Effects of Current
117
a
b
Fig. 4.17 a, b. Current acrtion in tidal flats. a Mya arenaria in life position uncovered by erosion of
about 20--30 cm of mud, by lateral migration of tidal channel. Note the residual bivalve shells which
collect as a "pavement" on the erosional surface. b Large current-produced ripples, covered by small
ripple marks. German Bight north of Weser estuary. Ripples migrate ocean-ward (to the left) , hence
they are produced by currents of the outgoing tide. [Photos E. S.)
a
b
Fig. 4.18 a, b. Water a and sand b movement at current ripple marks. Sand is eroded at the luff side
(sector 2) and migrates to the ripple crest. From there sand is transported on the bottom or in
suspension (white arrows) downward, forming lee laminae . Horizontal bottom vortex in the ripple
valley removes fine grains, concentrates coarse material , including shells. Therefore ripples move
over coarser-grained base laminae. [H. E. Reineck, 1961 , Senckenbergiana Lethaea, 42: 51 J
117
a
b
Fig. 4.17 a, b. Current acrtion in tidal flats. a Mya arenaria in life position uncovered by erosion of
about 20--30 cm of mud, by lateral migration of tidal channel. Note the residual bivalve shells which
collect as a "pavement" on the erosional surface. b Large current-produced ripples, covered by small
ripple marks. German Bight north of Weser estuary. Ripples migrate ocean-ward (to the left) , hence
they are produced by currents of the outgoing tide. [Photos E. S.)
a
b
Fig. 4.18 a, b. Water a and sand b movement at current ripple marks. Sand is eroded at the luff side
(sector 2) and migrates to the ripple crest. From there sand is transported on the bottom or in
suspension (white arrows) downward, forming lee laminae . Horizontal bottom vortex in the ripple
valley removes fine grains, concentrates coarse material , including shells. Therefore ripples move
over coarser-grained base laminae. [H. E. Reineck, 1961 , Senckenbergiana Lethaea, 42: 51 J