Hotspot: California on the Edge
Mountains of California's Sierra Nevada
Sierra Nevada
Range of Snow and Ice

The Sierra Nevada, which means “snowy range” in Spanish, stretches 400 miles (644 km) along California’s eastern flank. It is bounded on the west by the Great Central Valley and on the east by the Great Basin. In cross section, the Sierra is shaped like a triangular wedge, with a much steeper slope on the east side than the west.

About 1.8 million years ago, not long after the Sierra Nevada began to rise, the Earth’s atmosphere turned cold, marking the beginning of the Pleistocene, the Last Ice Age. Glaciers grew high in the mountains and moved slowly down former stream channels, carving U-shaped valleys as they advanced. Small glaciers still exist at the highest elevations in the Sierra Nevada, where they continue to scour the landscape.  The plants and animals that live at high elevations have special adaptations for surviving the cold, harsh environments.

Massively Cool
The massive formation of exposed rock, called a batholith, makes up most of the Sierra Nevada. Composed mainly of granite, quartz and diorite, it has been and continues to be carved by glaciers.

Another form of erosion called exfoliation causes relatively thin sheets of granite to peel off from the exposed surface of the batholith. These clean breaks in the rock create dramatic sculptures like Half Dome in Yosemite National Park.  

Glaciers leave their signature
Glaciers leave physical evidence of their passage. This rock was scraped by glacial ice and has a shiny polish and scratches that indicate the direction the ice was moving.

Rivers of Ice
A glacier is a large mass of ice that acts like a river moving very slowly downhill. Mountain glaciers begin at high elevation where snowfall excceds snowmelt. In these accumulation zones, snow at the bottom gets compacted by the weight o the new snow above, causing it to turn into dense glacial ice.

Once the ice is thick enough, the glacial mass will move under its own weight. A glacier carves the land as it advances, pushing huge amounts of rock debris, called till, ahead of it and off to the sides. These deposits are called moraines.

What’s that noise?
The sounds you hear were recorded inside a glacier. Listen to the sounds of ice crackling, popping, banging and squeaking as it melts and changes in size.

Why is glacial ice blue?
Glacial ice appears blue because compacted ice absorbs most wavelengths of light and reflects blue light. The deepest blue ice is found in crevasses.

The Pleistocene – The Last Ice Age
About 1.8 million years ago, the Last Ice Age began. A period of long-term cooling of Earth’s climate resulted in the expansion of continental and polar ice sheets and mountain glaciers.

As much as 30% of all the continents were covered by glaciers and parts of the northern oceans were also frozen.

During this Ice Age, no massive ice sheets covered California. Glaciers were confined to high mountains, advancing and retreating 4 to 7 times.

The Last Ice Age ended just 10,000 years ago, but several small glaciers still persist in California at high elevations.

High concentrations of gases like methane and carbon dioxide in the atmosphere trap heat radiating from Earth. Decreased levels of these gases, especially carbon dioxide, could have contributed to the onset of ice ages by cooling Earth’s surface.

The Ice Age Cometh?
Variations in Earth’s orbit around the sun cause cooling of the planet. These changes in Earth’s orbit, tilt and orientation to the sun all affect the amount of solar radiation striking Earth. The Milankovitch theory of climate change suggests that these variations occur on 100,000-year cycles and drive the onset of ice ages.

Earth wobbles on its axis, like a spinning top when it starts to slow down. This affects the tilt of Earth and determines the severity of seasons in the northern or southern hemisphere. The greater the tilt, the more severe the winters and summers. Over 41,000 years, Earth’s tilt will vary between 21.50 degrees and 24.50 degrees. Today, Earth’s tilt is at 23.50 degrees.

Earth's orbit around the sun is not quite circular. At certain times of the year, it is slightly closer to the sun. The combination of tilt and nearness to the sun are thought to control the growth and retreat of ice sheets.

Pleistocene Relicts
Species that lived during the Last Ice Age and still persist today are called Pleistocene relicts. Their present-day distribution is a remnant of their wider range during the Last Ice Age. The grizzly bear and California condor are both Pleistocene relicts – one is extinct in California, the other is barely clinging to existence.

Hope for the California Condor?
They once ranged widely over the dry foothills and mountain ranges of central and southern California where they nested in caves and cliff overhangs.

Destruction of habitat, poaching and lead poisoning led to the California condor’s decline. By 1985 populations plummeted to less than 9 birds in the wild.

At that time, conservation biologists decided to capture the remaining birds to protect and breed them.

Captive breeding programs have released birds back into the wild, but their long-term survival is still in question. Today, 85 of the 222 birds in existence, are in the wild. 
The California condor is the largest land bird in North America.
An adult can weigh up to 25 pounds (11 kg) and have a wing span up to 9.5 feet (3 m). 
Condors can soar for hours at altitudes of 15,000 feet (4572 m), cover hundreds of miles, and reach speeds over 55 mph (89 kph). Like most other vultures, condors eat carrion.

Grizzly Bears in California
Grizzly  bears once roamed the valleys and mountains of California, probably in greater numbers than anywhere else in the continental United States. It was the largest and most powerful mammal in the state. 

Fueled by the discovery of gold, California’s population grew rapidly and humans and grizzlies came into contact more frequently. 

Less than 75 years after the discovery of gold, every grizzly bear in California was gone. The last wild grizzly was killed in 1922.

The story of “Monarch”
Monarch was captured from the wild and died of old age in captivity in 1911. After his death, he was donated to the Academy by the de Young Museum. He remains a legacy to the once mighty California grizzlies.

The California grizzly bear was designated the official state animal in 1953 and appears on the state flag.

Cool Planet
Cool summers in the northern hemisphere, where most of earth's land mass is located, allow snow and ice to remain until the next winter. This contributes to the development of large ice sheets over hundreds to thousands of years.

More ice reflects more of the sun's energy back into space, causing additional cooling. It appears that the amount of carbon dioxide in the atmosphere falls as ice sheets grow, also adding to the cooling of the climate.

Living in a Cold World
Glacial refugia are like habitat islands, where organisms that were adapted to the colder climate of the ice ages, became isolated and still survive today.

Refugia are typically found at high latitudes or high altitudes on mountain tops. But, a few other special habitats, like caves or shaded, cool canyons at lower elevations may also serve as refugia for cold-adapted species. All these areas have a cool or cold microclimate in common, similar to ice-age conditions.

 Many species found here occur nowhere else in the world. These species often occupy a limited geographical area and tolerate only a very narrow range of environmental conditions. 

Over time, as climatic changes alter the environment, future generations must either find more favorable conditions, or adapt to the new climatic conditions. If they do either, they may survive; if they can do neither, extinction is inevitable.

Cold Adaptations
Watermelon snow
Chlamydomonas nivalis

These single-celled green algae are eaten by cold-tolerant animals and bacteria of snowfields and glaciers. Where heavily concentrated, they play an important role in the melting of snow and glaciers. Their red color is caused by secondary pigments produced to protect chlorophyll from damaging ultraviolet light.  

Ice crawlers

Ice crawlers are nocturnal insects that tolerate cold climates and high altitudes, thriving in temperatures just above freezing. Unlike their close relatives, the crickets and grasshoppers, they cannot hear nor can they make any sounds. Many live in snow melt areas or caves where they scavenge plant and animal material on snow fields.

Super Cool

Academy researcher Dave Kavanaugh studies this group of cold-adapted beetles that were once more widely distributed. Populations became isolated in glacial refugia as glaciers retreated and their icy habitat was fragmented into small patches. Nebria have amazing adaptations that allow them to survive the cold temperatures and short growing season at high elevations. Their metabolism is adapted to work very well at low temperatures. They synthesize a chemical which lowers the freezing point of water, and prevents the beetle from freezing solid. These super-cooled beetles regularly withstand subzero winter temperatures.

These beetles come out only at night to feed on organisms living in, or stranded on, permanent snowfields and glaciers. They avoid predation by being nocturnal. If active during the day, these dark beetles would be highly visible against the white snow and easily spotted by predators like the gray-crowned rosy finches.

Slow Change
Species become well-adapted to their environment and to each other only through a slow process of evolution that happens over many generations.

Adaptation is the way living organisms cope with environmental stresses and pressures. Organisms adapted to their environment are able to get food, deal with physical conditions of temperature, humidity, and light, defend themselves from natural enemies and reproduce. All these ecological pressures direct evolutionary change and continued
adaptation. Adaptations happen slowly, over long periods of time, in response to changes in an organism's environment. If changes occur too quickly or are too extreme, the organism may not be able to adapt.

In the face of rapid and dramatic changes, such as destruction of their habitat, loss of food source, or introduction of a new predator, organisms must relocate, if they can, or die. Their departure or loss may trigger other dramatic changes in the rest of their community.

In nature, no organism lives in isolation from its surroundings. Each interacts with its environment and the other organisms that live there. These relationships are fundamental to their survival and the functioning of the ecosystem as a whole. 

Organisms that have inhabited a particular ecosystem for a long time are usually well-adapted to each other and their environment. Their interactions and interdependence may be complex and indirect because of the network of shared interrelations in the entire ecosystem.  Species may affect each other by providing or sharing resources or battling common threats. 

When species live in close association with each other for a long time it is called symbiosis. When the relationship benefits both species it is called mutualism.

Made for Each Other
Clark’s nutcracker lives at high altitudes, between 4900 - 12,000 feet (1494 m - 3658 m), in the Sierra Nevada. It plays a major role in the dispersal of whitebark pine seeds which make up most of its diet. Nutcrackers eat, harvest and store clusters of ripe seeds, called caches.

Seeds are recovered using visual cues and spatial memory for up to 10 months after being cached. Clark’s nutcrackers can recover seeds even if the ground is covered with snow. Not all caches are recovered and large numbers of seeds germinate and establish seedling whitebark pine trees.

In addition to pine seeds, birds will eat insects, bird nestlings, berries and amphibians, including mountain yellow-legged frogs. It is unclear if the bird is dependent on the frogs, but it seems to prey on them in mid-summer and in years when the whitebark cone crop is low.

The harvesting and caching behavior of Clark’s nutcracker is critical for the dispersal of whitebark pines seeds.

Each year from late August to early October, a single nutcracker can cache about 32,000 seeds and will fly as far as 7 miles (11.3 km) away from harvest site to do so.
A special pouch under their tongue, called a sublingual pouch, can hold as many as 150 seeds to be eaten, fed to young or cached. Using their large bills, birds rip open and pry seeds from their shells. They dig holes for caches in open disturbed ground, and favor recently burned areas.

The pine seeds, which are high in fat, carbohydrates and protein are an important food source for many animals including woodpeckers, nutcrackers, jays, ravens, chickadees, chipmunks, finches, bears, and ground squirrels.

The whitebark pine is a hardy sub-alpine conifer that tolerates poor soils, steep slopes and windy exposures at high elevations. It provides shade, moisture and shelter from wind, which facilitates the establishment of conifers and understory vegetation.
Whitebark pine trees depend on Clark’s nutcrakcer for seed dispersal. It’s cones do not open even when ripe. The large wingless seeds, which are not dispersed by wind, must be opened by animals including Clark’s nutcracker.
Bird-dispersed pines develop a distinctive form with a flat top shrubby canopy. Vertically oriented branches with horizontal cones at the tips make the cones more visible to birds like the Clark’s nutcracker.

Vulnerable to Extinction

Why do some species become threatened or endangered and not others? A species is considered to be endangered when its numbers become so reduced that it is likely to go extinct in the near future.

Species can become extinct throughout their entire range or in just part of their geographic range. The loss of a species reduces the biodiversity of its community, affects other species and irreversibly alters the ecosystem.

yosemite toad
Learn more about the Yosemite Toad >

Certain characteristics make species more vulnerable to extinction. Organisms that require large areas to obtain food and mates, that have special diet or habitat requirements, or that have very slow reproductive rates are more vulnerable to extinction.

Small, isolated populations of organisms with limited ability to disperse are also at higher risk. If their only habitat is catastrophically altered or destroyed, they may become extinct.

Too Fast to Recover?
Extinction is a natural process that has been occurring ever since early forms of life evolved. Species have evolved and disappeared throughout geologic time. The inability to adapt to climate change, competition or predation leads to extinction.

Five times in Earth’s past history environmental conditions changed drastically and led to mass extinctions of  over 70% of all species alive at that time.  Each time, however, the surviving species continued to evolve and diversify, leading to greater biodiversity.
Today, most biologists think we are in the middle of a 6th mass extinction event, and this time humans are directly responsible. 

The rate at which species are disappearing is increasing at an alarming rate. Humans are altering the world’s habitats faster than most species can adapt.

Cause and Effect
Mountain yellow-legged frogs were once common in high elevation lakes and streams of the Sierra Nevada. For almost 9 months, both tadpoles and adults spend the winter and early spring under the ice of frozen lakes. They emerge shortly after the snow melts. During the day adults prey on a variety of invertebrates in the water and on land.

Historically, 99% of the high elevation lakes and ponds of the Sierra Nevada were fishless. Cascading waterfalls from hanging valleys carved by glaciers created physical barriers to fish moving upstream to these lakes. These fishless lakes and the land surrounding them teemed with life.

In the 1900's intentional fish introductions for sport- fishing dramatically altered these alpine lake ecosystems. Fish devoured mountain yellow-legged frog tadpoles and adults causing major declines in frog populations. Today, the mountain yellow-legged frog has vanished from 80% of its historic range. It is estimated that populations of these frogs have declined over 90% in the last few decades.

A natural predator of mountain yellow-legged frogs is the mountain garter snake, Thamnophis elegans elegans. The introduced rainbow trout, Oncorhynchus mykiss iridea, caused a major decline in frog populations, and in turn, negatively effected populations of mountain garter snakes. Other predators known to feed on mountain yellow-legged frogs that could be impacted by the disappearance of frogs are coyote, black bear and Clark's nutcracker. If nutcracker populations are negatively affected by the loss of frogs, there may be serious implications for whitebark pine distribution in the Sierra Nevada. Loss of the mountain yellow-legged frog may cascade to lower parts of the food web.

The disappearance of tadpoles could cause changes in the abundance of algae found
in lakes that could in turn alter the abundance of aquatic insects that feed on algae.
Mountain yellow-legged frogs have a short, raspy call.  They call primarily underwater during the day, but may also call at night.

The frogs on display in the exhibit were raised in captivity from field-collected eggs.

Fishing for Solutions
A 7-year study conducted by researcher Roland Knapp and his team from the University of California's Sierra Nevada Aquatic Research Laboratory, removed non-native fish populations from several high elevation lakes. After fish removal, frog populations increased rapidly.

Restoring an Alpine Ecosystem
Following fish removal, lakes with only 20 adults and 20 tadpoles of mountain yellow-legged frogs now have over 1500 of each life stage. The abundance of invertebrate fauna in the lakes increased more than ten-fold in the absence of the fish predation.

Recovery of the aquatic ecosystem has also positively impacted the surrounding terrestrial ecosystem, particularly for birds like gray-crowned rosy finches, that feed on caddisflies, mayflies and other invertebrates.

Vectors of Disease
In the last few years, a newly identified fungal pathogen poses a new threat to mountain yellow-legged frogs. The disease is causing major die-offs of frog populations even from lakes that were never stocked with fish.

UC Berkeley researchers Cherie Briggs, Vance Vredenburg and other colleagues are studying the mode of transmission of this disease. The outcome of this research will help shape recovery efforts for the mountain yellow-legged frog.

Metal of Madness
The discovery of gold in 1848 launched the largest global human migration in history. Over half a million people from around the world descended on California, changing its biological diversity and culture forever. 

The Academy has been a leader in environmental conservation since it was founded in 1853. Academy staff were among the first to study the environmental ravages caused by placer mining in the foothills of the Sierra Nevada, leading to major changes in mining regulation and extraction practices.

The Mother Lode
California's gold deposits were formed under great pressure and heat in the rocks of the Sierra Nevada. Circulating ground water, heated by magma, intruded into fractures in Earth's crust.

The hot water dissolved minerals from surrounding rocks. As the water reached cooler rocks, fine particles of gold and quartz separated out, leaving veins of deposits in the rock.

The "Mother Lode" is an area of active faults that extends along the western flank of the Sierra Nevada. The gold for which California is famous was deposited in fractures and broken rocks along these faults.

There are two types of gold deposits in the Sierra Nevada, lode and placer.
Lode gold,
found in the rocks in which it formed, must be dug out of the ground. It is often in the form of crystalline wires or leaves, held fast inside white milky-quartz. To remove the gold, the rock is crushed and then washed with water which sweeps away light-weight particles and leaves the heavy gold behind.

When rock erodes, particles of gold and other minerals wash down streams, along with sand and gravel. These deposits are called placers which tend to be in the form of flakes and nuggets. Gold is extremely dense and can be separated from other particles in sediment by panning and other simple methods.

Mountain Building - Millions of Years in the Making

The building of the Sierra Nevada is a complex process that began about 60 million years ago. Volcanoes lay where the range is today, marking the western edge of North America.
Around that time, the boundary between the tectonic plates began to change. The subduction of the Pacific plate under the North American plate, which fed the volcanoes with molten rock, slowed and eventually stopped.

Residual heat, combined with pressure from below, changed the older volcanic rocks above into metamorphic rocks.

Huge chambers of magma deep in the Earth cooled and slowly crystallized into granitic rock. As they solidified, these lighter plutonic rocks, began to rise, “floating” up through the older, dense volcanic rock. Hydrothermal fluids penetrated the old rocks, sometimes depositing minerals like gold and milky quartz.

Boulders to Sand
As the ancient mountains slowly rose, overlying older rocks and younger volcanic rocks  were eroded by wind, water and ice. A network of streams flowing from the mountains carried the particles of broken rock westward into the ancestral Sacramento River system.
The long basin between the rising mountains to the east and coastal range to the west gradually filled with these sediments, forming the Great Central Valley. In some places, the accumulated sediments are up to 10,000 feet (3048 m) thick.

The rocks types displayed here, granodiorite, gabbro, sandstone and slate, all contributed to the thick layers of sediment in the Great Central Valley. Deposits of these rocks on the west slope of the Sierra Nevada are still eroding and contributing sediment to the Valley today.

Sandstone is sedimentary rock composed of grains eroded from other rocks. Its color is determined by the minerals it contains.

Granodiorite is part of the batholith and makes up most of the rock in the Sierra Nevada. It is similar to granite in texture, but contains more feldspar and less quartz.

Gabbro is a dark, coarse-grained, intrusive igneous rock chemically equivalent to basalt. It forms when molten magma is trapped beneath Earth's surface and cools slowly into a hard, coarsely crystalline mass.

Slate (large slab in back) is fine-grained, layered metamorphic rock. Its parent rock is usually a fine-grained sediment like shale or claystone.

Fire Suppression
Restoration of natural fire cycles is important to restoring and protecting biodiversity in the Sierra Nevada. After a fire, the sequence of recovery of native plants provides wildlife with a variety of ecological opportunities.

Global Warming
As concentrations of greenhouse gases in the atmosphere increase temperatures, glaciers are retreating at an accelerated rate. California’s small mountain glaciers are particularly sensitive to warming.

In 1913, after a bitter battle, Congress gave San Francisco permission to dam the Tuolumne River at Hetch Hetchy Valley in Yosemite National Park. Following this action, legislation was passed that made it illegal to ever put a dam in any national park again.
Studies by the University of California, Davis show it may be possible to restore the valley by impounding Tuolumne River water at existing dams further downstream.

What You Can Do

  • Plan an educational adventure in the Sierra Nevada with Yosemite Institute:
  • Learn more about potential restoration of the Hetch Hetchy Valley:

Above: Mountains of California's Sierra Nevada

Glaciers - massively cool

Pleistocene - the last ice age

Pleistocene relicts

California Condor

Grizzly Bear

Living in a cold world


Nutcracker & Pine

Clark's nutcracker

Endangered - vulnerable to extinction

yosemite toad

Mountain yellow-legged frog

Gold - metal of madness

Boulder to Sand


Fire Suppression

Global Warming


What You Can Do