Sand-wave structures of both progressive (downstream migrating) and regressive (upstream migrating) types occur within the same pyroclastic-surge deposit at both Roccamonfina volcano (southern Italy) and Sugarloaf Mountain (Arizona, United States). Both "wet" and "dry" surge deposits are described that contain examples of regressive and progressive sandwave structures. At Sugarloaf Mountain progressive and regressive sand waves occur within metres of each other within the same bedset, indicating that penecontemporaneous deposition of the two different sand-wave structures occurred in close proximity. On the basis of this evidence it is suggested that the migration direction of sand waves in these pyroclastic-surge deposits is controlled by the flow regime of the surge cloud rather than the temperature and/or moisture content.

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