The Geologic Story of Yosemite Valley
By N. King Huber
In the footsteps of François E. Matthes (1922, revised 1929 and 1938)
U.S.
Geological Survey Bulletin 1595, 64p.
Generalized Geologic Map of the Yosemite National
Park Area
Bedrock Geology of the Yosemite Valley Area,
Yosemite National Park, California
YOSEMITE COUNTRY
For its towering cliffs, spectacular waterfalls, granite domes
and spires, glacially-sculpted and polished rock, and beautiful
alpine scenery, Yosemite National Park is world famous. Nowhere
else are all these exceptional features so well displayed and so
easily accessible. Artists, writers, tourists, and geologists flock
to Yosemite - and marvel at its natural wonders. Yosemite Valley
itself is deeply carved into the gently sloping western flank of
the Sierra Nevada, the longest, the highest, and the grandest single
mountain range in the United States outside of Alaska. And although
other valleys with similarities exist, there is but one Yosemite
Valley, the “Incomparable Valley” of John Muir, California's
most famous naturalist.
A SCENE OF WORLDWIDE FAME
Simply stated, Yosemite Valley, only 7 miles long and nearly 1
mile wide, is a flat-floored, widened part of the canyon of the
Merced River. But this broad rock-hewn trough with roughly parallel
sides, is boldly sculptured and ornamented with silvery cataracts.
From the valley floor at an elevation of 4,000 feet, the magnificent
cliffs rise 3,000 to 4,000 feet higher to forested uplands on either
side (fig. 1).
Once you enter Yosemite Valley its grandeur is overwhelming. Looking
eastward up the valley from its lower end you are struck by the
immensity of the sheer profile of El Capitan, the most majestic
cliff in the valley (fig. 2). Projecting
boldly from the north wall, its top rises 3,000 feet above the valley
floor. Directly opposite stand the Cathedral Rocks, over 2,500 feet
high, which also jut into the valley. Between the west end of this
promontory and the Leaning Tower, Bridalveil Fall leaps 620 feet,
its abundant spray commonly suffused with rainbows (fig.
3).
Eastward beyond the narrows at El Capitan and Cathedral Rocks,
the valley abruptly widens, and in an embayment on the south are
the Cathedral Spires, among the frailest rock shafts in the valley
(fig. 4). On the north are
the Three Brothers (fig. 5),
whose gabled summits rise one above another, all built architecturally
on the same angle. The highest, known as Eagle Peak, rises nearly
3,800 feet above the valley floor. Across the valley stands Sentinel
Rock, a finely modeled obelisk with a pointed top (fig.
6).
A mile farther up the valley, on the north side, are Yosemite Falls,
dramatically booming among clouds of mist during the spring and
early summer snowmelt (fig.
7). The Upper Fall, 1,430 feet high, would alone make any valley
famous; it is the highest unbroken leap of water on the continent.
The Lower Fall, which descends 320 feet, seems insignificant by
comparison, yet it is twice as high as Niagara Falls. The entire
chain of falls and intermediate cascades drops 2,425 feet. Ribbon
Fall, west of El Capitan, descends 1,612 feet, but it is confined
in a sheer-walled recess and does not make a clear leap throughout.
Farther up the valley, on the north side, are the Royal Arches,
sculptured one within another into an inclined rock wall that rises
1,500 feet (fig. 8). An
enormous natural pillar, the Washington Column, flanks them on the
right, and above them rises a smoothly curving, helmet-shaped knob
of granite called North Dome. Facing the Royal Arches on the south
wall, stands Glacier Point providing a matchless view of the valley
from its summit, which stands 3,200 feet above the valley floor.
At the head of the valley, as if on a pedestal, stands Half Dome,
the most colossal and recognizable rock monument in the Sierra Nevada,
smoothly rounded on three sides and a sheer vertical face on the
fourth (fig. 9). From its
summit, over 4,800 feet above the valley, you look southeast into
Little Yosemite Valley, which is broad floored and has granite walls
more gently sloping than in its larger namesake. From Little Yosemite's
western portal, guarded by Liberty Cap, the Merced River descends
by a giant stairway, making two magnificent waterfalls, Nevada Fall,
dropping 594 feet, and Vernal Fall, dropping 317 feet. Looking northward
from Half Dome's summit, the view is into Tenaya Canyon, a chasm
as profound as Yosemite Valley itself, yet the pathway of only a
small brook. To the northeast, Clouds Rest, the loftiest summit
in the vicinity of Yosemite Valley, rises to 9,926 feet; beyond,
spreads the vast panorama of the High Sierra.
The present Yosemite Valley is the result of many different geologic processes operating over an incomprehensible length of time measured in millions of years. These processes are by themselves not unique, but their unrivaled interaction has created this “Incomparable Valley.” The accumulated observations, studies, and interpretations by many individuals through the years allow us to reconstruct much of the valley's geologic history, adding to our appreciation of its scenic majesty.
ROCK, THE SCULPTOR'S MEDIUM
For any form of sculpture, whether of finely-chiseled statues or
massive landforms, the resulting product is strongly dependent on
the nature of the material being worked upon. For Yosemite Valley
that material is granite. Indeed, granite forms the bedrock of much
of the Sierra Nevada, including most of Yosemite National Park.
Granite, in the broad sense of the term (granitic rock),
is a rock with a salt-and-pepper appearance due to random distribution
of light and dark minerals. The mineral grains are generally coarse
enough to be individually visible to the naked eye.
Throughout the park granitic rock varies considerably in the relative proportions of the individual light and dark minerals, and these compositional differences are represented by a variety of specific names, such as granodiorite and tonalite, in addition to “true” granite as defined by geologists.
From a distance, all of Yosemite Valley's granitic rock looks the same. But it actually consists of individual rock bodies, each with their own characteristic mineral composition and texture, that is, the coarseness of their crystals and uniformity or variation in grain size. All of these variations, in turn, affect the rock's resistance to abrasion, fracturing, and weathering, all important to the sculptor and the end product. The imposing cliffs of El Capitan and Cathedral Rocks, for example, are composed of a particularly tough and resistant variety of granitic rock.
THE ROLE OF JOINTS
The bedrock structures having the greatest effects on Yosemite's
landform development are joints. Although granitic rock is unbroken
on a small scale, on a larger scale the rock is broken by joints,
which are more or less planar cracks commonly found as sets of parallel
fractures in the rock. Regional-scale joints commonly determine
the orientation of major features of the landscape, such as the
planar face of Half Dome, the series of parallel cliffs at Cathedral
Rocks, and the westward sloping faces of the Three Brothers. In
contrast, smaller, outcrop-scale joints determine the ease with
which rock breaks and erodes. Joints are of overwhelming influence
on landform development in granitic terrain because they form greatly
contrasting zones of weakness in otherwise homogenous, erosion-resistant
rock and allow access for water and air to enter and aid in the
weathering and disintegration of rock.
The type of jointing that most influences the form of Yosemite's
landmarks, however, is the broad, onion-shell-like sheet jointing
formed by a process referred to as exfoliation. Granitic
rocks originate at considerable depth within the Earth while under
great pressure from overlying rock that may be miles thick. As the
overlying rock erodes away, the decrease in the pressure that once
confined the granitic rock causes it to expand toward the Earth's
surface. When the outer expanding zone exceeds the strength of the
rock, it cracks away from less expanded rock beneath and bursts
loose as a sheet; subsequent cracks release successive layers of
expanding rock. Because the expansion that forms these sheets takes
place perpendicular to the local surface, the shape of sheets generally
reflects topography, with curved surfaces following hill and valley.
Sheet joints also tend to parallel the walls of canyons and cliffs
that may appear to be unbroken monoliths, such as El Capitan, but
which may have fractures behind and parallel to the cliff face.
The curved upper surfaces of North Dome and Half Dome, and the undulating
surface of Clouds Rest, are magnificent examples of sheet joints
or sheeting (figs. 8,
9).
Admiring Yosemite Valley's intricately sculptured walls as they
appear today and knowing something about the granitic rock from
which they were carved, we can look back in time to speculate on
the evolution of the valley's formation - its geologic history.
A STORY THAT BEGAN MILLIONS OF YEARS AGO
The last touches to Yosemite Valley's architecture were applied
relatively recently, geologically speaking. But the rock from which
the valley is carved originated mainly during the Cretaceous period,
about 100 million years ago, when dinosaurs roamed the Earth. At
that time molten rock, magma, generated deep within the
Earth, rose upward within the Earth's crust, or upper layer, and
crystallized far beneath the surface to form granitic rock along
a linear belt that was to become the future Sierra Nevada. The granitic
terrain that makes up the Sierra, once thought to have only local
variations in one huge mass of rock, is actually made up of a mosaic
of individual rock bodies that formed from repeated intrusions of
magma over many millions of years.
Some of the magma broke through to the surface, building a string of volcanoes atop hidden granitic roots, and we can perhaps envision an ancient majestic mountain range somewhat like the modern Cascade Range along the coast of our Pacific Northwest. Because of the high elevation of this ancestral range, however, the volcanic and other rocks covering the granite were soon eroded away, and by Late Cretaceous time, about 70 million years ago, the granitic rocks became exposed at the Earth's surface. By middle Cenozoic time, a few tens of millions of years ago, so much of the upper part had been removed that in the vicinity of Yosemite the surface of the range had a low relief of only a few thousand feet.
Later, the continental crust east of the Sierra Nevada began to stretch in an east-west direction, developing into a series of north-south-trending valleys and mountain ranges. Through a combination of uplift of the Sierran block and down-dropping of the area to the east, the Yosemite region acquired a tilted-block aspect with a long, gentle slope westward to the Central Valley of California and a short, steep slope separating it from the country to the east.
AGENTS OF EROSION
Erosion, simply stated, is the removal of earth materials from high areas to lower areas, modifying the landscape in the process. Two agents of erosion are chiefly responsible for sculpting the present Yosemite landscape - flowing water and glacial ice: flowing water had the major role, and glacial ice added additional touches. The general Sierran landforms were all well established before glaciation, and the major stream drainage's provided the avenues along which the glaciers would later follow. Some of the glacial modifications, however, were profound: the creation of alpine topography in the High Sierra, the rounding of many valleys from V-shape to U-shape, and the straightening of valleys in the process. Still another agent of erosion is simply gravity. The downslope movement of rock materials produces landslides and rockfalls. Although generally of local extent, such movement is important, particularly in mountainous terrain and on the over-steepened slopes in Yosemite Valley.
The Role of Flowing Water
The effectiveness of erosion by flowing water depends both on processes
of weathering - the breakdown of larger rocks into smaller individual
rock and mineral fragments that can be transported - and on stream
volume and velocity, which determines the size and amount of material
that can be transported. With the increasing late Cenozoic elevation
of the Yosemite region, the major streams coursing down its western
slope were rejuvenated and made more vigorous by their increased
slopes. Under these conditions the major streams cut canyons whose
channels became progressively deepened relative to the upland areas
between them, areas which even today retain comparatively moderate
relief. The upper basins and middle reaches of the Merced and Tuolumne
Rivers, for example, were later modified by glacial erosion, but
initial canyon cutting was accomplished solely by the action of
streams. Two sketches depict an artist's conception of the evolution
of the Merced River canyon at the site of the future Yosemite Valley
before the onset of glaciation (fig.
10).
The Role of Glaciers
The Yosemite landscape as we see it today strongly reflects the dynamic influence of flowing ice that long ago covered much of its higher regions. Geologists are still uncertain how many times ice mantled Yosemite, but at least three major glaciations have been well documented elsewhere in the Sierra Nevada. In the higher country, icefields covered extensive areas, except for the higher ridges and peaks. Lower down the western slope, at middle elevations, glacial tongues were confined to pre-existing river canyons, such as those of the Merced and Tuolumne Rivers. Thus our focus will be on the nature and activities of these valley glaciers, particularly as they apply to Yosemite Valley, and Hetch Hetchy Valley some 15 miles to the north, but remembering that the valley glaciers derive their flowing ice from icefields higher in the range.
In contrast to the sinuous V-shaped valleys of normal streams in
unglaciated mountainous terrain, glaciated valleys tend to be straighter
and have U-shaped profiles. Whereas a stream erodes the outsides
of bends preferentially and makes its course more sinuous, glacial
erosive force is concentrated on the insides of bends, removing
the protruding spurs of the original stream valley and leaving a
wider, straighter valley.
The resulting modification, in detail, depends on the nature and
structural integrity of the bedrock over which the glacier is flowing.
For granitic bedrock, the dominant structure of concern is jointing,
which controls the ease of removal of rock that is otherwise highly
resistant to glacial erosion.
YOSEMITE VALLEY AND ITS GLACIERS
Yosemite Valley has often been referred to as a “classic”
glacial valley. But what glacially-derived attributes does it display
to deserve that designation? A glacier tends to straighten a valley
and smooth its walls as it grinds past them. But the walls of Yosemite
Valley are extremely ragged, with many pinnacles and spires projecting
upward from them - Leaning Tower, Cathedral Spires, Sentinel Rock,
and Lost Arrow stand out strikingly. All of the waterfalls and lesser
cascades along the sides of the valley are ensconced in alcoves,
except for Upper Yosemite Fall, whose story will be told in upcoming
paragraphs. Eagle Creek and Indian Canyon Creek actually issue from
deep ravines. All of these seemingly anomalous features would doubtless
be obliterated by a glacier that filled the valley to its brim.
And yet glacial erratics - boulders transported and deposited by
a glacier - are found scattered above the valley's rim telling us
that a glacier indeed once filled the valley to its brim. How can
we explain this anomalous appearance if the valley was indeed shaped
by a glacier? The anomaly is even more apparent if we compare Yosemite
Valley with another “glaciated” valley of about the same
size and elevation, Hetch Hetchy Valley, which has comparatively
smooth walls and an absence of pinnacles and spires (fig.
11).
Little doubt exists that Yosemite Valley indeed represents a profound,
glacially-driven modification of the Merced River canyon, as no
other erosive agent could have accomplished such excavation. A glacier
filling the valley to its rim created the basic broad shape of the
valley and gouged out a deep bedrock basin whose bottom locally,
in its eastern part, lies more than 1,000 feet below the present
valley floor (fig. 12).
That glacial episode was named the El Portal glaciation by François
Matthes in his monumental Yosemite study, because he thought that
its glacier advanced down the Merced canyon to near the community
of El Portal, some 10 miles downstream from Yosemite Valley proper.
Today we correlate that glaciation with the Sherwin glaciation,
defined from studies along the east side of the Sierra Nevada, and
which name is now in general use. The Sherwin was the most extensive,
and longest-lived, glaciation documented in the Sierra. It may have
lasted almost 300 thousand years and ended about 1 million years
ago. A Sherwin-age glacier was almost surely responsible for the
major excavation and shaping of Yosemite Valley within the Merced
River canyon.
Later glaciations in the Sierra Nevada were of lesser areal extent
and briefer than the Sherwin. The best documented are the Tahoe
and Tioga glaciations, which probably peaked about 130,000 and 20,000
years ago, respectively; together they are equivalent to Matthes'
“Wisconsin” glacial stage, which he did not subdivide.
The last glacier in Yosemite Valley - Tioga in age - advanced only
as far as Bridalveil Meadow (fig.
12). At this location the forward movement of the glacier was
balanced by the melting of ice at its front, or terminus. A “terminal”
end moraine - a low ridge crossing the valley - was constructed
with rock debris transported by the glacier and deposited at its
terminus. The extent of the earlier Tahoe-age glacier in the valley
is uncertain, but evidence elsewhere in the Sierra, suggests that
it probably would have been somewhat longer than the Tioga. Nevertheless,
since the original excavation of Yosemite Valley by a Sherwin-age
glacier, no subsequent glacier has filled the valley to its rim,
a conclusion that has important consequences for the scenery.
From its terminus at Bridalveil Meadow, the ice surface of the Tioga glacier would have sloped upward toward the east end of the valley with the ice reaching a thickness of perhaps a little over 1,000 feet at Columbia Rock west of Yosemite Falls, 1,500 feet at Washington Column, and 2,000 feet in Tenaya Canyon below Basket Dome, as reconstructed by Matthes. Thus the Tioga and similar Tahoe glaciers could do very little to further modify or smooth the walls of Yosemite Valley. Above the ice surface of those glaciers, the valley walls have had a million years to weather: joints widened, rock fractured and crumbled, and waterfalls and cascades eroded back into alcoves and ravines. Thus the pinnacles and spires that seem so anomalous for a glacial valley actually had a million years to form and, being above the level of later glaciers, remain to amaze us today. In Tenaya Canyon, Tioga ice was thicker and reached farther up the walls, smoothing them and removing irregularities; no pinnacles and spires are found there.
Hetch Hetchy Valley on the Tuolumne River, otherwise similar to
Yosemite Valley, has comparatively smooth walls and an absence of
pinnacles and spires (fig. 11).
There the Tioga glacier was also less extensive than the Sherwin,
but unlike the glacier in Yosemite Valley, the Tioga glacier filled
Hetch Hetchy to the rim. Thus Hetch Hetchy Valley's walls were being
scraped and debris was removed from the valley with each glaciation,
including the last. The greater extent of the glacier in Hetch Hetchy
can be attributed to the fact that the drainage basin of the Tuolumne
River above Hetch Hetchy is more than three times as large as that
of the Merced River above Yosemite Valley. As a result, the much
larger icefield feeding the Tuolumne glacier was able to provide
the necessary volume of ice to fill Hetch Hetchy even though the
Tioga glaciation was regionally less extensive than the Sherwin.
The smaller Merced icefield was unable to provide sufficient ice
to fill Yosemite Valley, even though supplemented by ice from a
part of the Tuolumne drainage that flowed southwest over a low pass
into Tenaya Canyon.
LEAPING FALLS AND HANGING VALLEYS
Waterfalls leaping out from a valley's walls far above the valley
floor have long been considered evidence of a glacial origin for
the valley. The enormous Sherwin-age glacier that shaped Yosemite
Valley was able to excavate the central chasm to a greater depth
than smaller glaciers in side-entering tributaries. The result was
that some of the side valleys were left “hanging” with
waterfalls at their brinks. Since Sherwin time, most of the tributaries
have eroded their channels back into the walls to leave little more
than steep ravines with minor falls interrupted by chains of cascades,
such as those at Sentinel Fall. Bridalveil Fall is an exception,
although it also has receded back into an alcove from its original
position further out on the valley wall.
In contrast to Bridalveil Fall, Upper Yosemite Fall, although leaping
from a hanging valley, had a very different origin. Yosemite Creek
is the largest stream flowing into the north side of Yosemite Valley
and probably entered the Merced River canyon through a steep side
canyon before glaciation. After the Sherwin glacier deepened Yosemite
Valley, Yosemite Creek continued to enter the main valley through
that ravine, which lies just to the west of the present falls (fig.
7). Matthes recognized this and described “what appears
to be an old stream channel leading just to the west of the present
channel” (fig. 13).
At that time the site of the present Upper Fall hosted only a minor
ephemeral fall of short duration during spring runoff.
Matthes did not speculate on how or when Yosemite Creek was diverted
from that old channel into its present channel to create its Upper
Fall. He did, however, map a morainal complex that he attributed
to his “Wisconsin-age” glacier that flowed down Yosemite
Creek, but stopped about one-half mile short of the rim of Yosemite
Valley itself. A plausible explanation for the diversion of Yosemite
Creek into its present channel is that the stream was temporarily
blocked by glacial deposits and had to find a new way through the
intricate complex of nested moraines. As his Wisconsin glacial stage
includes both Tahoe and Tioga glaciations, Upper Yosemite Fall,
with its “newly” hanging valley, can be little more than
130,000 years old. And what a spectacular addition to Yosemite Valley's
architectural wonders it is!
YOSEMITE VALLEY'S GLACIAL FINALE
While the Tioga glacier was constructing its terminal moraine at Bridalveil Meadow, the climate apparently warmed slightly. The ice at the front of the glacier began to melt faster than the ice was moving forward, and the ice front, or “snout,” of the still-flowing glacier began to “retreat” up the valley. The climate cooled again; the ice front paused and temporarily stabilized just west of El Capitan Meadow. Here the glacier began to construct a new moraine, known as a “recessional” moraine because the glacier had receded from its terminal position. It remained at this location longer than it had at Bridalveil Meadow and the resultant El Capitan Moraine is larger in both volume and height. Eventually, the climate warmed abruptly, and the Merced glacier's snout retreated toward the head of the valley with no more recessional pauses, probably leaving Yosemite Valley by 15,000 years ago.
When the Tioga-age glacier departed from Yosemite Valley it left
behind a lake, which Matthes christened Lake Yosemite (fig.
14). It is likely that the advancing Tioga glacier had excavated
some of the pre-existing valley fill east of the El Capitan Moraine,
creating a shallow lake basin. The lake was in part dammed by this
moraine, with the Merced River flowing over a low spillway through
the moraine near the south valley wall. As the separate arms of
the Tioga glacier retreated up the Merced and Tenaya canyons, the
melt-water-swollen, debris-laden rivers issuing from their snouts
delivered large quantities of sediment to the lake basin. The lake
was soon filled in with this sediment, creating the relatively level
valley floor we see today. The resulting gentle slope allowed the
Merced River to develop a sinuous meander pattern across this broad
flood plain. A low-gradient, meandering stream is particularly susceptible
to over-bank flow during high water, and its flood plain is naturally
destined for periodic flooding.
THE ROCKS COME TUMBLING DOWN
The Tioga-age glacier did little to modify Yosemite Valley other than to remove pre-existing talus at the base of cliffs east of Bridalveil Meadow; all of the talus there now has accumulated in the last 15,000 years or so since the Tioga glacier departed. In contrast, the enormous talus slope west of El Capitan, known as the Rockslides, escaped the reach of the Tioga glacier. For the past million years, then, the rock walls of the valley that remained above the ice-level of the smaller post-Sherwin glaciers have weathered, joints have been enlarged, and rock has spalled off to form the very irregularly sculptured surface we see today. This geologic history provides the setting for abundant rockfalls. Every significant historical rockfall in Yosemite Valley has originated in vulnerable fractured rock from above the level of the Tioga glacier. Some rockfalls are quite large, but most are relatively small and gradually build up a cone of debris below the most active sites. Thus the size of a debris cone can reflect the volume or the frequency of individual falls, or a combination. The shattered rock high on the east side of Middle Brother provides material for a debris cone at one of the most active rockfall sites in the valley. Given the nature of the geologic setting, it is inevitable that such rockfalls will continue as part of ongoing geologic processes.
EPILOGUE
The geologic story of Yosemite Valley, as presented here, describes
our present understanding of the interplay of various geologic processes
that contributed to the valley's creation. But this is not the last
word. We are still learning about these various processes and their
effects on the evolution of the valley. And, indeed, these geologic
processes are far from finished. We are seeing only a brief period
in time in the landscape's ongoing history. Dynamic geologic processes
will continue to change the many faces of this “Incomparable
Valley.” Additional Reading
The Yosemite, by John Muir (1914). Sierra Club Cooks (Reprint,
1988). Collection of essays.
Geologic History of the Yosemite Valley, by Francois E. Matthes
(1930). U.S. Geological Survey Professional Paper 160. Dated,
but pioneering study of Yosemite region.
Bedrock Geologic Map of Yosemite Valley, by Frank C. Calkins (1985).
U.S. Geological Survey Map I-1639. Maps and describes the various
types of granitic rock in Yosemite Valley.
The Geologic Story of Yosemite National Park, by N. King Huber
(1987). Yosemite Association (Reprint, 1989). Geology for the
layman.
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