Geoscience Reference
In-Depth Information
angle, although these relationships have not been quanti-
fied: flutes are shorter and deeper on higher angle faces and
become more elongate as the facet angle declines (Max-
son, 1940; Sharp, 1949). In many cases, the scale of the
flutes increases from the base of the rock to the upper face.
Sometimes, smaller flutes develop within larger ones, sug-
gesting renewed cycles of erosion, the result of a change in
climate, sediment supply or wind energy. Flutes are very
common forms on ventifacts: rarer are scallops and heli-
cal scores.
Scallops
are U-shaped features that are open at
one end and closed at the other, and have similar length-
to-width ratios. They are present on some Martian rocks.
Helical forms
begin as shallow grooves, deepen and spiral
in a downwind direction and terminate in a sharp point.
In theory, they can be both right- or left-handed. Observa-
tions in the Mojave Desert suggest that the handedness of
the spiral is consistent across the face of the rock. Helical
forms have been observed in marble, basalt and granite, of-
ten where the wind velocity is accelerated, such as within
topographic saddles and near hillcrests. They form on the
upper faces of boulders and, like flutes, appear to increase
in scale up the rock. The most elongate element on a ven-
tifact is the
groove
. These subparallel forms are open at
both ends and best developed on surfaces gently inclined
or parallel to the wind. They are sometimes found on the
vertical sides of boulders, particularly in the cracks be-
tween adjacent rocks. Like flutes, they cut across mineral
grains and rock structures. Grooves vary greatly in scale,
from striae (fine lineations) (Figure 21.13), to grooves of
intermediate scale, to channels that may be several cen-
timetres or more in depth (Tremblay, 1961). Striae may
cross an entire outcrop, but in detail each lineament is
Figure 21.11
Basalts are often intensely pitted, as erosion
modifies pre-existing vesicles on high-angle, windward faces.
Pits merge over time and become larger. As the face angle
lowers near the upper surface of the rock, pits are elongated
to form flutes.
including pits, flutes, grooves, scallops and helical forms.
Pits
occur on the high-angle (55-90
◦
) windward surfaces
of boulders (Figure 21.11). The pits vary in form from
the more rounded and regular shapes, characteristic of
basalts, to more irregular ovoid forms, observed in tuffs
(Laity, 2009). The wind erodes and modifies pre-existing
indentations (such as vesicles in basalt) and softer miner-
als (as in the case of granites or tuffs).
As the angle of the facet decreases,
flutes
develop. In
form, they appear as 'arrowheads', open at one end and
closed at the other, which point in a downwind direction
(Figure 21.12). The form of the flute varies with the facet
Figure 21.13
Fine lineations (grooves) on a marble ventifact
parallel with the direction of the highest velocity winds. The
