Textbook: Origin, Distribution, and Systems of Cities | AP Human Geography - Grade 9 PDF Download

 Page 1


UNIT 6: CITIES AND URBAN LAND-USE PATTERNS AND PROCESSES 365
UNIT 6
Cities and Urban Land-Use 
Patterns and Processes
Chapter 15 Origin, Distribution, and Systems of Cities
Chapter 16 Urban Structure
Chapter 17 Urban Challenges and Sustainability
Unit Overview
Cities and the suburbs around them are constantly changing—in how they are 
laid out, how they work, and how large they are. One of the most basic questions 
geographers study is why people move into, within, or out of various parts of 
urban areas.
Models of Urban Areas
Geographers create models to show the distribution and size of cities. They 
identify patterns that help explain why cities grow to various sizes and how 
people in different cities are connected to each other. Other models help 
geographers analyze how cities are organized and develop. Cities generally 
have zones for commerce, housing, and other functions.
Urban Landscapes and Urban Challenges
People express their attitudes and values through the landscapes they build and 
how they organize social spaces. The choices people make, such as how closely 
they live to others and where to build an airport, reflect what they consider 
important.
Large concentrations of people can produce both great opportunities for 
progress and great challenges. Some challenges result from decline, such as the 
movement of industry out of cities. Others result from sustainability, such as 
how to keep air and water clean. 
ENDURING UNDERSTANDINGS
(PSO-6)  The presence and growth of cities vary across geographical locations 
because of physical geography and resources. 
(IMP-6)   The attitudes and values of a population, as well as the balance of power 
within that population, are reflected in the built landscape. 
(SPS-6)  Urban areas face unique economic, political, cultural, and environmental 
challenges. 
Source: AP® Human Geography Course and Exam Description. Effective Fall 2020. (College Board). 
Page 2


UNIT 6: CITIES AND URBAN LAND-USE PATTERNS AND PROCESSES 365
UNIT 6
Cities and Urban Land-Use 
Patterns and Processes
Chapter 15 Origin, Distribution, and Systems of Cities
Chapter 16 Urban Structure
Chapter 17 Urban Challenges and Sustainability
Unit Overview
Cities and the suburbs around them are constantly changing—in how they are 
laid out, how they work, and how large they are. One of the most basic questions 
geographers study is why people move into, within, or out of various parts of 
urban areas.
Models of Urban Areas
Geographers create models to show the distribution and size of cities. They 
identify patterns that help explain why cities grow to various sizes and how 
people in different cities are connected to each other. Other models help 
geographers analyze how cities are organized and develop. Cities generally 
have zones for commerce, housing, and other functions.
Urban Landscapes and Urban Challenges
People express their attitudes and values through the landscapes they build and 
how they organize social spaces. The choices people make, such as how closely 
they live to others and where to build an airport, reflect what they consider 
important.
Large concentrations of people can produce both great opportunities for 
progress and great challenges. Some challenges result from decline, such as the 
movement of industry out of cities. Others result from sustainability, such as 
how to keep air and water clean. 
ENDURING UNDERSTANDINGS
(PSO-6)  The presence and growth of cities vary across geographical locations 
because of physical geography and resources. 
(IMP-6)   The attitudes and values of a population, as well as the balance of power 
within that population, are reflected in the built landscape. 
(SPS-6)  Urban areas face unique economic, political, cultural, and environmental 
challenges. 
Source: AP® Human Geography Course and Exam Description. Effective Fall 2020. (College Board). 
366 CHAPTER 15: ORIGIN, DISTRIBUTION, AND SYSTEMS OF CITIES
Origin, Distribution, and 
Systems of Cities
Topics 6.1–6.4
Topic 6.1 The Origin and Influences of Urbanization
Learning Objective: Explain the processes that initiate and drive urbanization 
and suburbanization. (PSO-6.A) 
Topic 6.2 Cities Across the World
Learning Objective: Explain the processes that initiate and drive urbanization 
and suburbanization. (PSO-6.A) 
Topic 6.3 Cities and Globalization
Learning Objective: Explain how cities embody processes of globalization. 
(PSO-6.B)
Topic 6.4 The Size and Distribution of Cities
Learning Objective: Identify the different urban concepts such as hierarchy, 
interdependence, relative size, and spacing that are useful for explaining the 
distribution, size, and interaction of cities. (PSO-6.C)  
Cities are extremely local and intimate places…. At the same time 
they are the product of complex interactions with other places near 
and far away.
—David Lanegran, The Introductory Reader in Human Geography 
Source: David Palmer
Cities develop and change over time as illustrated by the contrast of the historic core and 
contemporary skyscrapers of London. (See topic 5.1 for factors that influence how cities 
develop.)
CHAPTER 15
Page 3


UNIT 6: CITIES AND URBAN LAND-USE PATTERNS AND PROCESSES 365
UNIT 6
Cities and Urban Land-Use 
Patterns and Processes
Chapter 15 Origin, Distribution, and Systems of Cities
Chapter 16 Urban Structure
Chapter 17 Urban Challenges and Sustainability
Unit Overview
Cities and the suburbs around them are constantly changing—in how they are 
laid out, how they work, and how large they are. One of the most basic questions 
geographers study is why people move into, within, or out of various parts of 
urban areas.
Models of Urban Areas
Geographers create models to show the distribution and size of cities. They 
identify patterns that help explain why cities grow to various sizes and how 
people in different cities are connected to each other. Other models help 
geographers analyze how cities are organized and develop. Cities generally 
have zones for commerce, housing, and other functions.
Urban Landscapes and Urban Challenges
People express their attitudes and values through the landscapes they build and 
how they organize social spaces. The choices people make, such as how closely 
they live to others and where to build an airport, reflect what they consider 
important.
Large concentrations of people can produce both great opportunities for 
progress and great challenges. Some challenges result from decline, such as the 
movement of industry out of cities. Others result from sustainability, such as 
how to keep air and water clean. 
ENDURING UNDERSTANDINGS
(PSO-6)  The presence and growth of cities vary across geographical locations 
because of physical geography and resources. 
(IMP-6)   The attitudes and values of a population, as well as the balance of power 
within that population, are reflected in the built landscape. 
(SPS-6)  Urban areas face unique economic, political, cultural, and environmental 
challenges. 
Source: AP® Human Geography Course and Exam Description. Effective Fall 2020. (College Board). 
366 CHAPTER 15: ORIGIN, DISTRIBUTION, AND SYSTEMS OF CITIES
Origin, Distribution, and 
Systems of Cities
Topics 6.1–6.4
Topic 6.1 The Origin and Influences of Urbanization
Learning Objective: Explain the processes that initiate and drive urbanization 
and suburbanization. (PSO-6.A) 
Topic 6.2 Cities Across the World
Learning Objective: Explain the processes that initiate and drive urbanization 
and suburbanization. (PSO-6.A) 
Topic 6.3 Cities and Globalization
Learning Objective: Explain how cities embody processes of globalization. 
(PSO-6.B)
Topic 6.4 The Size and Distribution of Cities
Learning Objective: Identify the different urban concepts such as hierarchy, 
interdependence, relative size, and spacing that are useful for explaining the 
distribution, size, and interaction of cities. (PSO-6.C)  
Cities are extremely local and intimate places…. At the same time 
they are the product of complex interactions with other places near 
and far away.
—David Lanegran, The Introductory Reader in Human Geography 
Source: David Palmer
Cities develop and change over time as illustrated by the contrast of the historic core and 
contemporary skyscrapers of London. (See topic 5.1 for factors that influence how cities 
develop.)
CHAPTER 15
6.1: THE ORIGIN AND INFLUENCE OF URBANIZATION 367
6.1
The Origin and Influence of 
Urbanization
Essential Question: What are the processes that initiate and drive 
urbanization? 
T he permanently inhabited portion of the earth’s surface—what the classical 
Greeks called the ecumene—is a variety of community types with a range of 
population densities. As humans increasingly settled in permanent locations, 
classifications of settlements emerged: 
• Rural areas (farms and villages) with low concentrations of people
• Urban areas (cities) with high concentrations of people
• Suburbs that are primarily residential areas near cities
Factors Driving Urbanization 
A settlement is a place with a permanent human population. The first agricultural 
settlements appeared around 12,000 years ago. Before that, people survived by 
hunting and gathering, so they lived in temporary or movable shelters. The first 
permanent settlements were small enough that the inhabitants could all farm and 
subsist on the surrounding fields. Over time, in several places around the world, 
small agricultural settlements began to develop characteristics that made them the 
first true urban settlements, or cities:
• the presence of an agricultural surplus
• the rise of social stratification and a leadership class or urban elite
• the beginning of job specialization
A food surplus became available as irrigation, farming, and domestication 
of animals and plants developed. These changes enabled increasing numbers 
of people to live in the same location. A ruling class emerged to control the 
products that were accumulated and the people living in the community. 
Because not everyone was needed to produce food, some people specialized in 
making things, such as tools, weapons, and art. Others specialized as accountants 
or religious leaders—the first members of a service sector. As a result, cities 
developed as economic centers of services, manufacturing, and trade. 
Urbanization
The process of developing towns and cities is known as urbanization, an 
ongoing process that does not end once a city is formed. Urbanization also 
Page 4


UNIT 6: CITIES AND URBAN LAND-USE PATTERNS AND PROCESSES 365
UNIT 6
Cities and Urban Land-Use 
Patterns and Processes
Chapter 15 Origin, Distribution, and Systems of Cities
Chapter 16 Urban Structure
Chapter 17 Urban Challenges and Sustainability
Unit Overview
Cities and the suburbs around them are constantly changing—in how they are 
laid out, how they work, and how large they are. One of the most basic questions 
geographers study is why people move into, within, or out of various parts of 
urban areas.
Models of Urban Areas
Geographers create models to show the distribution and size of cities. They 
identify patterns that help explain why cities grow to various sizes and how 
people in different cities are connected to each other. Other models help 
geographers analyze how cities are organized and develop. Cities generally 
have zones for commerce, housing, and other functions.
Urban Landscapes and Urban Challenges
People express their attitudes and values through the landscapes they build and 
how they organize social spaces. The choices people make, such as how closely 
they live to others and where to build an airport, reflect what they consider 
important.
Large concentrations of people can produce both great opportunities for 
progress and great challenges. Some challenges result from decline, such as the 
movement of industry out of cities. Others result from sustainability, such as 
how to keep air and water clean. 
ENDURING UNDERSTANDINGS
(PSO-6)  The presence and growth of cities vary across geographical locations 
because of physical geography and resources. 
(IMP-6)   The attitudes and values of a population, as well as the balance of power 
within that population, are reflected in the built landscape. 
(SPS-6)  Urban areas face unique economic, political, cultural, and environmental 
challenges. 
Source: AP® Human Geography Course and Exam Description. Effective Fall 2020. (College Board). 
366 CHAPTER 15: ORIGIN, DISTRIBUTION, AND SYSTEMS OF CITIES
Origin, Distribution, and 
Systems of Cities
Topics 6.1–6.4
Topic 6.1 The Origin and Influences of Urbanization
Learning Objective: Explain the processes that initiate and drive urbanization 
and suburbanization. (PSO-6.A) 
Topic 6.2 Cities Across the World
Learning Objective: Explain the processes that initiate and drive urbanization 
and suburbanization. (PSO-6.A) 
Topic 6.3 Cities and Globalization
Learning Objective: Explain how cities embody processes of globalization. 
(PSO-6.B)
Topic 6.4 The Size and Distribution of Cities
Learning Objective: Identify the different urban concepts such as hierarchy, 
interdependence, relative size, and spacing that are useful for explaining the 
distribution, size, and interaction of cities. (PSO-6.C)  
Cities are extremely local and intimate places…. At the same time 
they are the product of complex interactions with other places near 
and far away.
—David Lanegran, The Introductory Reader in Human Geography 
Source: David Palmer
Cities develop and change over time as illustrated by the contrast of the historic core and 
contemporary skyscrapers of London. (See topic 5.1 for factors that influence how cities 
develop.)
CHAPTER 15
6.1: THE ORIGIN AND INFLUENCE OF URBANIZATION 367
6.1
The Origin and Influence of 
Urbanization
Essential Question: What are the processes that initiate and drive 
urbanization? 
T he permanently inhabited portion of the earth’s surface—what the classical 
Greeks called the ecumene—is a variety of community types with a range of 
population densities. As humans increasingly settled in permanent locations, 
classifications of settlements emerged: 
• Rural areas (farms and villages) with low concentrations of people
• Urban areas (cities) with high concentrations of people
• Suburbs that are primarily residential areas near cities
Factors Driving Urbanization 
A settlement is a place with a permanent human population. The first agricultural 
settlements appeared around 12,000 years ago. Before that, people survived by 
hunting and gathering, so they lived in temporary or movable shelters. The first 
permanent settlements were small enough that the inhabitants could all farm and 
subsist on the surrounding fields. Over time, in several places around the world, 
small agricultural settlements began to develop characteristics that made them the 
first true urban settlements, or cities:
• the presence of an agricultural surplus
• the rise of social stratification and a leadership class or urban elite
• the beginning of job specialization
A food surplus became available as irrigation, farming, and domestication 
of animals and plants developed. These changes enabled increasing numbers 
of people to live in the same location. A ruling class emerged to control the 
products that were accumulated and the people living in the community. 
Because not everyone was needed to produce food, some people specialized in 
making things, such as tools, weapons, and art. Others specialized as accountants 
or religious leaders—the first members of a service sector. As a result, cities 
developed as economic centers of services, manufacturing, and trade. 
Urbanization
The process of developing towns and cities is known as urbanization, an 
ongoing process that does not end once a city is formed. Urbanization also 
368 HUMAN GEOGRAPHY: AP
®
 EDITION
involves the causes and effects of existing cities’ growth. Describing a region as 
urbanized indicates that cities are present there. A common statistic associated 
with regions, countries, and even continents is percent urban—an indicator of 
the proportion of the population that lives in cities and towns as compared to 
those that live in rural areas. 
Urbanization is one of the most important phenomena of the 19
th
 and 20
th
 
centuries, and geographers continue to study its development. Today, more 
than 50 percent of the world’ s population lives in cities. Demographers estimate 
that by the year 2030, 60 percent will live in cities, and nearly 70 percent by 
2050. Most of those people will be in the less-developed countries (LDCs) of 
the world’s periphery and semiperiphery. While urbanization can be positive 
for both individuals and societies, the challenges may be overwhelmingif a city 
is not prepared to grow or if urbanization occurs too rapidly.
Influence of Site and Situation on Cities
The location of where cities develop is a critical aspect of urban geography. 
The concepts of site and situation, introduced in Topic 1.4, play a key role in 
explaining this process. Site describes the characteristics at the immediate 
location—for example, physical features, climate, labor force, and human 
structures. In contrast, situation refers to the location of a place relative to 
its surroundings and its connectivity to other places. Examples would include 
near a gold mine, on the coast, or by the railroad. Important factors of site and 
situation today are different than past cities. 
The site and situation of a city influences its function. Specialized functions 
of cities include defense, religion, trade, education, finance, transportation, 
government, manufacturing, retirement, entertainment, residential housing, 
or service centers. Larger cities often have multiple functions. Cities near 
natural ports, such as Boston or New York City, started as centers of trade but 
provide multiple functions today. 
Early City-States
Historically, a city-state consisted of an urban center (the city) and its 
surrounding territory and agricultural villages. A city-state had its own political 
system and functioned independently from other city-states. The population in 
the surrounding villages and territory received services and protection from 
the urban center. These communities were often raided by other groups for 
their wealth. As a result, defense was a primary consideration, and military 
leaders evolved into political rulers, or kings.
Early city-states emerged in several locations around the globe in an urban 
hearth, or area generally associated with defensible sites and river valleys in 
which seasonal floods and fertile soils allowed for an agricultural surplus:
• the Tigris-Euphrates V alley (Mesopotamia) in modern Iraq
• the Nile River V alley and Nile Delta in modern Egypt
• the Indus River V alley in modern Pakistan
• the Huang-He floodplain in modern China
Page 5


UNIT 6: CITIES AND URBAN LAND-USE PATTERNS AND PROCESSES 365
UNIT 6
Cities and Urban Land-Use 
Patterns and Processes
Chapter 15 Origin, Distribution, and Systems of Cities
Chapter 16 Urban Structure
Chapter 17 Urban Challenges and Sustainability
Unit Overview
Cities and the suburbs around them are constantly changing—in how they are 
laid out, how they work, and how large they are. One of the most basic questions 
geographers study is why people move into, within, or out of various parts of 
urban areas.
Models of Urban Areas
Geographers create models to show the distribution and size of cities. They 
identify patterns that help explain why cities grow to various sizes and how 
people in different cities are connected to each other. Other models help 
geographers analyze how cities are organized and develop. Cities generally 
have zones for commerce, housing, and other functions.
Urban Landscapes and Urban Challenges
People express their attitudes and values through the landscapes they build and 
how they organize social spaces. The choices people make, such as how closely 
they live to others and where to build an airport, reflect what they consider 
important.
Large concentrations of people can produce both great opportunities for 
progress and great challenges. Some challenges result from decline, such as the 
movement of industry out of cities. Others result from sustainability, such as 
how to keep air and water clean. 
ENDURING UNDERSTANDINGS
(PSO-6)  The presence and growth of cities vary across geographical locations 
because of physical geography and resources. 
(IMP-6)   The attitudes and values of a population, as well as the balance of power 
within that population, are reflected in the built landscape. 
(SPS-6)  Urban areas face unique economic, political, cultural, and environmental 
challenges. 
Source: AP® Human Geography Course and Exam Description. Effective Fall 2020. (College Board). 
366 CHAPTER 15: ORIGIN, DISTRIBUTION, AND SYSTEMS OF CITIES
Origin, Distribution, and 
Systems of Cities
Topics 6.1–6.4
Topic 6.1 The Origin and Influences of Urbanization
Learning Objective: Explain the processes that initiate and drive urbanization 
and suburbanization. (PSO-6.A) 
Topic 6.2 Cities Across the World
Learning Objective: Explain the processes that initiate and drive urbanization 
and suburbanization. (PSO-6.A) 
Topic 6.3 Cities and Globalization
Learning Objective: Explain how cities embody processes of globalization. 
(PSO-6.B)
Topic 6.4 The Size and Distribution of Cities
Learning Objective: Identify the different urban concepts such as hierarchy, 
interdependence, relative size, and spacing that are useful for explaining the 
distribution, size, and interaction of cities. (PSO-6.C)  
Cities are extremely local and intimate places…. At the same time 
they are the product of complex interactions with other places near 
and far away.
—David Lanegran, The Introductory Reader in Human Geography 
Source: David Palmer
Cities develop and change over time as illustrated by the contrast of the historic core and 
contemporary skyscrapers of London. (See topic 5.1 for factors that influence how cities 
develop.)
CHAPTER 15
6.1: THE ORIGIN AND INFLUENCE OF URBANIZATION 367
6.1
The Origin and Influence of 
Urbanization
Essential Question: What are the processes that initiate and drive 
urbanization? 
T he permanently inhabited portion of the earth’s surface—what the classical 
Greeks called the ecumene—is a variety of community types with a range of 
population densities. As humans increasingly settled in permanent locations, 
classifications of settlements emerged: 
• Rural areas (farms and villages) with low concentrations of people
• Urban areas (cities) with high concentrations of people
• Suburbs that are primarily residential areas near cities
Factors Driving Urbanization 
A settlement is a place with a permanent human population. The first agricultural 
settlements appeared around 12,000 years ago. Before that, people survived by 
hunting and gathering, so they lived in temporary or movable shelters. The first 
permanent settlements were small enough that the inhabitants could all farm and 
subsist on the surrounding fields. Over time, in several places around the world, 
small agricultural settlements began to develop characteristics that made them the 
first true urban settlements, or cities:
• the presence of an agricultural surplus
• the rise of social stratification and a leadership class or urban elite
• the beginning of job specialization
A food surplus became available as irrigation, farming, and domestication 
of animals and plants developed. These changes enabled increasing numbers 
of people to live in the same location. A ruling class emerged to control the 
products that were accumulated and the people living in the community. 
Because not everyone was needed to produce food, some people specialized in 
making things, such as tools, weapons, and art. Others specialized as accountants 
or religious leaders—the first members of a service sector. As a result, cities 
developed as economic centers of services, manufacturing, and trade. 
Urbanization
The process of developing towns and cities is known as urbanization, an 
ongoing process that does not end once a city is formed. Urbanization also 
368 HUMAN GEOGRAPHY: AP
®
 EDITION
involves the causes and effects of existing cities’ growth. Describing a region as 
urbanized indicates that cities are present there. A common statistic associated 
with regions, countries, and even continents is percent urban—an indicator of 
the proportion of the population that lives in cities and towns as compared to 
those that live in rural areas. 
Urbanization is one of the most important phenomena of the 19
th
 and 20
th
 
centuries, and geographers continue to study its development. Today, more 
than 50 percent of the world’ s population lives in cities. Demographers estimate 
that by the year 2030, 60 percent will live in cities, and nearly 70 percent by 
2050. Most of those people will be in the less-developed countries (LDCs) of 
the world’s periphery and semiperiphery. While urbanization can be positive 
for both individuals and societies, the challenges may be overwhelmingif a city 
is not prepared to grow or if urbanization occurs too rapidly.
Influence of Site and Situation on Cities
The location of where cities develop is a critical aspect of urban geography. 
The concepts of site and situation, introduced in Topic 1.4, play a key role in 
explaining this process. Site describes the characteristics at the immediate 
location—for example, physical features, climate, labor force, and human 
structures. In contrast, situation refers to the location of a place relative to 
its surroundings and its connectivity to other places. Examples would include 
near a gold mine, on the coast, or by the railroad. Important factors of site and 
situation today are different than past cities. 
The site and situation of a city influences its function. Specialized functions 
of cities include defense, religion, trade, education, finance, transportation, 
government, manufacturing, retirement, entertainment, residential housing, 
or service centers. Larger cities often have multiple functions. Cities near 
natural ports, such as Boston or New York City, started as centers of trade but 
provide multiple functions today. 
Early City-States
Historically, a city-state consisted of an urban center (the city) and its 
surrounding territory and agricultural villages. A city-state had its own political 
system and functioned independently from other city-states. The population in 
the surrounding villages and territory received services and protection from 
the urban center. These communities were often raided by other groups for 
their wealth. As a result, defense was a primary consideration, and military 
leaders evolved into political rulers, or kings.
Early city-states emerged in several locations around the globe in an urban 
hearth, or area generally associated with defensible sites and river valleys in 
which seasonal floods and fertile soils allowed for an agricultural surplus:
• the Tigris-Euphrates V alley (Mesopotamia) in modern Iraq
• the Nile River V alley and Nile Delta in modern Egypt
• the Indus River V alley in modern Pakistan
• the Huang-He floodplain in modern China
369 6.1: THE ORIGIN AND INFLUENCE OF URBANIZATION
Other urban centers also emerged in Mesoamerica (in modern Mexico) and in 
the Andean region of South America.
Examples of city-states through history include those of Classical Greece 
(Athens, Sparta, Corinth), those of the Middle Ages in Europe, and Venice and 
Italian city-states during the Renaissance. Monaco, a city-state located entirely 
within the boundaries of Italy, has endured to modern times. Vatican City 
and Singapore are also modern city-states, though they did not evolve from 
previous agricultural settlements, but from religious influence. City-states 
eventually coalesced to form early states and empires. The ancient Babylonian 
Empire grew from the original city-state of Babylon. 
Centers for Services
As cities grew, more people developed specialized skills other than producing 
food. This changed the relationship between cities and the areas around them. 
City residents depended on farmers for food. In return, people in cities focused 
on supplying services for their inhabitants and the inhabitants of surrounding 
regions.
Early cities often specialized in particular services. Some emerged as 
administrative centers from which the elite ruled. Others, often associated with 
important shrines, became religious centers. Defensive strongholds, university 
towns, and centers of specialized production—located at resource sites—also 
emerged.
Defining Cities
Most definitions of a city describe a place with a relatively high concentration 
of people. Cities are places where people come together to build a nucleated, 
or clustered, settlement. An urban area is usually defined as a central city 
plus land developed for commercial, industrial, or residential purposes, and 
includes the surrounding suburbs. 
Legal Definition of a City
Definitions of what constitutes a city vary greatly, but the easiest way to define a 
city is a higher-density area with territory inside officially recognized political 
boundaries. This definition is useful for determining the precise population, 
taxing residents, providing services, and establishing and enforcing laws. Most 
large cities today, as defined legally, share boundaries with adjacent cities, yet 
those boundaries are visible only on a map. On the ground, people leaving one 
city might have no idea they were entering another legal city.
Metropolitan Areas
A collection of adjacent cities economically connected, across which population 
density is high and continuous is a metropolitan area, sometimes called a 
metro area. Most large cities in the world today are really metro areas of a 
series of legally defined cities, but they are referred to using only the name of 
the largest city. For example, the metro area of Denver, Colorado, consists of