Loess formation generally involves the four main stages of production, deflation, transportation, and deposition of loess particles. Traditionally, loesses are classified as glacial, desert, or other types based only on source area characteristics without taking into account provenance and transportation. Research on loess genesis is often local with only a few attempts at systematically overviewing worldwide loess distribution. Both the local and few global studies lack information on the variability in loess thickness, continuity, and areal extent. This review integrates a large body of information on loess source areas and transportation pathways and the existence of desert transition zones. Three modes of loess genesis, (1) continental glacier provenance-river transport, (2) mountain provenance-river transport, and (3) mountain provenance-river transport-desert transition, were identified. Global distribution maps of provenance and transport pathways of major loess areas and their different genesis modes were meticulously prepared. Maps showing the spatial distribution, thickness, and continuity of loess deposits were also composed and the sizes of loess-covered areas of each continent were re-estimated. The main features related to the distribution of the loess deposits on each continent are summarized for different regions or as a whole depending on the coverage of the source maps and references.

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