It was a tough hike for 6 and 8 year olds -- up 600 slick granite steps to the top of a waterfall, getting thoroughly splashed in the process. This month marks the 150th anniversary of the Yosemite Grant Act, which set aside Yosemite Valley and the Mariposa Grove as the first protected wild land in the country -- the first time scenic, wilderness lands were set aside specifically for preservation and public use by the federal government.
Frank Calkins' work in Yosemite was preceded by Henry W. Turner, also of, the U.S. Geological Survey, who began mapping the Yosemite and Mount Lyell 30 minute quadrangles in 1897 and laid the foundation that Calkins' work was built on. Although Turner never completed this sizable assignment, he recognized the differing types of plutonic rocks and, for example, named the El Capitan Granite.
Calkins mapped the valley and adjacent areas of Yosemite National Park during the period 1913 through 1916, at the same time that Francois Matthes was studying the glacial geology of Yosemite. Calkins summarized the bedrock geology of part of Yosemite in the appendix of Matthes' classic volume "Geologic History of the Yosemite Valley" (1930), but his detailed bedrock map of the valley was never published during his lifetime. The ultimate perfectionist, Calkins never was satisfied that he had all the geologic contacts correctly located or that he understood completely enough the relations between all the geologic units. We still do not understand all of those relations, and although his map is being published now as a historical document, it remains the definitive geologic map of Yosemite Valley. Its publication, although belated, is a fitting memorial to his mapping talent.
The map itself remains unchanged but its explanation has been modified to reflect current nomenclature, and the names of some of the geologic units differ slightly from those in Calkins' earlier descriptions. The accompanying text is based in part on Calkins' writings, but also partly from the work of other students of Yosemite geology, notably Frank Dodge, Francois Matthes, Dallas Peck, and Clyde Wahrhaftig.
The topographic base used for this map is the original one Matthes prepared from plane-table surveys made in 1905 and 1906 and is a classic in its own right. Matthes' map is used here because it was the base used by Calkins' to compile his geology, and to avoid the considerable adjustment required to fit the geology to the current topographic map of the Valley prepared by photogrammetric methods. Calkins' desired that this base be used because he believed it had more "character" and gave the "feel" of the cliffs better than the more recent version-that it does, despite the increased geodetic accuracy of the latter. The location of roads and structures on Matthes' map was last revised in 1949, but no new highways have been constructed since then, so the map has not been further revised for this publication.
Yosemite Valley, one of the world's great natural works of rock sculpture, is carved into the west slope of the Sierra Nevada. Immense cliffs, domes, and waterfalls tower over forest, meadows, and a meandering river, creating one of the most scenic natural landscapes in North America (fig. 1). In Yosemite Valley and the adjoining uplands, the forces of erosion have exposed, with exceptional clarity, a highly complex assemblage of granitic rocks. The accompanying geologic map shows the distribution of some of the different rocks that make up this assemblage. This pamphlet briefly describes those rocks and discusses how they differ, both in composition and structure, and the role they played in the evolution of the valley.
Magma and its Products
The rocks that now form the walls and domes of the Yosemite Valley area, part of the lofty Sierra Nevada, originated from molten material-magma-buried miles below the Earth's surface. Cooling and crystallization of this deep-seated magma required millions of years. The resulting rock, composed of interlocking crystals of several kinds of minerals, is called a plutonic rock, named for Pluto, the Roman god of the underworld. Formation of the plutonic rocks of the Sierra occurred over a long timespan, as magma episodically rose from the Earth's interior, intruding older host rocks, and eventually crystallizing to create one of many individual bodies of rock called plutons. Many of these plutons are today exposed at the Earth's surface due to erosion of the once overlying older rocks.
These older rocks-slate, quartzite, marble, and metavolcanic rocks-formed by alteration, or metamorphism, from their original state-shale, sandstone, limestone, and volcanic rocks-with intense heat and pressure. The metamorphism both preceded and accompanied the intrusion of the plutons. Few remnants of these preplutonic host rocks remain near the present valley, although several small masses occur on the flanks of Sentinel Dome and in Indian Canyon. Great thicknesses of these rocks, however, do crop out a few miles to the west. Tightly folded beds of them are visible from the road turnout on Highway 140, 11 miles downstream from El Portal and about 1 mile west of the bridge crossing the South Fork of the Merced River (outside map area). Ancient metamorphic rocks are also widely exposed along the crest of the Sierra, notably on Mt. Dana and Mt. Gibbs (outside map area).
The numerous plutons composing the Yosemite Valley area, as well as the entire Sierra Nevada, together are called the Sierra Nevada batholith (from the Greek words bathos, deep, and lithos, rock). In the earliest geologic studies of the Sierra, the composite nature of the batholith was not recognized-the differences in rock types were thought simply to represent variations in one huge rock mass. The complex history of the batholith was only deciphered when individual plutons were recognized as separate units. Calkins' Yosemite study was one of the earliest to assign different names to different plutons yet at the same time recognize the composite nature of the batholith. The plutons exposed in the walls of Yosemite Valley were intruded over a period of about 30 million years (approximately from 120 to 90 million years ago) during part of the Cretaceous Period. The Sierra Nevada batholith, however, is composed of hundreds of individual plutons, and construction of this extensive batholith may have taken as much as 130 million yers.
The plutonic rocks of the valley and adjacent uplands are composed of a variety of minerals. Five of these minerals make up most of each rock variety; quartz, two varieties of feldspar (potassium feldspar and plagioclase), biotite, and hornblende. These minerals mainly comprise the elements silicon and oxygen, and all except quartz include aluminum as well. Other constituents of the feldspar include potassium, sodium, and calcium; biotite and homblende also include magnesium and iron.
Feldspar and quartz are translucent and appear light gray on fresh surfaces. On a weathered surface, the feldspars. turn chalky white, but the quartz remains light gray. Feldspar crystals have good cleavage-a property of breaking along planar surfaces that reflect light when properly oriented; quartz has no cleavage, so it breaks along curved, or conchoidal, surfaces. Biotite, black mica, commonly appears as hexagonal crystals that can be split with a knife into thin flakes along one perfect cleavage direction. Homblende is much harder than biotite and commonly occurs as greenish-black elongate rod- shaped crystals; it has good cleavage in two directions that intersect to form fine striations along the length of the rods, making them look like bits of charcoal. Other minerals are present in minor amounts; the most distinctive is sphene, which occurs in small amber wedge-shaped crystals. With a little practice, all of these minerals can be readily identified with a small magnifying glass.
Plutonic rocks consisting chiefly of the light-colored minerals quartz and feldspar and only a minor amount of dark minerals are loosely called granitic rocks. Granitic rocks, such as granite, granodiorite, and tonalite, differ primarily in the relative proportions of these minerals (see fig. 2). For example, granite, in the technical sense of the term, contains much quartz, both potassium feldspar and calcium-rich feldspar (plagioclase), is generally light colored with only a small percent of dark minerals. Granodiorite is similar but has about twice as much plagioclase as potassium feldspar and has more dark minerals. Tonalite has even less potassium feldspar and more dark minerals. Additional compositional data indicate that rocks previously called quartz monzonite by Calkins (1930) are granodiorite under the rock classification currently in use. In contrast to granitic rocks, diorite and gabbro contain mostly plagioclase and dark minerals with little or no quartz or potassium feldspar; the plagioclase in gabbro has more calcium than it does in diorite.
Because most granitic rocks contain minerals of about equal size, they are said to have a granular texture. But some granitic rocks have crystals of one mineral considerably larger than the others; these oversized crystals are called phenocrysts (from the Greek words meaning "to appear" and "crystal") and the texture of such a rock is described as porphyritic. In Sierran granites the mineral that most commonly occurs as phenocrysts is potassium feldspar, with crystals sometimes as much as 2 or 3 inches long.
Certain field relations help determine the relative ages of individual plutons. For example, hot magma associated with younger plutons commonly intrudes older plutons along cracks, or fractures, and solidifies to form sheets, or dikes, along those fractures in the older rocks. Additionally, younger plutons commonly contain inclusions, or fragments, of the older rock, which became embedded in the still-molten magma. Determining the specific age of a given rock in million of years requires measurement of the amount of radioactive decay of certain elements such as uranium, thorium, potassium, and rubidium. From these measurements and the known rates of decay, one can approximate the amount of time elapsed since the rock crystallized or cooled below a temperature that permitted the radiometric decay to start.