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 over a vent. A physical model of the resulting fluid flow field can be devised for any planetary environment 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. -from Authors