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those relations, and although his map is being published now as a historical
document, it remains the definitive geologic map of Yosemite Valley.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.
Introduction
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.
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