All articles by Wretch Fossil are here: http://www.wretch.cc/blog/lin440315&category_id=0
High Dune and Namib Dune in Gale Crater, Mars began as shoals (notes 1, 2) at lake bottom. After the lake dried up, the shoals became sand dunes. Note 1: “The origin of sand dunes is very complex, but there are three essential prerequisites: (1) An abundant supply of loose sand in a region generally devoid of vegetation (such asan ancient lake bed or river delta); (2) a wind energy source sufficient to move the sand grains; and (3) a topography whereby the sand particles lose their momentum and settle out.” (quoted from http://waynesword.palomar.edu/ww0704.htm#Origin )