Ventifact field trip
to Mojave Desert,
October 2012
Nathan Bridges, John Hopkins Applied Physics Lab
Julie Laity, Dept. of Geography, CSU Northridge
NOTES
October 13, 2012
Ventifact field trip to Mojave Desert, October 2012
ii
Ventifact field trip to Mojave Desert, October
2012
Nathan Bridges, John Hopkins Applied Physics Lab
Julie Laity, Dept. of Geography, CSU Northridge
KEY POINTS
Silver Lake
USE OF VENTIFACTS FOR DETERMINATION OF
WIND DIRECTION
1. Ventifacts provide a proxy for wind direction at both the local and regional scale.
2. The wind direction represented reflects the highest velocity winds in a given region.
3. A ventifact may indicate more than one wind direction. Secondary wind directions
represent
a. Differences in regional wind flow. For example, at the Little Cowhole
Mountains, winds seasonally shift from the north to the south.
b. Changes in wind flow associated with the passage of a front. This has been
observed in the Rasor Road area of the Mojave River Sink.
In either case, a dune may be found atop the hillcrest, which shifts up and
down the slopes, but does not change in net position.
4. Wind directions may be determined with respect to features on the ventifact.
a. The easiest and most reliable means of determining wind direction is to use
grooves or flutes on a large stable boulder.
b. Facets form (largely) perpendicular to the flow but are less reliable indicators
of direction
i. Facet formation requires considerable mass loss from the boulder (it is
rare to form facets on really large boulders and small faceted pebbles
move to easily; only medium sized rocks can usually be used).
Reliable rocks are rare.
Ventifact field trip to Mojave Desert, October 2012
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ii. Facets may develop from other
processes, such as rock splitting in deserts.
Above: Ventifact keels formed in bidirectional flow regime, Owens Lake area
(left) and Silver Lake (right).
5. As relict
ventifacts may be displaced from their original position by earthquakes (see ventifacts
along a fault, below), downslope movement, freeze-thaw processes, animals, or tree
roots (below), a large population of ventifacts is necessary to provide a reliable
measure of regional wind direction.
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1
2
Right: Basaltic
ventifacts displaced by
junipers, Larkin Lake,
Nevada
Below: Wide range of orientations along a fault
6. As local topography influences ventifact orientation, agglomerating several
populations of ventifacts is valuable in understanding regional wind flow (see below)
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FEATURE FORMATION
7. Owing to the acceleration of winds up hillslopes, the most well-developed ventifacts
are located near hill crests. On the crests,
a. There are more ventifacted rocks.
b. The rocks show larger grooves and better-developed flutes and facets.
8. Similarly, feature size usually increases up the face of a large rock (smaller flutes near
the rock base, increasing in size towards the top of the rock).
9. Features/textures include
a. Lineations
Fine lineations in marble,
Little Cowhole Mountains
b. Flutes
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Flutes at Silver Lake
Flutes in rhyolitic ignimbrite, Argentinean Altiplano. Note moat surrounding the
ventifact.
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c. Pits occur on near vertical faces
Pits in basalt exploit pre-existing vesicles
d. Helical scores
e. Dedoes (finger-like projections, with a harder inclusion at the tip)
Bishop Tuff, Aeolian Buttes
f. ‘Knobby’ texture, the result of harder inclusions
Knobby texture, Owens Valley, south
of Owens Lake
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g. Etching: differential erosion of layered rocks
Etching in ignimbrites, Rasor Road
h. Case hardening, forming a fluted rind that can spall off
Case-hardened fluted rind,
Bishop Tuff, Mono Basin
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Case-hardened fluted rind,
basalt, Lake Larkin, NV
i. Notches on rocks that are tall enough to reach the height of maximum abrasion
Field trip co-leader Bridges next to ignimbrite ventifact/yardang, Argentinean
Altiplano.
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Kinetic energy flux profiles. The blue curve is for purely suspended grains. The other three
are combinations of saltated particles, which dominate the distribution, and suspended
grains, with the curves having different mean saltation liftoff velocities (red curve is for
mean liftoff velocities and yellow and green curves are these velocities enhanced by a factor
of 2 and 4, respectively). Note that the profiles with saltated grains have distinct maxima at
a height of a few 10 s of centimeters and have much greater kinetic energy fluxes through
most of their height compared to the suspended case. Curves from Anderson (1986).
10. Fine grained/homogeneous rocks may facet, but develop few if any features;
heterogeneous rocks, including vesicular rocks, develop features such flutes, pits, and
lineations.
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Owens Valley,
south of Owens
Lake
11. Ventifacts surfaces may develop a sheen or polish, comparable to glacial polish. At
the microscale however, SEM images reveal a rough surface related to sandblasting.
12. Flutes often increase in size up the boulder.
LOCATION OF VENTIFACTS
Ventifacts are formed in locations where the wind is strong, vegetation is lacking,
and there is a supply of abrasive sediment.
13. The acceleration of winds by topography is one of the most important factors
determining ventifact presence in a region. Hence, ventifacts are present, and best
developed
a. In passes, such as the area south of Ludlow
b. At hill crests and within saddles of slopes
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Flutes at a hillcrest in the Mojave River
sink exceed a meter in length and are cm in
width
14. A source of sand is required. The most important sources are/were:
a. Rivers, such as the Mojave River and the Whitewater River
b. Lake shorelines. Ventifacts are commonly found near pluvial lakes, including
Silver Lake, Searles Lake, Lake Russell, etc.
15. Ventifacts are more than local “curiosities,” but are actually quite widespread in
California, allowing the opportunity to map them to detail regional sand transport
over time.
16. Ventifacts are found in both arid and periglacial/paraglacial settings in California.
Coastal abrasion appears to be less common.
a. Thus, ventifacts are widespread in the east-central Mojave Desert (for
example, at the Cady Mountains and the Little Cowhole Mountains); in
Death Valley, including sites other than Ventifact Hill; in the Panamint Valley
(allowing the tracing of sand travel to the north towards the star dunes at the
north end of the valley); and other locations.
b. Ventifacts are also common into the Great Basin east of the Sierra Nevada,
located on many of the glacial moraines, the Aeolian Buttes, areas extending
eastward into Nevada, and the area to the south of Owens Lake.
AGE OF VENTIFACTS
17. Most ventifacts are relict as indicated by:
a. Staining of the surface
b. The development of rock varnish.
c. The presence of lichens
d. “Pedestalling” of the hardened, abraded surface on moraines, as the rest of
the rock weathers away
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e. Displacement of the rocks by earthquakes, downslope movement, freeze-thaw
processes, animals, or tree roots
18. Abrasion is active in some locations, often near hillcrests. The lower parts of the
slopes may contain relict ventifacts. Locations in the Mojave Desert where active
ventifaction can be observed include
a. The Little Cowhole Mountains
b. The Bristol Mountains
c. The Pisgah lava flow
HEIGHT OF WIND EROSION AND FLUCTUATIONS IN ABRASION OVER
SHORT TIME PERIODS
19. The height of wind erosion on a ventifact may exceed a meter (general zone of
saltation) owing to
a. Moat formation, which propels sand to higher levels on the ventifact
b. Sand which is propelled from an upwind ventifact onto a downwind rock
c. Increases in the level of the surface as dunes move across surfaces
J.E. Laity, N.T. Bridges / Geomorphology 105 (2009) 202–217
. 13. Schematic of ventifact abrasion processes based on observations made over several hours during a high wind event in the Mojave River Sink area (Fig. 4). Even in the absence
and shadows, lee side abrasion was not observed. Sand that flows up sand ramps in front of boulders or is projected from one boulder to another increases the height of abrasion in
field relative to that predicted from kinetic energy fluxes (Fig. 12) alone.
20. Wind flow during a storm event is complex.
a. As sand passes over the rocky surface, some rocks are buried, while others are
re-exposed.
The field studies during a high wind event at Rasor Road (Mojave
Regardless of rock lithology, ventifacts show the anticipated relationship
sert, CA) and observations of post and pole abrasion in the LCM
between maximum abrasion and feature formation on the windward
owed several reasons for elevated heights of sand abrasion: 1) a
face, and
Ventifact field trip to Mojave Desert, October 2012
12 a facet that slopes away from the wind (Smith,1984; Laity,1987,
nsiderable rise in the overall level of the sand surface from
1994). These results support those of other researchers who have
nsport; 2) sand bouncing from hard surfaces (scoured bedrock or
undertaken process-based field research, including McKenna-Neuman
ntifacts) to levels greater than predicted when saltation occurs on
and Gilbert (1986) and Mackay and Burn (2005).
nes; 3) sand hitting angled rock faces and rebounding upward into
By contrast, Whitney and Dietrich (1973) argue that backside
e higher velocity air stream, and impacting ventifacts many meters
vortices entraining dust abrade the leeward side of rocks as effectively
wnwind at higher levels than predicted from saltation consideraas the front side. The experiments used uncompressed and comns alone; and 4) sand ramp and moat formation in front of high
pressed air in a glass tube and therefore had no controlled boundary
b. The wind direction may completely reverse following the passage of a front.
c. Sand may be aligned in ribbons, with some ventifacts exposed, and others
buried (see photo below).
d. There is considerable interaction within the field of ventifacts itself: the
ventifacts cannot be considered in isolation
Rasor Road: Wind speed to 23 m/s
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VENTIFACTS ON MARS
These are very common at the Pathfinder, Spirit, Opportunity, and Curiosity sites.
Basically, anywhere on Mars where we’ve roved, we’ve seen abraded rocks. Below are some
examples.
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Ventifacts and abrasion textures at the Mars Exploration Rover sites (a) The fluted rock
“Mazatzal” at the Spirit site (Pancam composite color). (b) Fluted rocks at the Spirit site (Pancam
composite color, Sol 584). (c) Bedded rocks that have preferentially abraded along weak layers,
Columbia Hills, Spirit site (Pancam, Sol 754). (d) “Tails” in the lee of resistant nodules within
sulfate-rich soft rock at the Opportunity site (Hazcam, Sol 142). (e) Faceted and fluted rocks at
the Spirit site (Pancam, Sol 585). (f) Faceted rocks near bedforms at the Spirit site (Pancam, Sol
620). (g) Faceted rocks at summit in Columbia Hills, Spirit site (Pancam, Sol 1344). h)
Microscopic image of rock texture in vicinity of (c) (image width is 3 cm, Sol 753).
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Gale Crater
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