Sequoia National Park

As our journey through the Sierra Nevada continues, we will now explore Sequoia National Park, the home of the world's largest trees by volume. These organisms, known as giant sequoias, occur naturally only in groves on the western slopes of the Sierra Nevada. They thrive in this environment, particularly between 5,000 and 7,000 feet in elevation, due to its humid climate which is characterized by dry summers and snowy winters. The soil composition most conducive to giant sequoia growth is granitic-based alluvial soil. Alluvial means that the granitic rock of the Sierra Nevada is weathered into a fine-grained soil, rich in minerals, and then deposited by water flowing over flood plains or in river beds. Thus, the giant sequoia groves are almost always located in deep, well-drained sandy soil with an average pH of 6.5 (slightly acidic).

Depicted above is a giant sequoia known as the General Sherman Tree. By volume, it is the largest known living single stem tree on the planet. However, the General Sherman Tree is not the tallest tree on Earth; at 396 feet, that is a distinction which belongs to the Hyperion Tree, a Coast redwood. General Sherman is not the widest tree on Earth; both the largest cypress and largest baobab have a greater diameter. Nor is the General Sherman the oldest known living tree on Earth; the Prometheus tree, a Great Basin bristlecone pine, holds that distinction at approximately 5,000-years-old. But with a height of 275 feet, a diameter of 25 feet, an estimated bole volume of 52,513 cubic feet, and an estimated age of 2,300-2,700 years, it is among the tallest, widest, and longest-lived trees on planet Earth. 

Another incredible aspect of Sequoia National Park is its intricate cave systems. There are at least 240 known caves in the park, many with endemic cave fauna; the most impressive of these caves is Crystal Cave. Crystal Cave has formed within marble of the Sequoia Pendant, one of many metamorphic pendants trending northwest across the Sierra Nevada. A pendant is the downward extension of the surrounding rock that protrudes into the upper surface of an igneous intrusive body; in this case, the intrusive body is the granitic batholith which comprises the Sierra Nevada. The Sequoia Pendant is approximately 16.7 miles long and 1.24 miles wide. Crystal Cave has formed in a narrow lens of coarsely crystalline, vertically bedded marble. Most passages in Crystal Cave have developed on strike, parallel to the axis of the marble lens and the metamorphic pendant. Many of the cave walls are composed of schist, a medium-grade metamorphic rock. With its ornate marble polished by subterranean streams and decorated with stalactites and stalagmites, Crystal Cave is a natural treasure to behold.

This picture shows the incredible stalactite formations in Crystal Cave. The cave is located on the western slope of the Sierra Nevada in Sequoia National Park. The Sierra Nevada is a unique location for the formation of caves. Below the glacially sculpted range crest, the broad western slope of the range descends in a series of undulating low relief upland surfaces punctuated by deeply incised river canyons. The vast majority of caves in the area have formed in proximity to these rivers. Additionally, the climate of the Sierra Nevada is distinctly Mediterranean, with cool, wet winters and hot, dry summers. As a result, groundwater recharge for the caves of the Sierra Nevada is usually allogenic, meaning it is derived from sinking streams that originate in granitic catchments. These areas of drainage are often steep, dropping several thousand meters over very short (6.2-18.6 miles) distances. Snowfall in the winter provides much of the precipitation in the southern Sierra Nevada. Temperatures increase rapidly in the springtime which results in a dramatic seasonal run-off with stream discharges rising two or even three orders of magnitude over the course of only a few days to weeks. Although it is less frequent, much larger floods can occur due to unseasonably warm rainfall onto a dense snow pack. Recent events of this nature have occurred within the study area in 1957, 1969, 1997 and 2002. Because of the steep granitic catchments that supply water to the caves of the Sierra Nevada, and the dominance of sinking stream recharge and flooding, sediment flux through cave passages is high, complicating cave conduit development.

Crystal Cave and adjacent small satellite caves are situated at elevations ranging from 4,547 feet to  4774 feet, and occupy the lower 210 feet of a marble ridge approximately 525 feet long and 131 feet wide. Contained within this narrow ridge, there are 3 miles of surveyed cave passages. A high percentage of voids within the bedrock, approximately 11%, have been produced as a result of the extensive cave passages and the small size of the marble lens. The cave and ridge are located between Yucca and Cascade creeks, two tributaries of the North Fork of the Kaweah River. These streams have incised as much as 820 feet into the surrounding bedrock in the vicinity of Crystal Cave, forming deep canyons with steep (up to 55°) hillslopes. Although they are located near the western limit of Pleistocene glaciation, these drainages have remained free of ice during periods of glaciation, so their rugged relief is solely a product of stream incision.

Figure A: This map represents the location of Crystal Cave in the southern Sierra Nevada.
Figure B: Plan view of the cave showing passage layout, hydrology, and geology. Satellite caves shown in dark gray; streams shown by light gray lines, dashed where intermittent or inferred.
Image courtesy of: http://www.caves.org/pub/journal/PDF/V67/v67n2-Despain.pdf

This is a profile view of Crystal Cave and surrounding caves showing pronounced cave levels. The highest level cave, Bear Den, formed first, while the lowest level cave, Entrance Passage, contains an active vadose stream and several shallow phreatic pools. A vadose system is water that is located in the zone of aeration in the earth's crust above the ground water level, while phraetic pools result directly from the groundwater. 

Six distinct levels constitute the Crystal Cave system (the term “level” here refers to a specific phase of cave development and does not imply a lack of passage gradient). The vertical relief of these levels is 210 feet. The largest passages in the cave are contained within four of the levels; these are primarily large, low gradient tubes and wide canyons. The other two levels have smaller, shorter passages with more prominent vadose incision in passage floors. All but one of the levels are connected by steep, narrow, meandering vadose canyons. The highest levels of the cave are located near the eastern, upstream edge of the marble; the subsequent lower levels exhibit a progressive shift of cave development toward the western, downstream edge of the marble. A perennial stream can be found at the lowest level of the cave. An extensive breakdown collapse is superimposed upon previously intact passages in the northern upstream portion of the cave. This collapsed area extends from base-level pools in the lowest sections of Crystal Cave to the overlying hillslope. Included among these collapsed areas are the Dusty Breakdown Cave and the northernmost 213 feet of Crystal Cave, north of the Fault Room.
Image courtesy of: http://www.caves.org/pub/journal/PDF/V67/v67n2-Despain.pdf

Works Cited

Allen, Casey. Professor Allen's Geog 1202 Syllabus. University of Colorado Denver, 2013

Despain, Joel D., and Greg M. Stock. “Geomorphic History of Crystal Cave, Southern Sierra Nevada, California.” Journal of Cave and Karts Studies, v. 67, no. 2, p. 92-102.

NPR. "The World's Tallest Tree is Hiding Somewhere in California." National Public Radio, 08 April, 2011. Accessed 14 April 2013.

NPS. "The General Sherman Tree." National Park Service. Last Updated 23 April 2013. Accessed 14 April 2013.

UANews. "Keepers of the Prometheus: The World's Oldest Tree." University of Arizona, 23 January 2013. Accessed 14 April 2013.

Perhaps one of the most mystifying sights in Sequoia National Park is this great sequoia growing around a boulder. If the boulder appears to be out of place to you, that's because it is. In fact, this boulder is a glacial erratic, meaning that it was carried to this location over dozens of miles by a glacier.

An awe-inspiring view of a giant sequoia grove. The sequoia in the middle of the photograph demonstrates that the soil in which it is growing has been moving down slope in a mass wasting process known as creep. Creep is the slow downhill movement of soil and regolith. The top layer of soil was likely moving faster than the soil beneath, causing the young sequoia to bend down slope. As the roots stabilized the soil, the process of creep was greatly diminished and the sequoia was then able to grow straight.

Yosemite National Park

Perhaps the most recognizable rock formation in the world, Half Dome in Yosemite National Park is a granite dome that rises almost 5,000 ft in the air and 8,900 ft above sea level. George Anderson was the first to reach the summit in 1875, despite an 1865 report declaring that it was "perfectly inaccessible, being probably the only one of the prominent points about the Yosemite which has never been, and never will be, trodden by human foot". Its distinct shape has lead scientists to believe that the other half has either eroded away or was simply never as well formed as the eastern slope. Half Dome rock is the sheerest cliff in North America with an astonishing 93% grade.

Not only is Yosemite National Park the most famous region of the Sierra Nevada, but with 3.7 million visitors per year, it has become one of the most iconic places on Earth. The uplifting of the Sierra Nevada caused two major changes to the landscape: streams became much steeper, which resulted in deeper and narrower canyons; and approximately 1 million years ago, glaciers that were formed in the higher alpine meadows began to move down the river valleys. 

Glaciers are formed when snowfall is exceeded by snowmelt over a number of years. The accumulation zone is the area in which the snow is undergoing a slow process of being transformed into ice. The area wehere the melting occurs is the ablation zone. The line between the two parts is known as the firn line ("firn" means "old snow" in German). If a glacier is in equilibrium, one third of the glacier will be ablation zone and two thirds will be accumulation zones. Even when a glacier is retreating, it will still flow forward, downhill, because they are so massive that the stress of gravity is greater than the force holding together the particles of water. At a depth of about 120 ft, internal deformation of the glacier begins to occer. Meltwater can also aid the movement of glaciers over rock surfaces, this is how they are able to abrade when carrying debris. 
Image courtesy of: http://www.indiana.edu/~sierra/papers/2008/gray_clip_image002.jpg

This diagram is depicting a rock formation known as roche mountenee (sheep rock), which is formed by glacial movement. Yosemite has many examples of this formation, including Lembert Dome in the Toulumne Meadows (shown in the image following this caption), which was given its shape by the Tioga glacier. A rouche mountonee is formed when a sufficiently large glacier reaches a hill. Instead of going around it, the glacier goes over it, leaving behind a smooth slope upstream and a steep slope downstream from where the boulders have been plucked by the departing ice.
Image courtesy of: http://www.indiana.edu/~sierra/papers/2008/gray_clip_image008.jpg

Lembert Dome in the Toulumne Meadows. For an explanation of how this rock formation got its shape, see the previous diagram and its caption.

It is estimated that the ice thickness in Yosemite Valley may have reached up to 4,000 ft during the early glacial episode. The downslope movement of these ice masses are what carved many of the landforms in Yosemite, including its scenic U-shaped valleys, jagged peaks, rounded domes, waterfalls, and moraines. Further evidence of  these ice masses can be seen in the park's abundance of glacially-polished granite. There have been at least four major glaciations that have occurred in the Sierra Nevada, locally called the Sherwin (also called the pre-Tahoe), Tahoe, Tenaya, and Tioga. The largest of the four were the Sherwin glaciers, which filled Yosemite and other canyons in the area. The retreating of these glaciers has often left recessional moraines that impounded lakes such as Lake Yosemite, which is a shallow,  5.5 mile-long lake that periodically covered much of the floor of Yosemite Valley.

One of Yosemite National Park's many majestic waterfalls.

It terms of the geological composition of the park, nearly every landform is cut from the granitic rock of the Sierra Nevada Batholith (as described in detail in my previous past, a batholith is a large mass of igneous rock that formed deep below the Earth's surface). Only about 5% of the landforms in Yosemite are metamorphosed (completely changed in form or nature) sedimentary and volcanic rock. Because these rocks once acted as a roof to the underlying granitic rock, they are often referred to as "roof pendants".

A breathtaking shot of Bridalveil Fall, one of the most prominent waterfalls in the Yosemite Valley. The waterfall is 617 ft in height and flows year round. The Ahwahneechee tribe believed that Bridalveil Fall was home to a vengeful spirit named Pohono which guarded the entrance to the valley, and that those who were leaving the valley mustn't gaze directly into the waterfall lest they be cursed. They also held the belief that inhaling the mist of Bridalveil Fall would improve an individual's chance of getting married. Many hanging valleys (tributary valleys with the floor at a higher relief than the main channel into which it flows) were left behind by the glaciers that carved Yosemite Valley. All of the waterways that feed these falls have carved the hanging valleys into steep cascades, with the exception of Bridalveil Fall. This particular waterfall still enters into the valley from the edge of a precipice, although that edge has moved back into an alcove from the original edge of the valley. The primary source of Bridalveil Fall is Ostrander Lake, which is approximately 9.9 miles to the south of Yosemite Valley.

Works Cited

Allen, Casey. Professor Allen’s Geog 1202 Syllabus. University of Colorado Denver, 2013. Web. 13 February 2013.

Guyton, Bill. Glaciers of California: Modern Glaciers, Ice Age Glaciers, Origin of Yosemite Valley, and a Glacier Tour in the Sierra Nevada: Vol. 59. California Natural History Guides. Berkeley: University of California Press; 1998. Retrieved 7 March 2013.

Hill, Mary. Geology of the Sierra Nevada: Vol. 80. California Natural History Guides. Berkeley: University of California Press; 2006. Retrieved 7 March 2013.

NPS. Half Dome Day Hike. National Park Service, 3 March 2013. Web. 7 March 2013.

NPS. Yosemite National Park. National Park Service, 11 June 2012. Web. 7 March 2013.

World of Earth and Science. “Batholith”. Encyclopedia.com, 2003. Web. 7 March 2013.

One last look back before we leave Yosemite.

Formation of the Sierra Nevada

The Sierra Nevada mountain range is one of the most geographically fascinating places on the face of the earth.  Although it is difficult to fathom, from 400 million to approximately 130 million years ago, an ocean used to cover the area that we now refer to as the Sierra Nevada. The formation of this glorious mountain range began with convergent plate interactions between the North American plate and the ancient Farallon plate. As the denser oceanic Farallon plate subducted beneath the North American plate during the late Paleozoic Era, the immense pressure and friction began to cause the crust of the Farallon plate to melt. As a result, plumes of plutonic rock started to float up towards the surface. The combined mass of these plutons eventually formed the Sierra Nevada batholith; with the earlier plutons forming the western half of the Sierra Nevada and the later plutons forming the eastern half. The result is one of the most spectacular granitic batholiths in the world.

According to the United States Geologic Survey, the definition of a batholith is a very large mass of intrusive (plutonic) igneous rock that forms when magma solidifies at depth. A batholith must have greater than 100 square kilometers (40 square miles) of exposed area.

The Sierra Nevada batholith was eventually emplaced during the Nevadan orogeny, a mountain building episode which occurred during the mid to late Jurassic (180-140 million years ago). Through the processes of uplifting and erosion, the batholith was, in the course of time, exposed on the surface. The resulting mountain chain has played a major role in the history of man, including being the driving force behind the California gold rush. The aforementioned immense pressure combined with mineral-rich solutions moving along cracks in the granitic batholith causes gold, copper, and other minerals to precipitate out.



Works Cited

Allen, Casey. Professor Allen’s Geog 1202 Syllabus. University of Colorado Denver, 2013. Web. 13 February 2013.

Resendes, Mary Ann. Geology of the Sierra Nevada. Central Sierra Historical Society, 2012. Web. 13 February 2013.

USGS. Geologic Glossary. US Geologic Survey Western Earth Surface Team Processes and the National Park Service, 10 February 2000. Web. 13 February 2013.

World of Earth and Science. “Batholith”. Encyclopedia.com, 2003. Web. 14 Feb. 2013.

An Introduction

This blog was initially created as a project for Professor Casey Allen's course, Physical Geography 1202, during the Spring semester of 2013.

 

I have elected to blog about the Sierra Nevada, a mountain range running through the states of California and Nevada. Having grown up in California, my experiences with this incredible granite batholith are many. I have explored some of its greatest geological wonders, including Yosemite and Kings Canyon; I have marveled at the power of glaciers while sunbathing on the shores of Lake Tahoe; and I have stood before the Giant Sequoia trees, humbled as if I had just bore witness to God Himself. Many of the pictures throughout this blog were taken by me during my extensive travels. The Sierra Nevada is a truly magical region of this great country, so come on a journey with me through one of nature's gifts to humanity!