Weather and erosion may be taking a heavy toll on some historic, manmade landmarks, but some of these same natural processes are responsible for creating some of the most spectacular natural sites in the country. Here is a look at 10 of the National Natural Landmarks that were created by weather and other forces of nature.
The Flume Gorge, Franconia Notch State Park, New Hampshire
Franconia Notch as a whole is a National Natural Landmark, a mountain pass carved by glaciers in the White Mountain National Forest of New Hampshire. Nestled in the Franconia Notch is the Flume, a spectacular gorge of carved granite and basalt with cascading waterfalls and steep rocky walls that extend up to 800 feet.
According to the New Hampshire Parks Service, the Flume continues to be eroded and reshaped by the running streams in the summer and the ice and frost in the winters. Other natural elements, such as trees and other plant life growing off the walls of the gorge, are also slowly altering this area 200 million years in the making.
Shelta Cave, Huntsville, Alabama
With a depth of 2,500 feet into the Earth, the Shelta Cave is a vast cavern harboring a large underground lake. Limestone caves such as this are formed by water eroding limestone.
According to the National Speleological Society, the lake in the Shelta Cave is incredibly biodiverse. Some species are exclusively found in this particular body of water, including several beetles, crayfish and other arthropods.
Caverns of Sonora, Texas
The Caverns of Sonora are especially known for the breathtaking collection of unusual helictites and other crystalized formations.
Forged primarily of limestone, the caverns are located on a fault. This has allowed gases and acidic water to rise into the cave and dissolve the limestone.
The formations within the cave are very fragile and take years and years to form. The Caverns of Sonora are about 100-million-years old, and the helictites are still growing.
Pilot Mountain, Pilot Mountain State Park, North Carolina
Pilot Mountain, located in Pilot Mountain State Park in North Carolina, is one of the nation's most famous monadnocks, or stand-alone mountains.
According to the North Carolina Division of Parks and Recreation, the mountain stands 2,421 feet.
These unusual mountains are typically created when glaciers or other eroding processes grind away other parts of a range, leaving behind these singular peaks.
Hell's Half Acre, Bingham and Bonneville counties, Idaho
This vast area, extending 125 miles, is a fully exposed lava flow. This particular lava flow is a pahoehoe, characterized by the smooth, billowy surface it creates as as it cools.
The last eruption here, according to the Idaho Department of Commerce, occurred 4,100 years ago. Trails now wind their way through the landscape, offering visitors a glimpse of what was once a molten lake of basaltic rock topping 2,000 F.
Aniakchak Crater, Lake and Peninsula Borough, Alaska
The National Parks Service states that, "The Aniakchak Crater, located within Aniakchak National Monument, is one of the largest explosive craters in the world, averaging 6 miles wide and 2,000 feet deep."
This formation, called a caldera, was created by volcanic eruption, leaving a crater-shape in its wake. Located inside the crater is Surprise Lake, which is made from warm springs and serves as the source of the Aniakchak River.
Unique wildlife, such as salmon species, thrive in Surprise Lake.
The crater itself is home to abundant wildlife, such as grizzly bears, and is available for hiking and camping tours.
Creelsboro Natural Bridge, Russell County, Kentucky
According to the Natural Arch and Bridge Society, the Creelsboro Natural Bridge is the largest natural bridge east of the Rockies at 104 feet long. The bridge was formed, and is continuing to be eroded and reformed, by water flowing from the Cumberland River.
This process has taken more than 400 million years to bring the formation to its current state.
Valley of Fire, Valley of Fire State Park, Nevada
Nevada's Valley of Fire are formations of thrust faulting, wherein erosion and movement along a break in the Earth's crust create "reverse" faults where older rock ends up on top of newer rock.
In the Valley of Fire, so named because of the vibrant red colors of the exposed rock, vast canyons and wind-molded protrusions mark the landscape.
The Nevada Department of Conservation states that the formation of this area began 150 million years ago. The area is now home to a variety of cacti, birds, tortoises and coyote, among other native desert species.
Devil's Millhopper, Devil's Millhopper Geological State Park, Florida
At 120 feet deep, Florida's Devil's Millhopper sinkhole creates a "miniature rainforest."
Streams that dip down into the cavity of the sinkhole are absorbed into the ground, giving life to lush vegetation. In the sinkhole, fossilized marine animals, shells and shark teeth have offered historical glimpses into the area's past for scientists.
Sinkholes are fairly common in Florida, as water dissolves its underground bed of limestone, weakening and giving way to the earth above it.
Dunes Nature Preserve, Indiana Dunes State Park, Indiana
The National Parks Service calls the Dunes Nature Preserve, "the best remaining example of undeveloped and relatively unspoiled dune landscape along the southern shore of Lake Michigan, a portion of which is known as the 'Birthplace of American Ecology.'"
Over the course of thousands of years, winds and other elements have formed these massive sand dunes, some that tower over 200 feet tall. The dunes area spans 2,182 acres.
The region is rich in diverse flora and fauna that has been studied by biologists for more than a century.
Honorable Mention: The Grand Canyon, Grand Canyon State Park, Arizona
One of America's most famous landmarks, the Grand Canyon does not fall on the official list of National Natural Landmarks but earns an "honorable mention" spot on our list for its popularity, receiving 5 million visitors every year.
Formed over the course of 6 million years for the inner canyon, to a possible 70 million years since it began, the Grand Canyon was carved away by flowing waters, including the Colorado River.
The National Parks Service lists it as 277-river-miles-long, up to 18 miles wide and a mile deep.