There are some important differences between the way that primary volcaniclastic material behaves during transport and deposition and the terrigenous clastic detritus considered in earlier chapters. An important physical control on sedimentation is that the settling velocity is proportional to fragment size, shape and density (4.2.5). Unlike terrigenous clastic material, the density of pyroclastic particles is very variable. In particular pumice pyroclasts may have a very low density and can float until they become waterlogged (Whitham & Sparks 1986). Grading in pyroclastic deposits may show both normal and reverse grading of different components in the same bed. Lithic fragments and crystals will be normally graded, with the coarsest material at the base. Pumice pyroclasts deposited in water may be reverse graded because the larger fragments will take longer to become waterlogged and hence will be the last to be deposited, resulting in reverse grading. Three primary modes of transport and deposition are recognised: falls, flows and surges, but it should be noted that all three can occur associated with each other in a single deposit.