Grey tuffs of late Pleistocene age form broad fans radiating from the Laacher See basin. They were derived from phreatomagmatic outbursts, and transported in turbulent pyroclastic flows, in contrast with the underlying white pumice tuffs of air fall origin. Flow origin of the grey tuffs is inferred from the well‐bedded plane parallel to cross‐bedded tephra characteristic of base surge deposits, and a variety of other sedimentary structures, as well as grain size distributions. We recognize a tentative sequence of five main kinds of dune structures or cross‐bedded strata. With some reservations these may be compared with the high flow‐regime alluvial bedforms produced experimentally in flumes. Most of the cross‐bedded structures in the Laacher See deposits resemble antidunes, with steep stoss sides and very low‐dipping lee sides. Upcurrent migration of antidune crests is dominant close to the source, but changes to downcurrent migration at greater distances, presumably because of decay in flow energy. The most spectacular cross‐bedding is somewhat similar to chute and pool structures formed under experimental condition in alluvial flumes, but not recognized in ancient sedimentary rocks. We suggest that these structures of the Laacher See tuffs formed during deposition from phreatic pyroclastic flows of very high flow energy and high sediment concentration. The antidunes apparently formed at lesser flow velocity than chute and pool structures, although interpretation of velocity conditions by examination of the deposits is difficult because of other factors such as the cohesiveness of wet material erupted by explosive phreatic volcanic activity. The large wave lengths of the dune‐like structures, however, suggest unusually high velocities. The Laacher See magmas were of phonolitic to tephritic composition, and may have erupted with greater explosive energy and in greater volume than comparable basaltic eruptions.

Volcanic base surges from Taal volcano (1965, 1966) and Capelinhos volcano (1957–1958) showed all transitions from: (1) small radial surges of nearly pure white steam puffing out from the base of tall, vertically rising eruption columns charged with abundant black particles, to (2) much larger, dark gray, particle‐laden base surges fed by subsidence of the eruption column. Radial dispersal of volcanic base surges occurs essentially in three stages: (1) Surges of white steam with only a few solid particles spread immediately from the base of the emerging eruption column as water vapor concentrated along its periphery escapes and condenses; (2) Black plumes of solid particles shoot radially on ballistic trajectories from the walls of the rising eruption column as it is torn apart by internal steam bursts; and (3) The turbulent mixture of solid particles, water, and air tumbles en masse to the ground, and much of it flows away on the heels of the steam surge and incorporates it. Deposits from the particle‐laden surges consist mainly of chilled sideromelane accumulated in characteristically cross‐bedded dunelike layers with poor sorting. These alternate with air‐fall layers, many of which are rich in accretionary lapilli. At Taal, cross bedding developed from the deposition of dunelike bed forms that were eroded and buried by the debris of the next passing base surge. The dune shapes and form of internal cross bedding resemble antidunes that develop in high‐flow regimes on sandy beds of sediment‐laden streams, and from underwater density currents simulated in flumes. The cohesiveness of base‐surge deposits, shown by bedding sags and mud‐plastered walls and trees, is a major factor in preserving the antidune bed forms, but is not a factor in their initial development.

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