The Rockies, Sierra Nevada, and Pacific Coast - Fourth Grade Social Studies
Stand on a rugged sea cliff in northern California watching emerald waves smash against ancient sea stacks, or gaze up at the sky-piercing granite crests of the Rocky Mountains crowned in permanent glaciers, and you will understand why the American West is considered a land of titanic geological drama. Encompassing eleven mainland states—plus the non-contiguous giants of Alaska and Hawaii—the West is the largest and most physically diverse geographic region in the United States. Here, tectonic plates grind against one another, lifting jagged mountain chains three miles into the clouds, opening deep fertile agricultural valleys, and shaping over a thousand miles of rocky, wave-swept Pacific coastline. The majestic physical features of the West have guided pioneer trails, inspired national parks, and formed the natural framework for booming modern cities.
The Mountain Spines: Rockies, Cascades, and Sierra Nevada
The West is dominated by three colossal mountain systems that run generally north-to-south across the continent.
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| THE THREE GREAT MOUNTAIN SYSTEMS OF THE WEST |
| |
| ROCKY MOUNTAINS CASCADE RANGE SIERRA NEVADA |
| MT, WY, CO, UT, NM WA, OR, Northern CA Eastern CA & NV |
| * Continental Divide * Volcanic chain * Massive block |
| * Jagged granite peaks * Glaciated volcanoes * Yosemite peaks|
| * 3,000 miles long * Mount Rainier, St. Hel.* Mount Whitney |
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The Rocky Mountains and the Continental Divide
The Rocky Mountains form the longest mountain system in North America, stretching more than three thousand miles from northern British Columbia in Canada down into New Mexico. In Colorado alone, more than fifty mountain summits rise above fourteen thousand feet in elevation (affectionately called "fourteeners"). Running along the jagged crest of the Rockies is the Continental Divide (also called the Great Divide). An imaginary hydrological dividing line, the Continental Divide dictates the flow of all running water on the continent: rain or melting snow falling on the eastern side of the divide drains toward the Atlantic Ocean or Gulf of Mexico, while water falling on the western side drains toward the Pacific Ocean!
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| THE CONTINENTAL DIVIDE |
| |
| /\ |
| / \ [CREST OF ROCKY MOUNTAINS] |
| / \ |
| WESTERN SLOPE / \ EASTERN SLOPE |
| Rain & snowmelt drains / * \ Rain & snowmelt drains
| WEST toward the / (DIVIDE) \ EAST toward the |
| PACIFIC OCEAN / \ ATLANTIC OCEAN |
| (Colorado & Columbia) ~~~~~~~~~~~~~~~~~~ (Mississippi Basin) |
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The Cascade Range: Sleeping Volcanic Giants
Running through Washington, Oregon, and northern California, the Cascade Range is part of the Pacific "Ring of Fire," a horseshoe-shaped zone of active volcanoes and earthquakes encircling the Pacific Ocean. Unlike the folded granite peaks of the Rockies, the Cascades are a chain of towering volcanic cones topped with massive glaciers, including Mount Rainier (14,411 feet), Mount Hood, Mount Shasta, and Mount St. Helens (which famously erupted in a cataclysmic blast in May 1980).
The Sierra Nevada: The Snowy Range
In eastern California sits the Sierra Nevada (Spanish for "Snowy Mountain Range"). The Sierra is a single, gigantic tilted block of granite four hundred miles long. It contains Mount Whitney, which at 14,505 feet is the tallest mountain peak in the contiguous forty-eight United States. Glaciers carved the high granite peaks of the Sierra into world-famous wonders like Yosemite Valley, where sheer vertical stone cliffs like El Capitan and Half Dome rise thousands of feet above roaring waterfalls.
The Great Basin: A Land with No Ocean Outlet
Between the Sierra Nevada on the west and the Rocky Mountains on the east lies a vast, bowl-shaped physical province known as the Great Basin.
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| THE GREAT BASIN DRAINAGE |
| |
| SIERRA NEVADA ROCKY MTNS |
| (Western Rim) (Eastern Rim) |
| /\ /\ |
| / \ THE GREAT BASIN (Nevada & Utah) / \ |
| / \ \ / / \ |
| / \ `---> SINK LAKES <----' / \ |
| (Great Salt Lake) |
| |
| INTERNAL DRAINAGE: Rivers NEVER reach the ocean! All water flows into |
| inland salt lakes or evaporates in the desert air. |
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Internal Drainage and the Great Salt Lake
Covering nearly all of Nevada and western Utah, the Great Basin is characterized by internal drainage. Because high mountain rims completely enclose the basin, rivers and streams that flow down from the mountains have no physical outlet to any ocean. Instead, river water flows into shallow inland lakes and marshes, where it can only escape through evaporation into the hot, dry air. As the water evaporates, it leaves behind dissolved mineral salts. This process created the Great Salt Lake in Utah, which is up to eight times saltier than the ocean—so salty that swimmers can easily float on top of the water without sinking!
Fertile Valleys: The Central Valley and Willamette Valley
Flanked by towering mountain ranges are some of the most productive agricultural valleys in the world.
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| WESTERN AGRICULTURAL VALLEYS |
| |
| CALIFORNIA CENTRAL VALLEY: 450-mile-long valley between the Sierra |
| Nevada and Coast Ranges; deep alluvial |
| soil; produces 25% of the nation's food! |
| |
| WILLAMETTE VALLEY (Oregon): Lush, rain-fed valley between the Cascades|
| and Coast Range; historic goal of pioneers|
| on the Oregon Trail; berries and hazelnuts|
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California's Central Valley: America's Salad Bowl
Stretching 450 miles down the center of California, the Central Valley is a flat agricultural marvel divided into the Sacramento Valley in the north and the San Joaquin Valley in the south. Formed by millions of years of river silt washing down from the surrounding mountains, the valley possesses deep, rich soils. Fed by massive federal and state irrigation aqueducts that channel Sierra Nevada snowmelt, the Central Valley produces over four hundred different commercial crops, supplying more than two-thirds of the nation's fresh fruits and nuts and one-third of its vegetables.
The Pacific Coastline: Cliffs, Bays, and the San Andreas Fault
The coastline of the West is marked by dramatic cliffs that plunge directly into the cold Pacific Ocean.
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| THE PACIFIC RIM AND THE SAN ANDREAS FAULT |
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| PACIFIC TECTONIC PLATE NORTH AMERICAN TECTONIC PLATE |
| (Sliding Northwest at 2 in/year) (Moving Southeast) |
| ^ | |
| \ v |
| ================================= |
| THE SAN ANDREAS FAULT LINE |
| Friction locks plates together ===> Tension snaps ===> EARTHQUAKE! |
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Living on a Plate Boundary: The San Andreas Fault
Unlike the passive Atlantic coast, the Pacific coast sits directly along an active tectonic plate boundary. In California, the Pacific Plate and the North American Plate slide past each other along an 800-mile fracture zone called the San Andreas Fault. As the plates move, rock edges lock together until built-up friction suddenly snaps, releasing seismic shockwaves that cause earthquakes. Historic earthquakes, like the catastrophic 1906 San Francisco earthquake, forced western architects and civil engineers to pioneer earthquake-resistant building codes and flexible bridge designs.
Chapter Practice Exercises
Question 1: What is the Continental Divide, and how does it determine where river water flows in North America?
Question 2: Contrast the geological formation of the Cascade Range with that of the Rocky Mountains.
Question 3: Explain the concept of internal drainage in the Great Basin and describe how the Great Salt Lake became so salty.
Question 4: What is the San Andreas Fault, and what natural hazard does it create for residents living along the Pacific coast?
Solutions and Explanations
Solution 1: The Continental Divide is an imaginary hydrological dividing line that follows the highest crests of the Rocky Mountains. It determines river drainage: all precipitation falling on the eastern side drains eastward toward the Atlantic Ocean or Gulf of Mexico, while all precipitation falling on the western side drains westward toward the Pacific Ocean.
Solution 2: The Cascade Range is a volcanic mountain chain formed along the Pacific "Ring of Fire," composed of glaciated volcanic peaks and cones created by tectonic subduction. The Rocky Mountains, in contrast, were formed primarily by massive tectonic plate compression and collision that buckled and uplifted ancient granite and sedimentary bedrock.
Solution 3: Internal drainage means that rivers and streams inside a closed basin have no geographic outlet to any sea or ocean because they are encircled by mountain rims. In the Great Basin, water flows into shallow lakes and evaporates in the arid air; as water continuously evaporates, dissolved mineral salts are left behind, making the Great Salt Lake several times saltier than the ocean.
Solution 4: The San Andreas Fault is an 800-mile geological transform boundary in California where the Pacific Plate and the North American Plate grind past each other horizontally. As the plates lock and suddenly slip, the released energy triggers earthquakes, creating a serious seismic hazard for coastal cities like San Francisco and Los Angeles.