Lava flows cool as they move allowing their surface to freeze solid. The constant movement below this surface can break it up into a rough mass of jumbled broken rock. Sometimes larger surface areas that are thicker can behave like rafts that are dragged along by the flow.
Both features are visible in this image of an ancient lava flow in Cerberus Planitia. Isolated rafts that are still high-standing are visible and frozen into the flow. The rough areas show where the flow was fastest and have merged in places forming the large chevron-shaped features we see here.
Written by: HiRISE Science Team (19 February 2014)
More info and image formats at http://hirise.lpl.arizona.edu/ESP_034887_1870
Image: NASA/JPL/University of Arizona