In that moves down the other side

In this lesson, we’ll discuss two very different types of deserts located in the Southwestern United States. The Colorado Plateau and the Basin and Range Province each have special characteristics and features that make them unique.

Deserts of the Southwestern United States

If you were to travel around the United States, you would encounter a variety of different habitat types, including rain forests, mountains, coastal areas, plains and deciduous forests. In the Southwestern United States, the majority of the land is considered desert. Deserts are defined as regions with very low rainfall, usually less than 25 centimeters of rain per year.

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This region of the country is dominated by desert conditions, partially because it is located near the 30; North latitude line. Land near this latitude line is prone to desert conditions due to global air circulation patterns that bring in very dry air. This dry air gives the region the general characteristics of having a great deal of sunlight, minimal rain and high levels of evaporation.The formation of deserts in the Southwestern United States is also a result of rain shadow zones, which are the downwind sides of mountains that receive limited rainfall. As warm, moist air passes over a mountain range, it cools and expands, which causes condensation and precipitation. This leaves the air that moves down the other side of the mountain to be void of moisture and creates a dry habitat. There are several mountain ranges, including the Sierra Nevada range, that create rain shadow zones in the Southwestern United States.

Although the entire desert region shares common characteristics, you would notice some major differences if you were to travel across it. The region is actually divided into two areas, the Colorado Plateau and the Basin and Range Province. This division is based on major differences in geologic structures.

Colorado Plateau

The Colorado Plateau is one of the two desert regions in the Southwestern United States.

This region is centered around a spot known as the Four Corners, where you can stand in Utah, Colorado, Arizona and New Mexico all at once! The Colorado Plateau has the nickname ‘Red Rock Country’ because of the large, brightly colored rock formations that cover the land.This region is dominated by plateaus, which are large, flat-topped areas that are elevated above the surrounding ground and have cliffs at their edges. These plateaus can be formed from layers of rocks, such as sandstone and limestone or layers of lava flow. Plateaus can reach over 1,500 meters above sea level and are very vulnerable to erosion due to their high level of exposure to the natural elements.

Over time, as a result of erosion from moving water or wind, a plateau can be transformed into different types of geologic features. First, as erosion removes rocks from the base and cliffs of the plateau, some areas of land are left behind, resulting in the formation of a mesa. A mesa is a broad, flat-topped hill with cliff edges. Mesas are basically a small, isolated portion of elevated land that was once connected to the larger plateau, but now stands separated.

If erosion of a mesa continues, it will eventually break up into smaller areas of land. These geologic features are known as buttes, which are very narrow hills, with a flat top and steep sides. If a butte continues to erode, it will eventually be worn down to the flat ground and no longer be elevated.The Colorado Plateau is also known for a unique geological feature known as monoclines. Monoclines are step-like folds in the rock layers. Instead of the land having a gradual slope, the monocline creates more rigid layers that resemble steep steps.

Basin and Range Province

The Basin and Range Province is the other major desert region in the Southwestern United States.

This region covers the majority of Nevada and portions of Southern California, Utah, Arizona, New Mexico and the western tip of Texas.The region is dominated by mountain ranges that are separated by flat valley floors. The multiple mountain ranges cause many rain shadow zones and the resulting dry climate. The alternation between mountain range and flat valley floor is caused by faults, which are fractures in the ground where movement occurs. Movement along the fault lines has caused the valleys between the mountains to drop down.

Similar to the plateaus in the Colorado Plateau region, the mountain ranges in the Basin and Range Province are susceptible to severe erosion. In this region, the majority of erosion is due to heavy rains that fall as clouds pass over the mountains. This heavy rain causes erosion of the steep mountain slopes and carries the deposits down to the base in a flashflood or mudslide. The eroded sediment causes the creation of alluvial fans, which are large, fan-like piles of sediment.As erosion continues and alluvial fans grow in size, they can create a bajada, which is a broad, gently sloping surface formed by the joining of individual alluvial fans. As erosion continues, a pediment will eventually form, which is a gently sloping surface covered in gravel and large rocks. This formation will occur high up the mountain, above the bajada, and will be clear of fine sediment that has already washed down the mountainside.

Over long periods of time, a steep mountain slope will be transformed due to erosion and will shape the landscape of the desert.Like the mountains, the valleys of the Basin and Range Province also go through changes. When excess water from heavy rain collects on the bottom of the valley, it creates a playa lake. These are shallow, temporary lakes that contain fine clay from the mountainside. When the water evaporates after a few days, it will create a playa, which is a flat surface of hard, mud-cracked clay. This cracked surface is a characteristic that is very common in this type of desert.

Lesson Summary

Now, let’s review deserts of the Southwestern United States. First, deserts are defined as regions with very low rainfall, usually less than 25 centimeters of rain per year. The deserts of the Southwestern United States are formed due to their location near the 30° North latitude line and as a result of rain shadow zones, which are the downwind side of mountains that receive limited rainfall.This large desert region shares a common climate but is divided into two different regions based on geologic features. The Colorado Plateau is centered around the Four Corners and is dominated by plateaus, which are large, flat-topped areas that are elevated above the surrounding ground and have cliffs at their edges. As a result of erosion, over time, a plateau can be turned into a mesa, which is a broad, flat-topped hill with cliff edges and eventually a butte, which is a very narrow hill, with a flat top and steep sides.

The Colorado Plateau is also known for a unique geological feature known as monoclines, which are step-like folds in the rock layers.The Basin and Range Province is the other major desert region in the Southwestern United States and covers the majority of Nevada. This region is characterized by mountain ranges separated by flat valley floors. The alternation between mountain range and flat valley floor is caused by the movement of faults. Erosion of mountain ranges results in the formation of sediment deposits known as alluvial fans, which can combine into bajadas. Extreme and continued erosion can result in the formation of a pediment, which is a gently sloping surface covered in gravel and large rocks.

Heavy rainfall causes playa lakes to form in the valleys, which evaporate and leave cracked, clay surfaces known as a playa.Although both the Colorado Plateau and the Basin and Range Province are both considered deserts, they have their own unique physical features and landscape based on their geologic features. If you visit the deserts of the Southwestern United States, you will get to see two very different habitats that share a common desire for a rain shower.

Learning Outcomes

By the end of this lesson you should be able to:

  • Explain what climate and geological conditions contribute to the Southwestern United States’ desert climate
  • Describe the Colorado Plateau and the characteristics of the region
  • Talk about the Basin and Range Province and how it is affected by erosion
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