At this cache you’ll be looking at well developed
crossbeds and Moki marbles in an outcrop of the Navajo
Sandstone. The Navajo Sandstone is made up of as much as 2,500
feet cross-bedded quartz sand. Variations in the composition
of the cement that holds together the sand grains give the
dramatic coloration of the Formation.
The sand of the Navajo Sandstone was deposited by an ancient
wind blown (eolian) coastal and inland sand dune field during the
early Jurassic. This dune field covered much of Utah and extended
into adjacent states. These ancient dunes were buried, compacted,
and lithified (turned into rock) to form the characteristic
cross-beds of the Navajo Sandstone.
Cross-beds: (main coordinates)
Cross-beds are formed by layers of sand grains as they build up
into a dune then collapse. Wind piles sand up the gentle windward
side of a dune. When the dune becomes too steep to support itself,
it collapses creating the angled layers. This process also slowly
inches the dune in the direction of the wind.
Image Source USGS:
http://www2.nature.nps.gov/geology/usgsnps/dune/dune.html
Over time, the next dune migrates over the
first, burying the first and preserving the crossbeds. The
USGS Western Coastal & Marine Geology website has some
downloadable movies to demonstrate the process
(http://walrus.wr.usgs.gov/seds/Movie_list.html).
At the coordinates, the nearly vertical outcrop face across the
small drainage has excellently preserved and easily recognizable
cross-beds.
Moki Marbles (N 37 12.801 W 113 38.767)
At this location numerous dark circular raised patches can be seen
in the Navajo Sandstone. These features are called Moki (Moqui)
Marbles. Rain migrating through the sandstone slowly dissolves iron
within the sandstone and collects along the bedding planes. As
more and more iron ions are brought and deposited
along the same layers, the iron begin to form concentric rings
around each other forming spheres.
These spheres tend to be more resistant to weathering and thus
tend to stand out above the surrounding sandstone. In places, the
Moki Marbles are so well developed that they remain intact after
the sandstone has eroded out from around them, leaving fields of
these spheres. For a photo see
(http://www.utahphotowild.com/small/pages/small3.htm)
Logging requirements:
Send me a note with :
- The text "GCZ9C7 Petrified Dunes of Snow Canyon" on the first
line
- The number of people in your group.
- Look across the little canyon at the crossbeds and figure out
which way the sand was moving when it was buried (based only on
this one outcrop)
The above information was compiled from the
following sources:
- Higgins, Janice M. Geology of
Snow Canyon State Park, Utah in Geology of Utah’s Parks and
Monuments, 2003 Utah Geological Association Publication 28 (second
edition) D.A. Sprinkel, T.C. Chidsey, Jr. and P.B. Anderson,
editors
- Miek, Robert F., et. al., Geology of Zion
National Park, Utah in Geology of Utah’s Parks and Monuments, 2003
Utah Geological Association Publication 28 (second edition) D.A.
Sprinkel, T.C. Chidsey, Jr. and P.B. Anderson, editors
- USGS,
http://www2.nature.nps.gov/geology/usgsnps/dune/dune.html