Them that make ripples and them that don’t

A Piece of Mars: Higher ground is to the left. You’re seeing a tan layer sandwiched between two gray layers in this 0.96×0.54 km (0.6×0.34 mi) scene. Large ripples have accumulated in the lowest area to the right, which is the floor of an old river channel. Ripples have also formed on the gray upper layer. But not the middle tan layer – maybe it’s too fine-grained to erode into sand grains, or maybe it erodes too slowly to allow any eroded sand grains to pile into ripples before they’re blown away. (HiRISE ESP_048196_1995, NASA/JPL/Univ. of Arizona)