Arches
Why So Many Arches?
Arches National Park has the densest concentration of natural stone arches in the world. There are over 2,000 documented arches in the park, ranging from sliver-thin cracks to spans greater than 300 feet (97 m). Why are there so many arches in this place? How do they form? And what is an arch, anyway?
First, you need the right kinds of rock. Sandstone is made of grains of sand cemented together by minerals, but not all sandstone is the same. The Entrada Sandstone was once a massive desert, full of shifting dunes of fine-grained sand. The grains are rounded so, when packed together, they formed a rock that is very porous (full of tiny spaces).
Crack it into parallel lines. Deep beneath the surface lies a thick layer of salts. Squeezed by the tons of rock above it, the salt bulged upward, creating long domes. The rock layers covering these domes were forced to crack, like the surface of freshly-baked bread, into a series of more-or-less parallel lines.
Next, add the right amount of rain. On average, the park receives 8-10 inches (18-23 cm) of precipitation a year. That might not sound like much, but it’s enough to keep the engines of erosion working 24 hours a day, 365 days a year.
Rainwater soaks into the porous Entrada sandstone easily and then slowly dissolves the calcite bonding the sand together— in other words, rotting the rock from the inside out. Water puddles at the bottom of the Entrada layer, just above a denser rock layer, where it erodes a cavity, like one between your teeth. In winter, water trapped between the layers expands when it freezes and contracts, prying the rock apart. If the park received too much precipitation, the sandstone could erode so quickly that arches might not have time to form. If it never rained here, the engines of erosion would stop.
Let the water do its work. As erosion happens, a variety of shapes begin to appear. To be one of the park’s official stone arches, a hole must have an opening of at least three feet (1 m) in any one direction. There is no requirement for width; many arches in the park are so skinny you have to place your cheek against the rock to see any light through them.
Is a window a special kind of arch? Not really. “Windows” are arches that are particularly large, are located on a high wall or fin, or “frame” a particularly scenic view beyond.
What about bridges? A natural bridge spans a waterway—or somewhere water once ran. Very few bridges exist at Arches, but Natural Bridges National Monument, just two hours south (112 miles), has three tremendous examples of this feature. All of them are visible from their paved scenic road or by hiking trail.
Make sure your rocks don’t rock and roll. Luckily, earthquakes are rare in this area, otherwise these massive outdoor rock sculptures would splinter and collapse. The fact that over 2,000 still stand, tells us this area has been rather geologically stable for at least 50,000 years.
Lastly, pick the right time to visit. (You did.) The rock layers visible in the park today were once buried by over a mile of other rock that had to erode first to expose what lay beneath. Visitors one million years ago might have seen an endless flat plain dotted with vegetation. Imagine a visit far into the future, when these layers have fully worn away. What new rock shapes might you discover then?
Arches National Park has the densest concentration of natural stone arches in the world. There are over 2,000 documented arches in the park, ranging from sliver-thin cracks to spans greater than 300 feet (97 m). Why are there so many arches in this place? How do they form? And what is an arch, anyway?
First, you need the right kinds of rock. Sandstone is made of grains of sand cemented together by minerals, but not all sandstone is the same. The Entrada Sandstone was once a massive desert, full of shifting dunes of fine-grained sand. The grains are rounded so, when packed together, they formed a rock that is very porous (full of tiny spaces).
Crack it into parallel lines. Deep beneath the surface lies a thick layer of salts. Squeezed by the tons of rock above it, the salt bulged upward, creating long domes. The rock layers covering these domes were forced to crack, like the surface of freshly-baked bread, into a series of more-or-less parallel lines.
Next, add the right amount of rain. On average, the park receives 8-10 inches (18-23 cm) of precipitation a year. That might not sound like much, but it’s enough to keep the engines of erosion working 24 hours a day, 365 days a year.
Rainwater soaks into the porous Entrada sandstone easily and then slowly dissolves the calcite bonding the sand together— in other words, rotting the rock from the inside out. Water puddles at the bottom of the Entrada layer, just above a denser rock layer, where it erodes a cavity, like one between your teeth. In winter, water trapped between the layers expands when it freezes and contracts, prying the rock apart. If the park received too much precipitation, the sandstone could erode so quickly that arches might not have time to form. If it never rained here, the engines of erosion would stop.
Let the water do its work. As erosion happens, a variety of shapes begin to appear. To be one of the park’s official stone arches, a hole must have an opening of at least three feet (1 m) in any one direction. There is no requirement for width; many arches in the park are so skinny you have to place your cheek against the rock to see any light through them.
Is a window a special kind of arch? Not really. “Windows” are arches that are particularly large, are located on a high wall or fin, or “frame” a particularly scenic view beyond.
What about bridges? A natural bridge spans a waterway—or somewhere water once ran. Very few bridges exist at Arches, but Natural Bridges National Monument, just two hours south (112 miles), has three tremendous examples of this feature. All of them are visible from their paved scenic road or by hiking trail.
Make sure your rocks don’t rock and roll. Luckily, earthquakes are rare in this area, otherwise these massive outdoor rock sculptures would splinter and collapse. The fact that over 2,000 still stand, tells us this area has been rather geologically stable for at least 50,000 years.
Lastly, pick the right time to visit. (You did.) The rock layers visible in the park today were once buried by over a mile of other rock that had to erode first to expose what lay beneath. Visitors one million years ago might have seen an endless flat plain dotted with vegetation. Imagine a visit far into the future, when these layers have fully worn away. What new rock shapes might you discover then?