Along interstate 15 in southern Utah there is a lesser visited area of Zion National Park that many people, for one reason or another, choose to ignore. These folks speed away on their way north or south, eager to get wherever they are going, unaware of the geologic wonders that lie just off the asphalt slab of super highway.
Well, OK then. I am happy to keep the Kolob Canyons section of the Park all to myself.
|Taylor Creek in the Kolob Canyons section of Zion National Park|
About four miles from the Taylor Creek trailhead is a imposing edifice called Double Arch Alcove. Towering far above any hiker are a blind arch and an alcove formed high in the massive Navajo Sandstone.
How do these remarkable structures form, anyway?
At the base of the more permeable Navajo Sandstone (where we find the lower alcove), groundwater migrating downward comes in contact with the less permeable, slope–forming rocks of the Kayenta Formation. These clay–rich Kayenta beds form a significant barrier so that any moisture must now move outward instead of downward.
|Double Arch Alcove towers above Taylor Creek|
Stream migration (of Taylor Creek and all its previous incarnations) and sapping control the lower alcove. Sapping is a natural process of erosion that occurs, in our case, along the base of the Navajo Sandstone cliffs. It involves weakening of the rocks by seeping groundwater (moving laterally now) at the contact between the Navajo Sandstone and the underlying clay–rich Kayenta Formation.
|Desert varnish stains the walls of the lower alcove|
At this contact the seeping moisture evaporates at a much slower rate. This slower rate of evaporation causes the sandstone at the bottom of the arch to weather away more rapidly. Over time immense sheets and blocks of sandstone are loosened, the arch is undercut, and the entire structure expands.
A different sort of process controls the formation of the upper arch. What helps this arch–forming progression along is the presence of regular, parallel, vertical joints in the sandstone, providing a pathway in which water, ice, and frost are able to attack and weaken the rock.
As in other immensely cross–bedded sandstones, weathering in the form of exfoliation is the name of the game. Within the upper arch, slabs of rock peel off in a series of concentric layers, like the layers of an onion.
|Sapping and vertical joints are responsible for Double Arch Alcove|
So there we have it. Two arches, two different processes of formation. Two for the price of one!
Thanks to: Eves, R.L., 2005, Water, Rock, & Time: The Geologic Story of Zion National Park, Zion Natural History Association.