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Showing posts with label sand dunes. Show all posts
Showing posts with label sand dunes. Show all posts

Thursday, September 27, 2012

To the Edge of the Entrada Erg

The eponymous stone sculptures of Arches National Park in southeast Utah were once sand dunes -- part of the vast Entrada erg of middle Jurassic time (Delicate Arch, National Park Service photo).
The Entrada erg (dune field), now preserved as the Entrada Formation, was huge.  It covered much of Colorado, eastern Utah, northern New Mexico and northeast Arizona.  After millions of years of deposition, followed by burial, lithification, uplift and erosion, the ancient erg now is part of the spectacular sandstone scenery of the Colorado Plateau.
In the McInnis Canyons National Conservation Area near Grand Junction, Colorado, trails wind among domes and alcoves of the Entrada Formation -- remains of ca 175-million-year-old sand dunes.
Entrada sandstone in the McInnis Canyons area is especially amenable to tafoni development (cavities).
Weathering and erosion have brought out the beautiful cross-bedding, as well as tafoni, in this exposure of Entrada sandstone; Twenty-mile Wash, Grand Staircase - Escalante National Monument, southern Utah.
One of the many cross-bedded hoodoo inhabitants of Devils Garden, carved out of Entrada rocks; off the Hole-in-the-Rock Road east of Escalante, Utah.
Even the most extensive ergs do not go on forever, of course.  The Entrada erg was bordered on the north and west by ocean, and was periodically flooded and exposed with changes in sea level.  Instead of massive deposits of sand, there were sometimes low dunes, sometimes tidal flats.  The result is a very different Entrada, like the exposures below at Capitol Reef National Park, located about a half mile west of the east Park entrance.  Cross-bedded sandstone strata -- old dunes -- are present, but are only about a foot thick, and alternate with layers of siltstone, the remains of tidal flats.
Thin veins of gypsum run parallel to and cut across strata.
The Entrada (left center), capped by the greenish-brown Curtis Formation -- deposited during a later marine advance from the north.

 Sources and Additional Information

Doelling, H. H. et al.  2000.  Geology of Grand Staircase-Escalante National Monument, Utah.  Utah Geological Association Publication 28.

Fillmore, Robert. 2010. Geological evolution of the Colorado Plateau of eastern Utah and western Colorado, including the San Juan River, Natural Bridges, Canyonlands, Arches, and the Book Cliffs.  Salt Lake City: University of Utah Press.

Orndorff, R.L., Wieder, R.W. and Futey, D.G.  2006.  Geology underfoot in southern Utah.  Missoula, MT:  Mountain Press Publ. Co.

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Saturday, November 19, 2011

Dune Week: virtual field trip to the Oso Flaco dunes

Clastic Detritus and Galileo’s Pendulum have proclaimed and publicized this week as Sand Dune Week, after discovering multiple posts on the topic including singing sands and sand tsunamis.  Then looking for Detachment joined in with a virtual tour of Sand Mountain in Nevada.  Now there are more:  Earth Photo of the Day features dunes of Death Valley.  There is a post about kriiva in Estonia, puzzling landforms made of sand which may or may not be dunes.  I too am immersed in sand this week ... I'm reading Sand by Michael Welland of Through the Sandglass.  This calls for a virtual field trip to some dunes -- specifically the dunes of Oso Flaco on the Central Coast of California.
"The Oso Flaco" lies at the west end of the Santa Maria Valley.  It was named in 1769 by a group of hungry Spanish explorers for the skinny bear they killed and consumed there.  The Oso Flaco dunes are part of the Guadalupe - Nipomo Dunes Complex, 18 miles in length, in southern San Luis Obispo and northern Santa Barbara counties.  Mussel Rock Dune at the southern end of the complex is the highest on the west coast of the US at 500 feet.
Mussel Rock Dune viewed from the north.
Oceano Dunes, Ansel Adams, 1963



Sand dune landscapes are stark and unusual, and often oddly beautiful.  It is not surprising that there was an alternative community of mystics, writers and artists living in these dunes in the 1930s and 1940s -- the Dunites, who found the setting to be a source of creative energy.








By the 1960s, the dunes had become quite popular with dune buggy-ers, and when the environmental movement then arrived on the Central Coast, a battle began.



Off-road vehicles still crawl the northern part  -- the Oceano Dunes State Vehicular Recreation Area.  But the dune buggies were kicked out of the southern part, astounding considering the local culture.  Today this area is managed for conservation as state and county parks, and a wildlife refuge.
The Guadalupe - Nipomo Dunes Complex contains a wonderful diversity of ecosystems, including terrestrial, fluvial, lacustrine and marine.  There are three major zones:  active, partially-vegetated and stabilized dunes.  The oldest dunes underlie the Nipomo and Orcutt Mesas on either side of the broad valley of the Santa Maria River.
Plant-lovers surrounded by dunes lupines.  Photo by W. Kline.
To the west is the partially-vegetated zone, with extensive areas of grassland, shrubland and forest interspersed with lakes and active dunes.  Finally, open dunes with occasional plants rise over the beaches and ocean.
Beach at Oso Flaco.  Photo by W. Kline.




I will be posting a lot more about the dunes at Oso Flaco in May, when I’m there on vacation (whoo-eee!!).










Google Earth:  35.030452°  -120.621174°  (hikers’ causeway across Oso Flaco Lake)

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