Deposits of Sand: Ripples Versus Dunes - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 56

Fig. 5.8 Granule megaripples, with smaller accumulations of coarse
granules on lee of each megaripple, Great Sand Dunes National Park
and Preserve. Megaripples are coated by mm-sized granule particles.

Normal aerodynamic sand ripples are encroaching onto the megarip-
ples, from lower left in this view. Scale is 10 cm long. Photo J.
Zimbelman

describe large wind ripples coated by coarse particles, but
the term 'megaripple' is perhaps the one carrying the least
interpretive 'baggage' with its use.
Sharp (1963) described 'granule ripples' as consisting of
a surface concentration of grains[1 mm (Fig. 5.7 ), roughly
equivalent to Bagnold's 'ridges'. Sharp concurred with
Bagnold that impact creep, accompanied by significant
deflation of the sand surface, was the dominant process
involved in the formation of granule ripples. Ripples coated
by coarse-grained particles can grow to great size; wave-
lengths of 20 m and heights of 60 cm are reported by
Bagnold (1941, p. 155), and recently wavelengths of 43 m
and heights of [2 m are reported for gravel-covered ripples
in Argentina (Milana 2009; de Silva et al. 2011; see
Fig. 5.15 ). More typically, granule ripples have wave-
lengths in the range of *3 to 10 m and heights of *25 to

40 cm (Sharp 1963; Zimbelman and Williams 2006;

Zimbelman et al. 2012; see Fig. 5.7 ).
The coating of coarse particles, whether granules or
gravel, tends to be concentrated at the surface of the bed-
form, usually as a monolayer of the larger particles, with the
interior of the bedform comprised primarily of sand and
only occasional coarse particles (Fig. 5.16 ). For granule
ripples, the granules are most thickly accumulated at the
crest of the ripple (Sharp 1963).
Profiles measured perpendicular to the crest of the
features turn out to be valuable for distinguishing between
aerodynamic sand ripples, megaripples, and sand dunes.
Ripple profiles can be measured in the field from photo-
graphs (see Chap. 16 ) —a neat field trick is to use a
straight edge to cast a shadow on the ripple surface. The
profile shapes are distinct for each of the three classes of