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Fig. 4.12 Climbing ripples: in the lower part of the figure,
more of the stoss side of the ripple is preserved, resulting
Fig. 4.10 Migrating straight crested ripples form planar in a steeper ‘angle of climb’.
cross-lamination. Sinuous or isolated (linguoid or lunate)
ripples produce trough cross-lamination. (From Tucker
1991.)
deposition will occur on the stoss side as well as on the

lee side. Climbing ripples are therefore indicators of
rapid sedimentation as their formation depends upon
the addition of sand to the flow at a rate equal to or
greater than the rate of downstream migration of the
ripples.

Constraints on current ripple formation
The formation of current ripples requires moderate

%
& ' flow velocities over a hydrodynamically smooth bed
(see above). They only form in sands in which the
Fig. 4.11 In plan view current ripples may have straight, dominant grain size is less than 0.6 mm (coarse sand
sinuous or isolated crests.
grade) because bed roughness created by coarser sand
creates turbulent mixing, which inhibits the small-
scale flow separation required for ripple formation.
reduced leaving cross-laminae created by earlier Because ripple formation is controlled by processes
migrating ripples preserved. In this way a layer of within the viscous sublayer their formation is inde-
cross-laminated sand is generated. pendent of water depth and current ripples may form
When the rate of addition of sand is high there will in waters ranging from a few centimetres to kilo-
be no net removal of sand from the stoss side and each metres deep. This is in contrast to most other subaqu-
ripple will migrate up the stoss side of the ripple form eous bedforms (subaqueous dunes, wave ripples),
in front. These are climbing ripples (Allen 1972) which are water-depth dependent.
(Fig. 4.12). When the addition of sediment from the Current ripples can be up to 40 mm high and the
current exceeds the forward movement of the ripple, wavelengths (crest to crest or trough to trough