This evenly layered rock photographed by the Mast Camera (Mastcam) on NASA's Curiosity Mars Rover shows a pattern typical of a lake-floor sedimentary deposit not far from where flowing water entered a lake.

This is an example of a thick-laminated, evenly-stratified rock type that forms stratigraphically beneath cross-bedded sandstones regarded as ancient river deposits. These rocks are interpreted to record sedimentation in a lake, as part of or in front of a delta, where plumes of river sediment settled out of the water column and onto the lake floor.