Monday, August 27, 2018

Arches National Park, Moab, Utah

Water and ice, extreme temperatures, and underground salt movement are responsible for the sculptured rock scenery of Arches National Park. On clear, blue-sky days it is difficult to imagine such violent forces -- or the 100 million years of erosion -- that created this land boasting one of the world's greatest densities of natural arches. Over 2,000 cataloged arches range in size from a three-foot opening, the minimum considered an arch, to the longest, Landscape Arch, measuring 306 feet base to base. While several large arches are visible from the road, towering spires, pinnacles, and balanced rocks  -- perched atop seemingly inadequate bases -- vie with the arches as scenic spectacles here.

Today new features are being formed as old ones are destroyed. Erosion and weathering work slowly but relentlessly, creating dynamic landforms that gradually change through time. Change sometimes occurs more dramatically. In 1991 a rock slab 60 feet long, 11 feet wide, and four feet thick fell from the underside of Landscape Arch, leaving behind an even thinner ribbon of rock. Delicate Arch, an isolated remnant of a bygone fin, stands on the brink of a canyon, with the dramatic La Sal Mountains as backdrop.

The sheer walls of this canyon visitors of buildings lining a big city street. Rising majestically, these geologic "skyscrapers" tell the story of Entrada Sandstone. Entrada Sandstone began forming more than 150 million years ago as tidal flats, desert, and beach deposits. Over time, layers of rock, perhaps a mile thick, covered these deposits. Tremendous pressure from these rock layers compressed the buried sand into sandstone and cracked it. Erosion then removed the overlying rock layers and the Entrada began to weather. Within the past two million years, erosion of the cracks in the Entrada has left vertical slabs like the rock wall above right. These slabs, called fins, are the first step in arch formation.

Courthouse  Towers
The rise and all of an arch ~~ like all living things, arches have life cycles too. Starting as small holes in rock faces, they enlarge and eventually collapse from weathering and erosion. Water, whether from rain or snow, dissolves the natural cement (calcium carbonate) in the Entrada sandstone. Sand grains once "glued" together are separated and washed away; arches form, grow, mature, and fall. Although there are no major arches here at Courthouse Towers, the cycle is continuing. Look for Baby Arch in the rock wall to the left of Sheep Rock. Weathering over time will enlarge this growing arch until it finally collapses. (See picture below.)

Sheep Rock is part of an eroding rock wall or "fin" that probably contained arches (above). We can't be certain, but the sharp cleavage found on Sheep Rock and the shapes of other rock faces near it are clues that an arch once connected the "sheep" to the rock mass it faces.

Balanced Rock

The forces of erosion are sculpting more than arches. Balanced Rock clearly shows the various layers responsible for this amazing defiance of gravity. The caprock of the hard Slick Rock Member (topmost stone) of the Entrada Sandstone is perched upon a pedestal of mudstone. This softer Dewey Bridge Member (the base) of the Carmel Formation weathers more quickly than the resistant rock above. Eventually, the faster-eroding Dewey Bridge will cause the collapse of Balanced Rock. Balanced Rock Formation is 128 feet high; boulder height is 55 feet, and it weighs 3,500 tons.

Turret Arch

Yes, those are people up there

Looks like a future arch

These are considered the north and south windows. 

North Window and South Window
How arches are formed: In the Windows area, you can see many stages of arch formation. Look closely at the above pictures. Some arches are hard to see because of the rock walls behind them. Entrada Sandstone -- the rock in which arches are formed -- was deposited here as sand more than 150 million years ago. Over time it was buried by new layers, hardened into rock, and shaped by the powerful forces of erosion. These look a bit like they can be bridges, but the difference between arches and bridges is water. Bridges are formed by the erosive action of moving water. Arches are formed by other erosional forces -- mainly frost action and seeping moisture -- that also enlarge natural bridges once stream erosion forms them.

Double Arch

View from below

View from below

Native Americans used this area for thousands of years. The Archaic peoples, and later ancestral Puebloan, Fremont, and Ute peoples, searched the arid desert for food animals, wild plant foods, and stone for tools and weapons. They also left evidence of their passing on a few pictograph and petroglyph panels. The first non-native explorers came looking for wealth in mineral forms. Ranchers found abundant grasses for cattle and sheep. Disabled Civil War veteran John Wesley Wolfe and his son Fred settled here in the late 1800s. A weathered log cabin, root cellar, and corral give evidence of the primitive ranch they operated for over 20 years.

The Corral

Root Cellar

The Cabin

A nagging leg injury from the Civil War prompted John Wolfe to move west from Ohio, looking for a drier climate. He chose this tract of more than 100 acres along Salt Wash for its water and grassland -- enough for a few cattle. The Wolfes built a one-room cabin, corral, and a small dam across Salt Wash. In 1906, John's daughter Flora Stanley, her husband and their children moved to the ranch. Shocked at the primitive conditions Flora convinced her father to build a new cabin with a wooden floor -- the cabin seen today. The reunited family weathered a few more years in Utah and in 1910 returned to Ohio. John Wolfe died on October 22, 1913, in Etna, Ohio, at the age of 84.

Fiery Furnace
Most of the rocks in Arches National Park owe their brilliant color to presence or absence of iron. When iron oxides -- similar to a nail rusting -- it gives the rock a red color like Fiery Furnace, where the sandstone fins glow like flames at sunset. Bands of white occur where water has removed the iron or bleached the rock through chemical reaction. Black, brown, or deep metallic purple streaks on stone faces are created by iron oxide, manganese oxide, and clay interacting with bacteria and water. Green rocks form in an oxygen-poor environment, such as a shallow lake, where iron is in a reduced or ferrous state. Over millions of years the many colors of iron have painted the landscape of Arches into the work of art visible today.

Guess Who?

Skyline Arch
Arches usually form slowly, but quick and dramatic changes do occur. In 1940, a large boulder suddenly fell out of Skyline Arch, roughly doubling the size of its opening. 

Devils Garden Trail
This trail goes to seven arches, but I did not take the hike as it was over 7 miles long.

For most of Arches' long history, red rock arches did not exist. Salty inland seas, braided river systems, coastal plains, and sand dunes fill the chapters in our geologic history book. Marine fossils in the 300-million year old Honaker Trail Formation are remnants of an ancient sea. Cheerio-like discs of crinoid stems, lacy branches of bryozoans, and claim-like brachiopods tell the story of a warm, shallow sea teeming with life. Crinoids are animals that look like underwater flowers with feathery arms for collecting food. Trilobites, an early ancestor of the crab, crawled or swam among the corals. While crinoids, bryozoans, and brachiopods still exist, other animals, such as horn corals and trilobites, have been extinct for millions of years.

Fossils are not the only clues to the stories written in the rocks. Ripple marks reveal evidence of past running or lapping water. Both the main arch-forming layer, Entrada Sandstone, and the tan Navajo Sandstone, show diagonal lines called cross-bedding. These are the inner structure of ancient sand dunes frozen in time. Amazingly, geologists can figure out ancient wind direction by studying cross-bedding.

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