Laboratory Manual for Introductory Geology 4e

FIGURE 6.11 Ripple marks.
Steep side
Gentle side
(a) Asymmetric ripple marks in modern sand. The arrow indicates the
direction of the current that formed the ripples.
(b) Symmetric ripple marks in ancient sandstone. Note the
equally steep slopes on both sides of the ripples.
water, sand, silt, and gravel in a jar or graduated cylinder. Shake to mix thoroughly
and watch as the grains settle.
6.6.2b Ripple Marks, Dunes, and Cross Beds Ripple marks are regularly spaced ridges
on the surface of a sedimentary bed that form at right angles to the direction of the
current flow when sand grains are deposited by flowing air or water. Small ripple
marks are shown in FIGURE 6.11. Some ripple marks have systematically oriented
steep and gentle sides (Fig. 6.11a). These asymmetric ripple marks are produced
by a current that flowed from the gentle side toward the steep side. Symmetric
ripple marks have steep slopes on both sides and form from oscillating currents
(Fig. 6.11b). Larger mounds of sediments that resemble asymmetric ripple marks
in shape are called dunes. The largest dunes occur in deserts, where they build from
windblown sand, but smaller dunes can also form underwater.
In some sedimentary rocks, subtle curved surfaces, delineated by coarser and/or
denser grains, lie at an angle to the main bedding surfaces. These inclined surfaces,
called cross beds, form when sediment moves up the windward (or upcurrent) side
and then slips down the leeward (or downcurrent) side as dunes and ripples form
and migrate (FIG. 6.12).
6.6.2c Mud Cracks and Bedding-Plane Impressions Mud cracks are networks of
open fractures in the surface of a bed that form when mud dries out and shrinks.
Typically, mud cracks display a honeycomb-like pattern (FIG. 6.13a). Sand deposited
over the mud layer fills the cracks, so when the sediment lithifies, the shape of
the cracks remains visible (FIG. 6.13b).
Moving objects can leave impressions on unconsolidated sediment before the
bed hardens into rock, and the imprints of these objects can provide clues to animals
living in the sediment or processes acting on it. Imprints may be made by
raindrops falling on a muddy floodplain (see Fig. 6.13a), logs dragged by currents
along the bottom of a stream, worms crawling along the seafloor in search of food
(FIG. 6.14a), and the feet of animals (FIG. 6.14b). Turbulent water may also cut into
the unlithified sediment in the underlying beds.
6.6 SEDIMENTARY STRUCTURES: CLUES TO ANCIENT ENVIRONMENTS
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