This week on our national parks journey, we travel to Utah. The western state is home to several of the most popular national parks in America.

Today, we travel to the eastern part of the state. We visit a park filled with strange and beautiful colored rock formations. These formations have been created over millions of years. They curve and narrow and balance on top of one another. Some are extremely thin. Others have huge cracks, yet remain standing. Many of the rock formations seem to ignore the rules of gravity.

Welcome to Arches National Park!

Arches are formations that look like half a circle standing up. Sometimes they create a curved bridge between two large rocks.

Arches National Park contains the most natural rock arches of any place on Earth. The smallest arches are less than a meter across. The largest, called Landscape Arch, measures 93 meters from one base of the arch to the other.

Water and ice, extreme temperatures, and many layers of salt underground created these rock formations over hundreds of millions of years. The huge amount of salt is the remains of an ancient sea. It covered the area around 300 million years ago.

As time passed, the area filled with material left by rivers. Rocks buried the area. This created pressure on the underground salt bed.

Salt under pressure moves easily. It is not stable. The thick cover of rock created pressure on the salt. It moved this way and that, making the rock layers above move, as well.

A soft rock called sandstone moved upwards. It met with other, harder rocks. These sandstone structures continued to grow for about 150 million years.

Scientists say water is the most important element in creating arches. Water destroys the chemicals that keep rock particles together. The rock breaks as the water freezes and expands. Then the wind blows away the loose rock pieces.

Erosion from wind and weather slowly removed layers of younger rock. Most of the layers of rock seen today are pinkish-red in color. The rock is called Entrada Sandstone.

Experts say that most arches seen in the park today developed within the past million years. But the land formations continue to change slowly over time.

But sometimes, change happens quickly. In 2008, a formation called Wall Arch suddenly collapsed. Thousands of tons of sandstone crashed to the ground, creating a thunderous noise.

It is not easy to predict when arches might collapse. Wall Arch was not as thin as Landscape Arch, for example. Wall Arch also did not have visible cracks. But, many tiny breaks within the stone made the arch weaker than others.

Some formations within the park look as though the gentlest wind could send huge pieces of rock crashing to the ground. These are called balanced rocks. The most famous of all is simply called Balanced Rock.

This sandstone rock is the size of three school buses. It sits on top of a tower of darker rock called mudstone. The formation is about 39 meters tall. This bottom rock narrows sharply where it connects to the sandstone rock on top. Scientists call this a bridge. Mudstone weathers much more quickly than sandstone.

One day, far in the future, Balanced Rock will no longer be balanced. Just as Wall Arch did, Balanced Rock will collapse.

The names of the rock formations give you information about their appearance or structure. The most famous arch within the park is named Delicate Arch. "Delicate" means "easily broken or damaged." But it can also mean "very carefully and beautifully made."

Delicate Arch is a huge free-standing arch. It is 19 meters high and 13 meters wide. Humans have called it many different names throughout history. The name "Delicate Arch" first appeared in a 1934 article. Scientists described it as "the most delicately chiseled arch in the entire area."

Of the 2,000 stone arches within the park, Delicate Arch has become a symbol of the state of Utah. It is among the most famous geological features in the world.

Many of Arches' famous rock formations are easily seen from the single road that goes through the park. But Delicate Arch is not visible from a car. Some visitors choose to walk to two view points. But, even from the view points, Delicate Arch is still more than a kilometer away.

Many visitors choose to hike up to see the famous arch. The trail is 5 kilometers. It is the most popular hike in the park.

The walk is difficult and steep. It offers no shade. The sun is extremely strong. In summer months, the temperature can rise to 40 degrees Celsius. Park officials tell hikers to take at least two liters of water with them. In winter months, ice and water can make the sandstone trail extremely slick.

Along the way, hikers pass a wall of ancient animal drawings created by the Ute Indians. These are called petroglyphs. Petroglyphs exist throughout the park. They are a reminder of the long human history in this harsh environment.

Before reaching Delicate Arch, hikers must walk on a long and narrow rock ledge. The trail is often crowded with people.

But most say the heat and crowds are worth it. Delicate Arch stands alone above a natural sandstone bowl. It rises up dramatically from the flat land that surrounds it. On one side of the arch, the sandstone gently twists. On the other, a severe crack cuts through the narrowing rock.

Those who hike to its base appreciate and understand why it was once described as "delicate."

Most visitors to Arches National Park would agree that there is no other place in the world quite like it in the world. It is quiet and dreamlike, unpredictable and violent, old yet ever-changing. A trip to Arches is like visiting another world.

I'm Ashley Thompson.

And I'm Caty Weaver.

Ashley Thompson reported and wrote this story, with materials from the Learning English archive and the National Park Service. Caty Weaver was the editor.

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stable - adj. in a good state or condition that is not easily changed or likely to change

erosion - n. in a good state or condition that is not easily changed or likely to change

thunderous - adj. making a loud noise like the sound of thunder; very loud

chiseled - adj. having an attractive well-formed shape

geological - adj. relating to the rocks, land, processes of land formation, etc., of a particular area

shade - n. n area of slight darkness that is produced when something blocks the light of the sun

ledge - n. a flat rock surface that sticks out from a cliff