Pacific Crest Trail: Northern California. Jeffrey P. Schaffer

      Granitic rock walls above Smedberg Lake, Yosemite National Park, Section I

      Limestone, another type of sedimentary rock, is formed in some marine environments as a chemical precipitate of dissolved calcium carbonate or as cemented fragments of shells, corals and foraminifers. The individual grains are usually microscopic. If the calcium in limestone is partly replaced by magnesium, the result is dolomite.

      Since the PCT attempts to follow a crest, you’ll usually find yourself in an area being eroded, rather than in a basin of deposition, so you’ll find very ephemeral sediments or very old ones. The young ones may be in the form of alluvium, talus slopes, glacial moraines, or lake sediments. The old ones are usually resistant sediments that the intruding granitic plutons bent (folded), broke (faulted) and changed (metamorphosed).

      Metamorphic rocks

      A volcanic or a sedimentary rock can undergo enough alteration (metamorphism) due to heat, pressure, and superhot, corrosive fluids that it loses its original characteristics and becomes a metavolcanic or a metasedimentary rock. Metamorphism may be slight or it may be complete. A shale undergoing progressive metamorphism becomes first a slate, second a phyllite, then a schist, and finally a gneiss. The slate resembles the shale but is noticeably harder. The schist bears little resemblance and is well-foliated, with flaky minerals such as biotite or other micas clearly visible. The gneiss resembles granite, but has alternating layers of light and dark minerals.

      Hornfels is a hard, massive rock, common in parts of the High Sierra, formed by contact of an ascending pluton with the overlying sediments. It can take on a variety of forms. You might find one that looks and feels like a slate, but differs in that it breaks across the sediment layers rather than between them.

      Quartzite is a metamorphosed sandstone and resembles the parent rock. The spaces between the grains have become filled with silica, so that now if the rock is broken, the fracture passes through the quartz grains rather than between them as in sandstone. Metamorphism of limestone or dolomite yields marble, which is just a crystalline form of the parent rock. Check out Marble Mountain, in northern California, when you reach it.

      Geologic Time

      You cannot develop a feeling for geology unless you appreciate the great span of time that geologic processes have had to operate over. A few million years’ duration is little more than an instant on the vast geologic time scale (see following Geologic Time Scale). Within this duration a volcano may be born, die and erode away. Dozens of major “ice ages” may come and go.

      A mountain range takes longer to form. Granitic plutons of the Sierra Nevada first came into being about 240 million years ago, and intrusion of them continued until about 80 million years ago, a span of 160 million years. Usually there is a considerable gap in the geologic record between the granitic rocks and the older sediments and volcanics that they intrude and metamorphose—often more than 100 million years.

      Geologic History

      With the aid of a geologic section, like the one above, we can reconstruct in part the geologic history of an area. Our geologic section represents an idealized slice across the Sierra Nevada to reveal the rocks and their relations.

      Through dating methods that use radioactive materials, geologists can obtain the absolute ages of the two granitic plutons, the andesite flow, and the basalt flow, which respectively would likely be Cretaceous, Pliocene, and Holocene. The overlying, folded sediments intruded by the plutons would have to be pre-Cretaceous. The metabasalt could be dated, but the age arrived at may be for the time of its metamorphism rather than for its formation. A paleontologist examining fossils from the marble and slate might conclude that these rocks are from the Paleozoic era.

      Before metamorphism the Paleozoic slate, quartzite, metabasalt, and marble would have been shale, sandstone, basalt, and limestone respectively. The shale–sandstone sequence might indicate marine sediments being deposited on a continental shelf, then on a coastal plain. Lack of transitional rocks between the shale and the sandstone leads us to conclude that they were eroded away, creating a gap in the geologic record. We then have an unconformity between the two strata (layers), the upper resting on the erosional surface of the lower. The basalt, shale, and limestone sequence indicates first a localized volcanism, followed by a marine and then a shallow-water environment.

      These Paleozoic rocks remained buried and protected from erosion for millions of years until the intrusion of granitic plutons and associated regional volcanism. Radiometric dating would show that the quartz-monzonite pluton was emplaced before the granodiorite pluton. Field observations would verify this sequence because the latter intrudes the former as well as the overlying sediments. During the Mesozoic period, plutonism and volcanism were at times accompanied by mountain building. This occurred when large pieces of continental crust, which were riding atop a plate that generally was diving eastward beneath the edge of the continent, were transported toward the range. Being relatively low in density, this continental crust did not descend with the rest of the plate, and so was forced against the range. The resulting compression caused uplift, and the Paleozoic rocks became folded, metamorphosed, and often faulted. Until plutonism ceased about 80 million years ago, the Mesozoic Sierra Nevada was just a small part of a much longer range that extended continuously along the western coasts of North America and South America. The climate was mostly tropical, and both weathering and erosion were intense; so as uplift occurred, these processes removed much of the Paleozoic rocks.

      Geologic Time Scale

      Light Marble Mountain and dark Black Mountain, Section Q

      After plutonism ceased in California, late Cretaceous through early Tertiary faulting broke up the longer range and the Sierra Nevada became separated from the Klamath Mountains on the north, and the Coast, Transverse, and Peninsular ranges on the south. (This, and much that follows, cannot be deduced from the geologic section.) Before the breakup, the longer range was high, similar to today’s Andes, but with the faulting into smaller blocks there also was detachment faulting—the separation of upper crust from lower crust. This occurred when the lower continental crust, under tremendous pressure from the thick, overlying upper crust and from high heat flow below, started to flow laterally. The upper continental crust lacked sufficient heat and pressure to flow. Rather, this brittle layer detached at its base and was transported laterally, atop the flowing lower continental crust. Where the upper several miles of Sierran proper crust went is not yet known. In the southern Sierra, most of the upper crust was transported westward. Then, when the San Andreas fault system developed, it was transported northwest, slivering into linear blocks in the process.

      With the upper crust removed—more than 65 million years ago for most of the Sierra—the unburdened lower crust rose to heights that probably were a bit higher than today’s. In the ensuing millions of years, broad summits such as Mt. Whitney’s have been reduced through weathering and erosion by only a few hundred feet, if that. Back in those early days following detachment, the range already had achieved a largely granitic landscape, since most of the exposed lower crust was granitic. Because stepped topography

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