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).