Tunnel Arch - Explored!
This is a good example of an arch eroded entirely within the massive Slick Rock Member, part of the Entrada Sandstone. Although it's difficult to tell from this image, the arch is 8.1m wide, 6.7m tall and the fin within which it sits it is about 4.3m thick at the arch.
Tunnel Rock is located in the Devil's Garden, a section in the north of Arches National Park in Utah. To see the main features, the Devil's Garden Trail meanders between sheer walls of sandstone fins. The fins were created when vertical cracks in a thick layer of sandstone were eroded and widened by water -either scoured by run-off from rainfall and snow melt, or pried and exfoliated by ice expansion. These stone formations may only last a few thousand years - a short time on the geologic time scale.
The events that led to the arches, fins, and other rock shapes began about 300 million years ago, when seas periodically covered the area. The seas became trapped in low-lying areas and then evaporated, leaving salt beds up to 1,500m thick in some places. Sand, silt and clay subsequently accumulated on top of the salt deposits over millions of years. The uneven weight and pressure of these overlying sediments squeezed the salt into an anticline (a domed ridge). Overlying horizontal rock layers bulged upward and cracked vertically allowing rainwater to trickle down and dissolve the salt away.
As the salt receded, the overlying rock burden sank with it. Salt Valley, located to the immediate south-west, is an example of the resulting landform. At the edges of the valley, where Devil's Garden is located, the cracked rock was slightly pulled apart. Rain and snow soaked into the vertical cracks, which dissolved the cementing minerals and loosened grains of sand to be carried away by running water. As the cracks widened, tall fins were left standing. Weak zones in fins were either dissolved by naturally occurring acids in rainwater or wedged apart by freezing and thawing water, and openings developed into the various arches seen today.
Scanned from a negative.
Tunnel Arch - Explored!
This is a good example of an arch eroded entirely within the massive Slick Rock Member, part of the Entrada Sandstone. Although it's difficult to tell from this image, the arch is 8.1m wide, 6.7m tall and the fin within which it sits it is about 4.3m thick at the arch.
Tunnel Rock is located in the Devil's Garden, a section in the north of Arches National Park in Utah. To see the main features, the Devil's Garden Trail meanders between sheer walls of sandstone fins. The fins were created when vertical cracks in a thick layer of sandstone were eroded and widened by water -either scoured by run-off from rainfall and snow melt, or pried and exfoliated by ice expansion. These stone formations may only last a few thousand years - a short time on the geologic time scale.
The events that led to the arches, fins, and other rock shapes began about 300 million years ago, when seas periodically covered the area. The seas became trapped in low-lying areas and then evaporated, leaving salt beds up to 1,500m thick in some places. Sand, silt and clay subsequently accumulated on top of the salt deposits over millions of years. The uneven weight and pressure of these overlying sediments squeezed the salt into an anticline (a domed ridge). Overlying horizontal rock layers bulged upward and cracked vertically allowing rainwater to trickle down and dissolve the salt away.
As the salt receded, the overlying rock burden sank with it. Salt Valley, located to the immediate south-west, is an example of the resulting landform. At the edges of the valley, where Devil's Garden is located, the cracked rock was slightly pulled apart. Rain and snow soaked into the vertical cracks, which dissolved the cementing minerals and loosened grains of sand to be carried away by running water. As the cracks widened, tall fins were left standing. Weak zones in fins were either dissolved by naturally occurring acids in rainwater or wedged apart by freezing and thawing water, and openings developed into the various arches seen today.
Scanned from a negative.