was an important time. Roughly about then the climate began changing from one that was somewhat tropical to one that was drier and more seasonal. In the northern half of the Sierra Nevada the range was in part buried by extensive rhyolite-ash deposits. Furthermore, the San Andreas fault system was born, west of the modern coast of southern California. By 15 million years ago, California had acquired an essentially modern summer-dry climate; the northern half of the Sierra Nevada was buried under even larger amounts of andesitic deposits (burying the old, granitic river canyons); and the fault system was beginning to migrate eastward onto existing lands, thereby disrupting them. As today, lands west of any fault segment moved northward with respect to those on the east (right-lateral faulting). Also by 15 million years ago, the composite Sierra Nevada–Central Valley block had begun migrating from its location near the southwestern Nevada border, first west, then northwest, some 150–180 miles to its present location. Today on a very clear day, from Mt. Whitney’s summit you can see granitic Junipero Serra Peak, highest summit of central California’s outer coast ranges, about 170 miles west. Likewise, back then from the same summit, on a very clear day you could have seen the Grand Canyon plateau (no canyon yet), a similar distance east.
Most of the volcanic deposits in the northern Sierra Nevada were readily eroded, but the new canyons cut in such deposits were inundated by additional sediments. About 10–9 million years ago several massive outpourings of lava flowed westward from faults near the present Sierran crest. These faults were created by extension of the Great Basin lands, which before widespread down-faulting had been a rugged, mountainous highland. The floor of the Owens Valley sank, but the already high Sierra Nevada did not rise; the opposite-direction arrows along the fault in the idealized geologic section indicate only relative movement, not absolute up or down. Note that the fault cuts the bedrock but not the lateral moraine (an accumulation of debris dropped off the side of a glacier), and this indicates that no faulting has occurred since the moraine was deposited (or else it too would have been disrupted).
Significant parts of these lava flows still remain, and the remnants best preserved are those that lie directly atop old bedrock, as does the remnant of an andesite flow in the idealized geologic section. Such remnants stand high above the floor of today’s granite-walled canyons, which had been mostly exhumed of volcanic deposits before glaciation commenced. From this relation, geologists have concluded—incorrectly in my opinion—that major postflow uplift raised the flows to their present high positions, and that the steepened rivers then cut through thousands of feet of granite to their present low positions. According to this view, glaciers aided in the excavation, but misinterpretation of the field evidence has led geologists to infer major glacial erosion in some canyons, such as Yosemite Valley, and very little in others, such as the Grand Canyon of the Tuolumne River—two adjacent drainages both in Yosemite National Park.
The Sierra Nevada first experienced major glaciation about two million years ago, although it could have had episodes of minor glaciation long before that. These first large glaciers eroded the layer of rough, fractured, weathered bedrock, then retreated to leave behind much smoother surfaces. Where the bedrock floor was highly fractured and/or deeply weathered (in Yosemite Valley, the most extreme example, tropical weathering had penetrated some 2000 feet down), glaciers could excavate quite effectively, leaving behind bedrock basins that quickly filled with water each time the glaciers retreated, creating a bedrock lake, or tarn. (In some canyons a lake formed behind a terminal moraine, although such a lake exists not so much because of a moraine dam, but rather because of impervious bedrock that is buried by the moraine.) On the resistant, smoothed and polished bedrock, succeeding glaciers could do very little, despite a century of claims by glaciologists.
Some evidence for a lack of major glacial erosion lies along or close to the PCT, much of it in the southern half of the PCT. This is described in greater detail in Pacific Crest Trail: Southern California.
In the Sonora Pass to Echo Lake Resort section of the northern PCT, a descent from the Wolf Creek Lake saddle north takes you into the deep, glaciated East Fork Carson River canyon. After about two trail miles from there, and before you cross the river’s second tributary, there are remnants of volcanic deposits on the west slopes that descend to within 200 feet of the canyon floor. These remnants are dated at about 20 million years old, indicating that back then—before any supposed uplift and before any glaciation—the canyon was about as wide as it is today and almost as deep.
Mount Shasta, a volcano, rises beyond Bull Lake and Mt. Eddy, Section P
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