Large-scale pyroclastic flows (ignimbrites) are produced when a dense mixture of volcanic gas and pyroclasts is erupted into an atmosphere and forms a collapsed, fountain-like structure (Sparks et al., 1978) over a vent. A physical model of the resulting fluid flow field can be devised (Heslop, 1987) for any planetary environment (Heslop and Wilson, 1988) which allows the pressure and velocity of the ''dusty gas'' mixture emerging from the vent to be deduced. Calculation of the drag forces exerted by the dusty gas on relatively large clasts allows us to deduce the sizes of the largest clasts of a given density which can just be transported through the vent to contribute to a proximal lag breccia deposit. We find that ignimbrites on Earth are potentially capable of transporting out of the vent significantly larger clasts than those commonly found in lag deposits; Martian ignimbrite lag breccia deposits should contain near-vent clasts at least twice as large as those in terrestrial deposits produced in eruptions with the same volatile contents. Such clasts should be detectable from future Mars-orbiting spacecraft. Furthermore, the near-vent clast sizes in Martian ash fall deposits will be at least a factor of 100 times smaller than those in proxmal ignimbrite deposits. When coupled with other observations of the morphology of near-vent deposits, the detection of the presence of coarse clasts should provide a strong indication of the eruption mechanism which emplaced them. ¿ American Geophysical Union 1990