I have tried to minimize the use of technical terms in this volume, but some jargon is inevitable in any discussion of technical matters. Most of the strictly geologic terms that are apt to be stumbling blocks are defined where they first appear in the text; for case of reference, some of them are summarized here in this short glossary. Geologic time terms are not included (see fig. 7).

Alluvial fan. A sloping, fan-shaped mass of loose rock material deposited by a stream where it emerges from a canyon onto a broad valley or plain.

Alluvium. A general term for clay, silt, sand, and gravel deposited by running water, such as a stream.

Andesite. A volcanic rock of intermediate composition, with a silica (SiO₂) content generally of from 50 to 60 percent.

Arête. A narrow, serrate mountain ridge (fig. 56).

Basalt. The most common type of volcanic rock, generally fine grained, dark, and heavy, with a silica (SiO₂) content of no more than about 50 percent.

Batholith. A very large body of plutonic rock. The Sierra Nevada batholith is a composite of numerous smaller bodies (plutons) that represent repeated intrusions of granitic magma.

Bedding. The arrangement of sedimentary rock in beds or layers, reflecting the fact that water or wind spread sediment they deposit in thin sheets. The beds of sedimentary rock are these successively accumulated sheets.

Bergschrund. A deep crevice near the head of an alpine glacier that separates moving ice from the headwall of the cirque. It may be covered by or filled with snow during the winter, but visible and reopened in the summer (fig. 56).

Breccia. A consolidated rock composed of angular rock fragments.

Cirque. A bowl-shaped, theaterlike basin at the head of a glacial valley (fig. 56).

Cleavage. The tendency of a mineral to break along definite planes controlled by its molecular structure and producing smooth surfaces.

Columnar jointing. Joints that bound parallel prismatic columns, polygonal in cross section, formed by contraction during cooling in some lava flows, dikes, and volcanic plugs (a plug consists of solidified lava in an old volcanic conduit) (fig. 32).

Conglomerate. A rock, the consolidated equivalent of gravel.

Crust (of the Earth). The outermost of the concentric shells that make up the Earth. The crust is 4 to 5 mi thick beneath oceans and 20 to 35 mi thick beneath continents (fig. 25).

Dike. A sheetlike body of igneous rock that was intruded while molten into cracks in older rocks (figs. 13, 17).

Diorite. A plutonic rock composed primarily of plagioclase and dark minerals; generally fine grained (figs. 9, 12).

Erratic (glacial). A rock fragment, generally large, that has been transported from a distant source by the action of glacial ice (fig. 61).

Exfoliation. Any process by which concentric scales, plates, or shells of rock are successively spalled or stripped from the surface of a rock mass. It produces such diverse results as the spalling off of glacial polish a fraction of an inch thick and the formation of sheet joints many feet thick (figs. 39, 55).

Fault. A fracture in the Earth’s crust along which there has been movement parallel to the fracture plane.

Gradient. As applied to streams, the inclination of the bed.

Granitic rocks. Includes granite (in the technical sense), granodiorite, and tonalite (see fig. 9 for classification of plutonic rocks).

Grus. The fragmental products of granular disintegration of granitic rocks in place; granitic sand (fig. 45).

High Sierra. A term coined by J.D. Whitney (1868) to describe the higher region of the Sierra Nevada, much of it above timberline.

Igneous Rock. A rock formed by solidification of hot molten material, either at depth in the Earth’s crust (plutonic) or erupted at the Earth’s surface (volcanic).

Intrude, intruded, intrusion. Process by which magma invades or is injected into preexisting rock bodies.

Intrusive suite. A grouping of individual plutons or plutonic rock units having significant features in common and thought to have formed from the same parent magma.

Isotope. Any of two or more forms of an element with the same or very closely related properties and the same atomic number but different atomic weights. Some isotopes are radioactive and change to different isotopes at a constant rate (fig. 7).

Joint. A fracture along which there has been little or no movement parallel to the fracture plane.

Lithosphere. The rigid outer portion of the Earth comprising the Earth’s crust and the uppermost part of the upper mantle (fig. 25).

Magma. Naturally occuring molten rock generated within the Earth. Magma may intrude to form plutonic rock or be extruded to form volcanic rock.

Mantle (of the Earth). The intermediate of the concentric shells that make up the Earth; it lies beneath the crust. (fig. 25).

Metamorphic rock. Rock changed materially in composition or appearance by heat, pressure, or infiltrations at depth in the Earth’s crust.

Mineral. A naturally occurring, inanimate substance of definite chemical composition and distinctive physical and molecular properties. Minerals make up rocks.

Moraine. An accumulation of glacial till with an initial topographic expression of its own, commonly a ridge. Several varieties are described in the text (fig. 62).

Pegmatite. An exceedingly coarsely crystalline plutonic rock, commonly in dikes or pods a few feet across. Individual crystals are several inches to a foot or more across.

Phenocryst. A large crystal in an igneous rock, embedded in a finer grained matrix.

Plate (tectonic). A segment of the Earth’s crust in constant motion relative to other segments (fig. 26).

Pluton. A general term applied to any body of intrusive igneous rock of deep-seated origin, regardless of shape or size.

Plutonic rock. Igneous rock formed by solidification of magma deep within the Earth’s crust.

Porphyritic. Said of an igneous-rock texture with larger crystals scattered through a finer grained matrix.

Porphyry. A porphyritic rock with conspicuous phenocrysts in a very fine grained matrix (fig. 21).

Pre-Tahoe glaciation. Composite of the one or more major glaciations in Yosemite that preceded the Tahoe glaciation. Supersedes the El Portal and Glacier Point glaciations of Matthes’ usage in Yosemite.

Ptroclastic rock. Rock formed of ash or other fragmental material explosively ejected from a volcano.

Quartz monzonite. A granitelike plutonic rock containing about equal proportions of potassium feldspar and plagioclase and less than 20 percent quartz under the classification system now in use. Rocks in the Yosemite area containing more than 20 percent quartz that were previously called quartz monzonite are now classified as granite (fig. 9).

Rhyolite. A light-colored volcanic rock with a high silica (SiO₂) content of at least 70 percent.

Roche Moutonnée. A protruding knob of bedrock glacially eroded to have a gently inclined, striated upstream slope and a steep, rough, and hackly downstream side. Commonly translated as “sheep,” moutonnée is actually a French adjective meaning “fleecy”; the term was introduced into geology in 1786 in describing rounded Alpine hills whose repeated curves, taken as a whole and as seen from a distance, resemble a thick fleece (figs. 54, 70).

Schist. A crystalline metamorphic rock composed chiefly of platy mineral grains, such as mica, oriented so that the rock tends to split into layers or slabs.

Schlieren. Streaky concentrations of dark minerals in a granitic rock, caused by movement within the partially solidified magma (fig. 16).

Sedimentary rock. Rock formed by consolidation of sediment (gravel, sand, mud) deposited at the surface of the Earth.

Silica (SiO₂). Occurs as the natural mineral quartz, including various fine-grained varieties, such as chert. The element silicon (Si) also occurs in most rock-forming minerals (silicates), such as feldspars.

Subduction. The process wherein an oceanic plate converging with a continental plate is deflected downward and consumed into the mantle beneath the continental plate (fig. 28).

Tahoe glaciation. The intermediate of the three major glaciations recognized in Yosemite. Approximately equivalent to the earlier part of the Wisconsin glaciation of Matthes.

Talus. An accumulation of coarse, angular rock fragments derived from and resting at the base of a cliff or very steep slope.

Till. Glacially transported material deposited directly by ice, without transportation or sorting by water (fig. 60).

Tioga glaciation. The latest of the three major glaciations recognized in Yosemite. Approximately equivalent to the younger part of the Wisconsin glaciation of Matthes.

Trimline. A sharp boundary line marking the maximum upper level of the margins of a glacier. It commonly separates jagged cliffs above from glacially smoothed rock surfaces below (figs. 56, 57).

Tuff. Rock formed from consolidation of volcanic ash.

Next: References • Contents • Previous: Final evolution