HiRISE image PSP_001508_1245 covers a portion of a sinuous ridge on the floor of Argyre Basin in the southern hemisphere of Mars (55.0 degrees south, 319.2 east).

The ridge is one of a number of similar ridges that split and rejoin as they wind across the floor of the basin and around hills and mountains. It is unclear what process is responsible for formation of the ridges, but glacial, coastal, and fluvial processes have all been suggested.

For example, they may represent ancient coastal shorelines, sediments deposited in a river flowing under glacial ice, or an ancient river bed that has been left standing in relief as surrounding, probably finer grained sediments were subsequently eroded away.

The HiRISE image shows that the sediments forming the ridge include many large boulders that are often 1-2 meters or more in diameter. In addition, the sediments appear to occur in crude layers in a few locations. Such characteristics hold important clues to the process(es) responsible for formation of the ridge.

For example, if the ridge is the deposit formed by an ancient river then it may be difficult to account for the transport of so many large boulders.Written by: John Grant   (6 February 2007)