INTERFERENCE RIPPLES
Interference ripples (a.k.a. "ladderback
ripples") are uncommon sedimentary structures. They consist of
sediment surfaces showing two sets of ripple crests intersecting at high
angles. They form as ripples get developed on an already-rippled sediment
surface. They indicate the existence of two successive current
directions.
Interference ripples on modern shallow-water seafloor (western French Bay,
southwestern San Salvador Island, eastern Bahamas) (see video). The
seafloor sediments are composed of biogenic aragonite (CaCO3).
The prominent ridges are symmetrical ripple marks, formed by a dominant,
shallow-water, two-directional current. The smaller, ladder-like ridges
are symmetrical ripple marks formed by a subordinant, shallow-water,
two-directional current. The two sets of ripple marks form an interesting
network of seafloor ridges - interference ripples.
Interference ripples in quartzose sandstone (Berea Sandstone, lower
Kinderhookian Stage, lowermost Mississippian). The two successive current
directions indicated by the symmetrical ripples on this slab are left-to-right
and up-and-down. The ripple set with ridges oriented up & down formed
by a dominant shallow-water, two-directional current moving
perpendicular to the ridge crests (= left-and-right direction, when looking at
this upright slab).
The ripple set with ridges oriented left & right
formed by a subordinant shallow-water, two-directional current moving
perpendicular to their ridge crests (= up-and-down direction, when looking at
this upright slab).
OSU public display (Orton Geology Museum, Ohio State
University, Columbus, Ohio, USA).