Laboratory Manual for Introductory Geology 4e

FIGURE 6.12 Cross bedding, a type of sedimentary structure within a bed.
Position of dune
crest at Time 1
Current
Slip face
Wind
Cross-bed orientation
indicates wind direction at
the time of deposition.
Time
Position of dune
crest at Time 2
Cross bed
Erosion
Cross
bedding
Deposition Main
bedding
Main
bed
(a) Cross beds form as sand blows up the windward side of a dune
or ripple and then accumulates on the leeward side (the slip face).
Over time, the dune crest moves.
(b) A cliff face in Zion National Park, Utah, displays large cross
beds formed between 200 and 180 million years ago, when the
region was a desert with large sand dunes.
FIGURE 6.13 Formation of mud cracks.
(a) Mud cracks in modern sediment. The circular impressions
were made by raindrops hitting the mud before it dried.
(b) Mud cracks preserved in 180-million-year-old mudstone.
FIGURE 6.14 Impressions on bedding planes.
(a) Worm burrows on siltstone in Ireland (lens
cap for scale).
(b) Footprints of dinosaurs of varying sizes.